CS402 Short Notes PDF

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Virtual University of Pakistan http://vustudents.ning.com/ CS402 ShortJoin Us on http://vustudents.ning.com/ Notes FAQ's about Lectures 1 to 5 QNo1.What is the difference between the strings and the words of a language? Answer:A string is any combination of the letters of an alphabet where as the words of a language are the strings that are always made according to certain rules used to define that language.For example if we take Alphabet = { a , b } Here a , b are the letters of this alphabet. As you can see we can make a lot of strings from these letters a and b. For example a,b,aa,ab,ba,bb,aaa,aab,aba,baa,............................................................ and so on. But when we define a language over this alphabet having no a's and only odd number of b's. Then the words of this language would have only those strings that have only odd number of b's and no a's.some example words of our defined language are b , bbb , bbbbb , bbbbbbb ,...................................and so on. So we can say that all the words are strings but all the strings may not be the words of a language.Hence strings are any combination of letters of an alphabet and the words of a language are strings made according to some rule. QNo.2 What is the difference between an Alphabet and an element of a set. Whether Alphabet is an element of a set or it is a set itself? Answer:An Alphabet is a set in itself. The elements of an Alphabet are called letters. For example Binary Alphabet = {0,1} Here 0,1 are the letters of binary alphabet. Binary Alphabet is very important because it the Alphabet used by the computer. Set of Natural Numbers N={1,2,3,4,5,.........................................} Here 1,2,3........................................... are the elements of set of http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Natural Numbers. QNo.3 What is Null String ( ) ? Answer:The string with zero occurrences of symbols (letters) from ∑. It is denoted by (Small Greek letter Lambda) or (Capital Greek letter Lambda) , is called an empty string or null string. The capital lambda will mostly be used to denote the empty string, in further discussion. QNo.4 What is PALINDROME ? Answer:The language consisting of (Null String) and the strings s defined over an Alphabet such that Rev(s)=s. Some example words of this language are aa As Rev(aa) = aa aba As Rev(aba) = aba bbb As Rev(bbb) = bbb aabaa As Rev(aabaa) = aabaa bbbaaabbb As Rev( bbbaaabbb ) = bbbaaabbb It is to be noted that the words of PALINDROME are called palindromes. QNo5.What is the concept of valid and invalid alphabets ? Answer:While defining an alphabet of letters consisting of more than one symbols, no letter should be started with any other the letter of the same alphabet i.e. one letter should not be the prefix of another. However, a letter may be ended in the letter of same alphabet i.e. one letter may be the suffix of another. = { a , b } ( Valid Alphabet) = { a , b , cd } ( Valid Alphabet) http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ = { a , b , ac } ( Invalid Alphabet) QNo 6. What is ALGOL ? Answer:ALGOL (ALGOrithmic Language) is one of several high level languages designed specifically for programming scientific computations. It started out in the late 1950's, first formalized in a report titled ALGOL 58, and then progressed through reports ALGOL 60, and ALGOL 68. It was designed by an international committee to be a universal language. Their original conference, which took place in Zurich, was one of the first formal attempts to address the issue of software portability. ALGOL's machine independence permitted the designers to be more creative, but it made implementation much more difficult. Although ALGOL never reached the level of commercial popularity of FORTRAN and COBOL, it is considered the most important language of its era in terms of its influence on later language development. ALGOL’s lexical and syntactic structures became so popular that virtually all languages designed since have been referred to as "ALGOL - like"; that is they have been hierarchical in structure with nesting of both environments and control structures. QNo7. What are the Sequential Operators? Answer:Sequencing Operators: Sequencing operators a >> b Sequence Match a and b in sequence Sequential- Sequential-and. Same as above, a && b and match a and b in sequence a || b Sequential-or Match a or b in sequence The sequencing operator >> can alternatively be thought of as the sequential-and operator. The expression a && breads as match a and b in sequence. Continuing this logic, we can also have a sequential-or operator where the expression a || b reads as match a or b and in sequence. That is, if both a and b match, it must be in sequence; this is equivalent to a >> !b | b. QNo 8.What is Non-Determinism and Determinism and what is the difference between them ? http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Answer:Determinism means that our computational model (machine) knows what to do for every possible inputs. Non determinism our machine may or may not know what it has to do on all possible inputs. As you can conclude from above definition that Non-Deterministic machine can not be implemented ( used ) on computer unless it is converted in Deterministic machine. QNo 9. What is meant by equivalent FA's ? Answer:FA's that accept the same set of languages are called Equivalent FA's. QNo 10. What is the difference between Palindrome and Reverse function? Answer:It is to be denoted that the words of PALINDROME are called palindromes. Reverse (w) = w Example: ={a,b}, PALINDROME={ , a, b, aa, bb, aaa, aba, bab, bbb,...} If a is a word in some language L, then reverse (a) is the same string of letters spelled backwards, called the reverse of a. e.g reverse (xxx) = xxx reverse (623) = 326 reverse (140) = 041 QNo11.Define Kleene Star? Answer:Given , then the Kleene Star Closure of the alphabet , denoted by *, is the collection of all strings defined over , including It is to be noted that Kleene Star Closure can be defined over any set of strings. Examples If = {x} Then * = { , x, xx, xxx, xxxx, ….} If = {0,1} Then * = { , 0, 1, 00, 01, 10, 11, ….} If = {aaB, c} http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Then * = { , aaB, c, aaBaaB, aaBc, caaB, cc, ….} Note: Languages generated by Kleene Star Closure of set of strings, are infinite languages. (By infinite language, it is supposed that the language contains infinite many words, each of finite length) QNo12.Valid/In-Valid alphabets? Answer:Any alphabet is valid if any of its letter does not appear in the start of any other letter otherwise it is invalid. QNo13.What is Reverse of a string? Answer:Alphabet provides only a set of symbols. A string is a concatenation of these symbols. Reverse of the string means to write the string in reverse order. It has no effect on alphabet. Alphabet will remain same. QNo14.Differentiate Kleene Star Closure and PLUS? Answer:Given , then the Kleene Star Closure of the alphabet , denoted by *, is the collection of all strings defined over , including. Plus Operation is same as Kleene Star Closure except that it does not generate (null string), automatically. You can use other symbol for alphabet but we are mostly use sigma symbol. QNo15.Define Regular Expression? Answer:Regular Expression is the generalized form of any regular language through which you can construct any string related to that language. Take an example from your handouts L1 = { , a, aa, aaa, …} and L2 = {a, aa, aaa, aaaa, …} can simply be expressed by a* and a+, respectively. so a* and a+ are the generalized form of Languages L1, L2. And a* and a+ are called the regular expressions (RE) for L1 and L2 respectively. FAQ's about Lectures 6 to 10 Automata Theory FAQ's about Lectures 6 to 10 Q No.1 What is the concept of FA also known as FSM ( Finite State Machine) ? FA (Finite Automaton) is a finite state machine that recognizes a regular language. In http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ computer science, a finite-state machine (FSM) or finite-state automaton (FSA) is an abstract machine that has only a finite, constant amount of memory. The internal states of the machine carry no further structure. This kind of model is very widely used in the study of computation and languages. Q No.2 What is the difference between FA , TG , GTG. ? In every FA, we mark transitions with single letter of the given alphabet but in TG transitions can be marked with letters or strings (combination of letters). In every FA, every state shows transition for all letters of given alphabet but in any TG it is not necessary to show all transition for all letters of given alphabet. In TG, we may or may not show all letter transitions according to requirement. We can also show transitions on reading any strings in TGs but it is not possible in FA's. In GTG Directed edges connecting some pair of states are labeled with regular expressions. It may be noted that in GTG, the labels of transition edges are corresponding regular expressions. In TG we write strings and in GTG we are bound to write RE. Every FA is also a TG but not every TG is FA. Q No.3 What is the difference between FA's and TG's.Why we need TG's when we have FA's? The Transition Graphs (TG) differ from FA in the following areas TG's are generalizations of FA's. TG's can change state without an input ( Null transition). Can read more than one letter (words of the language they are accepting) along the transition edges at a time. Can have a regular expression as a edge label. Can have more then one start state. We have been given more freedom in TG's. But this freedom is on the cost of more memory and processing power it means that if we implement TG's on computer using some programming language it will need more memory and processing power of computer than used in the implementation of FA's. Q No.4 What is the concept of the Union of FA's ? When we take Union of two FA's it means that resultant FA's should accept all the words that were accepted by the two FA's individually. It is like taking union of two sets, the resultant set contain members of both sets. For example Let A ={1,3,5,7,9} and B = {0,2,4,6,8,10} then, A U B = { 0,1,2,3,4,5,6,7,8,9,10 } you can see that A U B contain elements of both sets similar is the case with FA's. Q No.5 What is the difference between is TG and GTG ? In TG, there are letter transitions for the strings. While in GTG, one can write whole RE as a transition from one state to another one. Q No.6 How one can create RE of a particular language? First thing about RE and FA is that there is no hard and fast formula or method to generate these. One can generate them by its mental approach. And this mental approach can be acquired through only PRACTICE. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Here are some useful tips to write RE's,  Let our language consist of the words of length three exactly over alphabet = {a,b} then it consists of the words L = {aaa, aab, aba,abb,baa,bab,bba,bbb}. Its RE can be simply written as RE = aaa + aab + aba + abb + baa + bab + bba + bbb which simply means that our language consists of only these words. So we can make RE for a finite language by writing its all words with + operator between them.  We should also keep the null string in our mind. If our language generates null string than our RE should also generate it) For example language having all the words of even length has null string in it as well so we can write its RE as follows RE = ((a+b)(a+b))* This RE also generates null string. If a language generates all strings starting with a. then strings will be of type a , aa, ab, aab, aaa, aba, abb,…. Here RE should start with ‘a’ and then all strings including null. So this will be (a + b)* and complete RE is a (a+ b)*. Similarly languages of strings ending in b will have RE (a + b)*b. Q No.7 What is the diagrammatically difference between FA's and TG's? The main differences between FA’s and TG’s are as follows    Q No.8 What is the corresponding FA for RE =aa((a+b)(a+b))* RE is aa((a + b)(a + b))*. Its corresponding FA is as follows. Q No.9 What is difference between FA's and NFA's. Are they opposite to each other ? FA stands for finite automata while NFA stands for non-deterministic finite automata In FA there must be a transition for each letter of the alphabet from each state. So in FA number of transitions must be equal to (number of states * number of letter in alphabet). While in NFA there may be more than one transition for a letter from a state. And finally every FA is an NFA while every NFA may be an FA or not. Q No.10 Differentiate between (a,b) and (a+b)? (a, b) = Represents a and b. (a + b) = Represents either a or b. FAQ's about Lectures 11 to 15 Q No.1 What is the difference between how’s FA and TG.Why we need TG's when we have FA's? The Transition Graphs (TG) differ from FA in the following areas http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/  TG's can change state without an input ( Null transition). We have been given more freedom in TG's. But this freedom is on the cost of more memory and processing power it means that if we implement TG's on computer using some programming language it will need more memory and processing power of computer than used in the implementation of FA's. Q No.2 What is the concept of the Union of FA's ? When we take Union of two FA's it means that resultant FA's should accept all the words that were accepted by the two FA's individually. It is like taking union of two sets the resultant set contain members of both sets. For example Let A ={1,3,5,7,9} and B = {0,2,4,6,8,10} then, A U B = { 0,1,2,3,4,5,6,7,8,9,10 } you can see that A U B contain elements of both sets similar is the case with FA's. Q No.3 What is the difference between GT and GTG ? In TG, there are transitions for the strings. While in GTG, one can write whole RE as a transition from one state to another one. Q No.4 How to create a RE of a particular Language? Regular expression is used to express the infinite or finite language, these RE are made in such a way that these can generate the strings of that unique language also for the cross check that the defined RE is of a specified language that RE should accept all the string of that language and all language strings should be accepted by that RE. Q No.5 How diagrams of FA's are created ? It depends upon the question how many states involve in a FA. There is not any formal procedure to design FA for a language. This ability just improves with time and practice. Every FA is also a TG but not every TG is FA. In every FA, every state shows transition of all letters of given alphabet but in any TG it is not must. In TG, we may or may not show all letters transition according to requirement. We can also show transitions on reading any strings in TGs but it is not possible in FAs. Q No.6 How one can create RE of a particular language? First thing about RE and FA is that there is no hard and fast formula or method to generate these. One can generate them by their mental approach. And this mental approach can be acquired through only PRACTICE. I am giving you few tips. I hope those will help you. If we have a finite language then it will always be regular and will not have * in RE. e.g. L={aaa, aba, bb}. L language generates given three strings then its RE will be (aaa + aba + bb}. So in finite language + of all strings can be it’s RE. If we have an infinite language, then there will be * in it’s RE. We should also keep the null string in our mind. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ For practice just try to create RE of simple languages. Don’t try to confuse yourself with complex languages. For example if a language generates all strings starting with a. then strings will be of type a , aa, ab, aab, aaa, aba, abb,…. Here RE should start with ‘a’ and then all strings including null. So this will be (a + b)* and complete RE is a (a+ b)*. Similarly languages of strings ending in b will have RE (a + b)*b. I hope now you will be able to generate the RE of simpler languages. Gradually, increase the complexity of languages to become a perfect in RE’s. Now as similar to RE, FA of finite language will not have any loop in it. If language is infinite then there will always be at least one loop in its FA. From RE, if you want to generate its FA, then first get the smallest strings and generate their FA and then gradually get the strings of bigger length and keep amending the created FA. After some practice, you will be able to generate the FA’s. And the last thing nobody can do the new task accurately for the first time. Practice is the key to success. In the start you will have lot of mistakes but after practice you will be able to clear all of them. Q No.7 What is the difference between FA's ,and TG's ? There are two or three big differences between FA’s and TG’s. In FA there can be maximum one initial or starting state while in TG there may be more than one initial state. In FA there can be transition for letters only while in TG transitions from a state to another one can be for strings. In FA there must be transition from each state for each letter (deterministic) while in TG there may be no transition for specific letter from a state and there may be more than one path for a string or letter from a state. Q No.8 What is the exact definition of FA ? Definition: A Finite automaton (FA), is a collection of the followings Finite number of states, having one initial and some (maybe none) final states. Finite set of input letters (Ó) from which input strings are formed. Finite set of transitions i.e. for each state and for each input letter there is a transition showing how to move from one state to another. Q No.9 What is the difference between TG and GTG ? In TG, there are transitions for the strings. While in GTG, one can write whole RE as a transition from one state to another one. For RE =aa((a+b)(a+b))* what will be its corresponding FA ? RE is aa((a + b)(a + b))*. Its corresponding FA is as follows. Q No.10 What is the difference between FA and NFA ? FA stands for finite automata while NFA stands for non-deterministic finite automata In FA there must be a transition for each letter of the alphabet from each state. So in FA number of transitions must be equal to (number of states * number of letter in alphabet). While in NFA there may be a transition for a letter from a state. In NFA there may be http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ more than one transition for a letter from a state. And finally every FA is an NFA while every NFA may be an FA. FA: NFA: Q No.11 What is the method to understanding FA's and NFA's Firstly we know that an FA is used to describe a language. Now a language consists of strings. FA will describe the specific language only if it accepts all the strings of that particular language and all the strings generated by the FA are in the language. So confirmation is of two ways. Now, how to traverse the FA. It is very easy. Every FA has one initial state (state with - sign). From every state of FA there is one transition for every letter of the alphabet. Read the string letter by letter and move according to transitions from state to state. If the string ends in the final state (state with a + sign), that particular string will be accepted otherwise rejected. So, every string ending in final state will be accepted by FA and will be a word of the language. For NFA, there may be no path or more than one path for a letter from a specific state. As similar to FA just start traversing from the initial state and if the string ends in the final state, it will be accepted. Remember, as there may be more than one path for a letter from a state. So any path can be used. Goal is to reach the final state. Remaining theory is same to the FA. Practice is the key to success. Just try simple FA's and NFA's in the start. FAQ's about Lectures 16 to 20 Q No 1.What is the concept of Nondeterministic Finite Automaton (NFA) ?Nondeterminism plays a key role in the theory of computing. A nondeterministic finite state automaton is one in which the current state of the machine and the current input do not uniquely determine the next state. This just means that a number of subsequent states (zero or more) are possible next states of the automaton at every step of a computation. Of course, nondeterminism is not realistic, because in real life, computers must be deterministic. Still, we can simulate nondeterminism with deterministic programs. Furthermore, as a mathematical tool for understanding computability, nondeterminism is invaluable. As with deterministic finite state automata, a nondeterministic finite state automaton has five components.  a set of states  a finite input alphabet from which input strings can be constructed  a transition function that describes how the automaton changes states as it processes an input string  a single designated starting state  a set of accepting states The only difference lies in the transition function, which can now target subsets of the states of the automaton rather than a single next state for each state, input pair. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Q No 2. If a language can be expressed in the form of FA than why it is needed to use NFA ? NFA stands for non-deterministic FA and this sort of structure has relaxation compared with FA. So it is rather more easy to represent a language using NFA. We have methods to convert NFA into FA's so sometimes it is easier to build NFA of a given language and than convert its NFA into FA using these methods rather than directly building an FA for a language which may be very difficult. Q No 3.How to made NFA corresponding to the closure of an FA ? While generating NFA corresponding to closure of an FA one should take care of the null string. Simple way to accept null string is declare initial state, final as well. But in this way a lot of other strings will also be accepted. Therefore, accurate way is draw another state. Declare the new state initial as well as final. Connect the new state with the states originally connected with the old start state with the same transitions as the old start state. Newly drawn diagram will be an NFA representing the language closure of the given FA Q No 4.What is the difference between Union of two FA’s , Concatenation of two FA’s and closure of two FA’s ? Consider two FA's given below a a b b a b a b Y2+ Y1- X2+ X1- FA1 FA2 Here FA1 accepts all strings ending in a and FA2 accepts all strings ending b. An FA corresponding to FA1UFA2 will accept all the strings ending in a or ending in b. for example, aba,bbaaab,bbb An FA corresponding to FA1FA2 will accept all the strings whose first substring belongs to FA1 and second substring belongs to FA2. for example, ababab, bbabbb. An FA corresponding to FA1* will accepts all the strings of FA1 including null string. if FA1 represents RE r1 then FA1* will correspond to RE r1*. FAQ's about Lectures 21 to 25 Automata Theory FAQ's about Lectures 21 to 25 Q No 1.How Moore and Mealy machine works in Computer Memory what is their importance in Computing ? Mealy & Moore Machines work in computing as incrementing machine & 1's complement machine etc. These operations as basic computer operations so these machines are very important. Q No 2.What is sequential circuit ? Sequential Circuit: http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ A sequential circuit contains a memory component. The memory component provides a state input. A flip-flop is often used as a memory component. The state variable indicates the states of the sequential machine, i.e. the status or stage or progress of the whole event. The state of a sequential circuit is indicated by the output of a flip-flop. A single flip-flop can be used to indicate two states (q=0 and q=1). When there are more than two states, additional flip-flops are used. Given n flip-flops, a total of 2n states can be represented. In other words, a sequential machine can be put into a number of different states depending on the particular inputs given. The output is a function of both the Present Inputs and the Present States. In addition to the outputs, the circuit must also generate an update to the memory components so that the state of the machine can also be changed with respect to the new inputs. The update is called the Next State Function and is also a function of the Present Inputs and the Present States. Both the output functions and the Next State Functions are combinational circuits. Z=f(X,St) S=g(X,St) The superscript t indicates the present time period while the superscript (t+1) indicates the next time period. The characteristic of a sequential circuit is completely defined by a state transition diagram that enumerates all possible transitions for every possible input combination. Q No 1.What is the concept of Pumping Lemma I and II and what is the difference between pumping Lemma 1 and pumping Lemma 2 ? In fact PLI & PLII are same (A way to recognize Non Regular language). The only difference is that the conditions in pumping lemma II are more stricter than Pumping Lemma I some language that are difficult to proof Non Regular by Pumping Lemma I are proved Non Regular by pumping Lemma II easily. Further mare in pumping lemma I we have to generate all words to of a language but in Pumping Lemma II we have to generate a single word to prove a language non regular. Explanation: Some languages like PALINDROME that are proved to be regular by first version due to some of their symmetrical words when we pump these words they remain to be the parts of the language like bbabb By pumping lemma 1 Let y = a Now repeating y three times results in bbaaabb That is also a valid word of PALINDROME so by pumping lemma I PALINDROME can not be proved non regular, so there was the need of pumping lemma version version 2. Now consider for the word bbabb if we take N=2 http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Then by pumping y (let we take it b ) two times results in bbbbabb That word is not in PALINDROME. But if we take N=3 and y = a Then by pumping y two times results in bbaaabb That word is in PALINDROME. So be careful in taking total no of states of the FA and also the repeating factor (y) to prove an infinite language non regular you need to prove only one word that is not part of the language. Q No 2. What is the significance of Pumping Lemma II ? The significance of 2nd version of 'pumping lemma' is that there are some infinite non regular languages like PALINDROME we can built FA that can accept there certain words but if we increase the length of their words that FA don't accept these words so by pumping lemma version I it is very difficult to prove them non regular but with the second version we can prove that a language is Non regular even it's some words may be accepted by some FA's. See page 195 of the book for further example. Q No 3.Moore and Mealy machine? 1. In order to run a string on a Mealy or Moore machine, you can take directions from transition table. Running string on Mealy or Moore machine is similar to running string on a FA. For example, if want to run abba on the machine, take start from initial state. Check what is the transition for a, what state it goes. After that check what is the path of b from that state and so on. In this way you will be able to run whole of the string. Note that there is no final state in Mealy or Moore machine. So there is no case of acceptance or rejection of string. You just have to determine what the output is. I hope that will clear your mind for further clarification please listens to your lecture carefully. 2. The string is taken for the testing purposes. You can take any sort of string and determine its output using machine. FAQ's about Lectures 31 to 35 Q No 1.What is the difference between semiword and word please also give an example regarding this? Word: A word is complete combinations of terminals only e.g. abba or ab or a or null string. Semiword: A semiword is a string of terminals (may be none) concatenated with exactly one nonterminal on the right i.e. a semi word, in general, is of the following form (terminal)(terminal) ------------- (terminal)(nonterminal) For example aaaaaaB , aabbaaaA , A. What is the difference between derivation tree and total tree ? A Derivation tree is the one that shows how to derive any specific word of the language described by CFG but Total Language Tree shows all words of the http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Language described by CFG on it Q No 2.What does mean the LANGUAGE IS CLOSED? When we say that a Language is closed it is always with respect to certain operation. A simple example may be that the set of integers is closed under addition. It means when we take two numbers from set of integers say 3, 7 the result of their addition would also be in the set of integers. Similarly if the result of an operation on the words of a language results in the word of the same language we say that the language is closed under that operation. Q No 3.What are the Productions? Productions are the grammatical rules and regulations. These rules express the behavior of CFG. Using production in CFG terminals are converted into non-terminals and when all the terminals are converted using productions, a word is acquired. Q No 4.What is the difference between concatenation and intersection of two FA's also what is the difference among Union of two FA's and addition of them? In intersection of two FA's only those strings are accepted which are independently accepted by both FA’s, while in concatenation of two FA’s only those strings will be accepted in which first part of string is accepted by first FA and remaining part of string is accepted by the second FA. While taking union of two FA’s one can represent it using + sign. So (FA1 U FA2) and (FA + FA2) both are same. There is no difference between them. FAQ's about Lectures 36 to 40 Automata Theory FAQ's about Lectures 36 to 40 Q No 1.What is the Difference between Nullable and Null production? How to make eliminate Nullable and for Null Productions from the CFG ? The production of the form nonterminal ® L is said to be null production. Example: Consider the following CFG S ® aA|bB|L, A ® aa|L, B ® aS Here S ® L and A ® L are null productions. A production is called nullable production there is a derivation that starts at Non Terminal and leads to L i.e. S -----------> aA | bB | aa A-------------> C | bb C--------------> L Here A nullable Non Terminal due to Nullable production A---------------> C as C leads to null. Example: http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Consider the following CFG S ® XY, X ® Zb, Y ® bW Z ® AB, W ® Z, A ® aA|bA|L B ®Ba|Bb|L. Here A ® L and B ® L are null productions, while Z ® AB, W ® Z are nullable productions. Method: Delete all the Null productions and add new productions e.g. Consider the following productions of a certain CFG X ® aNbNa, N ® L, delete the production N ® L and using the production X ® aNbNa, add the following new productions X ® aNba, X ® abNa and X ® aba Thus the new CFG will contain the following productions X ® Nba|abNa|aba|aNbNa Note: It is to be noted that X ® aNbNa will still be included in the new CFG. Method: Consider the following CFG S ® XY, X ® Zb, Y ® bW Z ® AB, W ® Z, A ® aA|bA|L B ®Ba|Bb|L. Here A ® L and B ® L are null productions, while Z ® AB, W ® Z are nullable productions. The new CFG after, applying the method, will be S ® XY X ® Zb|b Y ® bW|b Z ® AB|A|B W®Z A ® aA|a|bA|b B ®Ba|a|Bb|b Note: While adding new productions all Nullable productions should be handled with care. All Nullable productions will be used to add new productions, but only the Null production will be deleted Q No 2. Is it possible to make CFG for infix and postfix expression's using derivation tree ? Derivation tree is only used to derive words of language that is described by a CFG. Yes, we can create CFG for languages infix expressions, postfix expressions. Q No 3 what is the uses of push down automata in computing ? PDA is just an enhancement in FAs. i.e Memory is attached with machine that recognizes some language. FA is basic structure for most advanced electronic machines such as computer etc. Q No 4 What is difference between PUSH DOWN STACK and PUSH DOWN STORE ? No difference at all. Both terms are used to describe memory structure http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ attached with FAs to store some characters in it. Q No 5 How we can distinguish between "CFG" and "CNF" in the questions ? Chomsky Normal Form (CNF) If a CFG has only productions of the form string of two nonterminalsnonterminal --------- or one terminalNonterminal ----------- Then the CFG is said to be in Chomsky Normal Form (CNF). Thus if the given CFG is in the form specified above it will be called in CNF. Q No 6.What is meant by the terms stack consistence and input tape consistence ? Term Stack consistent means we can pop any character from the top of the stack only. PDA should not be able to pop any character other than that is present on the top of the stack. Term Tape consistent means we can read only the first letter on the tape not any other letter of the tape after the first one. Q No 7 What is the concept of unit production ? The productions of the form one Nonterminalone Nonterminal -------- Are called unit productions. For example A (Unit Produciton)S -------- a | bA-------- A. we can directly writeHere there is no need of Unit Production S S ---- a | b Q No 8 Why Context Free Grammars are called "Context Free? Context Free Grammars are called context free because the words of the languages of Context Free Grammars have words like “aaabbb”(PALINDROME). In these words the value of letters (a , b) is the same on whatever position they appear. On the other hand in context sensitive grammars their value depend on the position they appear in the word a simple example may be as follows Suppose we have a decimal number 141 in our language. When compiler reads it, it would be in the form of string. The compiler would calculate its decimal equivalent so that we can perform mathematical functions on it. In calculating its decimal value , weight of first “1” is different than the second “1” it means it is context sensitive (depends on in which position the “1” has appeared). i.e. 1*102 + 4*101 + 1* 100 = 14 (value of one is 100) (value of one is just one) That is not the case with the words of Context Free Languages. (The value of “a” is always same in whatever position “a” appears). Q No 9. What is Unit Production? http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ The production in which one non-terminal leads to only one non-terminal. Q No 10.What is Left most Derivation in CFG? It is a method of generation of strings from a CFG starting from left most letter of the string. FAQ's about Lectures 41 to 45 Q No 1.Give a example of converting a CFG to CNF? Consider the CFG given below S ABC A aa | b B c C d Its CNF will be S DC D AB A EE | b E a B c C d Q No 2.In the lecture 41 's example, we have converted PDA to conversion form and a word 'aaaabb' is derived from this conversion form PDA. What are the derivation steps. The PDA converted to conversion form has some specific features that are important to understand first. These features are The states named START, READ, HERE and ACCEPT are called joints of the machine. With the help of the conversion form we have been able to achieve that POP state has only one path out of it and the path taking (multiple paths) decisions take place only on the READ state. The word 'aaaabb' is generated as follows from the PDA START-POP4-PUSH $ This step pops $ and then pushes it to ensure that stack contains $ at the beginning. READ1-POP6-PUSH $-PUSH a As first time after reading "a" there is $ at the top of stack so we will follow path segment READ1-POP6-PUSH $-PUSH a READ1-POP5-PUSH a-PUSH a Now a is on the top of the stack so we will follow READ1-POP5-PUSH a-PUSH a READ1-POP5-PUSH a-PUSH a Again following same segment for a READ1-POP5-PUSH a-PUSH a Again following same segment for a http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ READ1-POP1- HERE-POP2 As we read b on input tape. READ2-POP1-HERE-POP2 As we read b on input tape. READ2-POP3-ACCEPT. As we read ∆ from the input tape Q No 3.How to differentiate between "wanted" and "unwanted branch" ? When we derive a word in Top down parsing beginning with the starting Non Terminal the branches of the tree that do not lead to our required word are left aside these branches are called unwanted branches. For example for CFG S----->AA A----->a | b If we want to generate the word "aa" we will leave the branch generated by the production A------>b. Q No 4.What is the difference between intersection and union of a language? Intersection of two languages will consist of all those words which are in both languages while union of two languages will consist of all those words which are present in at least one language. Symbol for intersection is and for union is U. Q No 5.What is the difference between Context free languages and regular languages? Regular languages can be represented by FA’s because we do not need any memory to recognize (accept or reject them on FA) them but there is another class of languages that can not be represented by FA’s because these languages require that we have some memory (with the help of memory we can store letters of the string we are checking so that we can compare them with next coming letters in the string). For example language anbn requires that we must store a’s and then compare their count with next coming b’s so that we can check whether a’s are equal to b’s or not. Due to this reason we use Context Free Grammars to represent them because we can5t write RE’s for them. So Context Free Languages represent a broader category this category also include regular languages as subcategory. It means that context free languages include regular languages as well as some other languages. Q No 6.What is the difference between Moore and Mealey machines? In Mealy Machine we read input string letters and generate output while moving along the paths from one state to another while in Moore machine we generate output on reaching the state so the output pattern of Moore machine contains one extra letter because we generated output for state q0 where we read nothing. Q No 7.What does the following terms mean i. STACK Consistent http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ ii. Y-able Paths iii. Working string iv. Semi Word means Stack consistence means that in the PDA converted in the conversion form, when we follow a path segment (which is formed by combining start, read or here state with next read, here or accept state on the path) along the PDA its pop state should have the path for the same letter that is present on the top of the stack at that stage. If this doesn’t happen our PDA will crash because in conversion form of the PDA the pop state has only one letter path, so if we could not be able to find that letter on the top of the stack our PDA will crash (if will not find path where to go from that state) Working string means the string present on the input tape. Y-able Paths means that when we follow a certain sequence of rows from the row table to generate a path for a word form start state to accept state. The path (sequence of rows) should be stack as well as joint consistent it means that rows should end at the same read or here state (join consistency ) and the rows should be able to pop the letter from the top that is indicated in the pop state of the row. Semi word is the string of terminals it may be null string ending with a Non terminals on the right. For example some semi words are aaS aabbA B Is Automata Theory is a Programming Subject or theoretical? Automata theory is the study of abstract computing devices, or "machines". This topic goes back to the days before digital computers and describes what is possible to compute using an abstract machine. These ideas directly apply to creating compilers, programming languages, and designing applications. They also provide a formal framework to analyze new types of computing devices, e.g. biocomputers or quantum computers What are practical Examples of the implications of Automata Theory and the formal Languages? Grammars and languages are closely related to automata theory and are the basis of many important software components like:  Compilers and interpreters  Text editors and processors  Text searching  System verification What are the Types of Automata?  The Types of Automata Theory are  Finite Automata http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/  Regular Languages  Linear-bounded Automata  Context Sensitive Languages  Push-Down Automata  Context Free Languages  Turing Machines  Recursively innumerable languages There are others as well like,  Random Access Machines  Parallel Random Access Machines  Arrays of Automata Question: How types of Automata Differ? Answer: They differ in the following areas Complexity (or Simplicity) Power In the function that can be computed. In the languages that can be accepted. Question: What is the difference between the alphabet and an element of a set? Answer: Alphabets is a set of letters nothing else but a set of strings (elements) can have more than one letters in one string. Question: Difference between Palindrome and Reverse function? Answer: The language consisting of and the strings s defined over such that Rev(s)=s. It is to be denoted that the words of PALINDROME are called palindromes. Reverse (w) = w Example: ={a,b}, PALINDROME={ , a, b, aa, bb, aaa, aba, bab, bbb,...} If a is a word in some language L, then reverse (a) is the same string of letters spelled backwards, called the reverse of a. e.g reverse (xxx) = xxx reverse (623) = 326 reverse (140) = 041 Question: Define Strings? Answer: Concatenation of finite letters from the alphabet is called a string. e.g If = {a,b} then a language L can be defined as L = {a, abab, aaabb, ababababababababab,...............} it's mean all words with a's more or equal to b's http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Question: Define empty or null strings? Answer: Concatenation of finite letters from the alphabet is called a string. Sometimes a string with no symbol at all is used, denoted by (Small Greek letter Lambda) or (Capital Greek letter Lambda) , is called an empty string or null string. Question: Difference between string and word? Answer: Any combination of letters of alphabet that follows rules of language is called a word. A string is a finite sequence of symbols from an alphabet. Question: There are as many palindromes of length 2n as there are of length 2n-1, please explain? Answer: If we try to create palindromes then middle elements (2 in even palindromes & 1 in odd palindrome) does not cause any change in no. of palindromes Defining the language PALINDROME, of length 2n and 2n-1 defined over S = {a,b} e.g if we take n= 2 for 2n Length (2n) = 4 and string can be written as {aaaa, abba, baab, bbbb} And if we take n = 2 for 2n-1 Length (2n-1) = 3 and string can be written as {aaa, aba, bab, bbb} Question: Define Kleene Star? Answer: Given , then the Kleene Star Closure of the alphabet , denoted by *, is the collection of all strings defined over , including. It is to be noted that Kleene Star Closure can be defined over any set of strings. Examples If = {x} Then * = { , x, xx, xxx, xxxx, ….} If = {0,1} Then * = { , 0, 1, 00, 01, 10, 11, ….} If = {aaB, c} Then * = { , aaB, c, aaBaaB, aaBc, caaB, cc, ….} Note: Languages generated by Kleene Star Closure of set of strings, are infinite languages. (By infinite language, it is supposed that the language contains infinite many words, each of finite length). Question: Why do not we can write" ba" in the set of PALINDROME while it is reverse of "ab". Answer: The language consisting of and the strings s defined over such that http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Rev(s)=s. It is to be denoted that the words of PALINDROME are called palindromes. Example For ={a,b}, PALINDROME={ , a, b, aa, bb, aaa, aba, bab, bbb,...} All two length string cannot satisfied the palindrome. aa and bb in palindrome but ba and ab are not in palindrome. Question: What are the steps of Recursive Definition of Languages? Answer: A recursive definition is characteristically a three-step process. First, we specify some basic objects in the set. Second, we give rules for constructing more objects in the set from ones we already know. Third, we declare that no objects except those constructed in this way are allowed in the set. Question: Strings that ending in "a " and strings containing exactly one "a". Answer: Its means all string ending in a e.g = {a, b} {a, aa, ba, aba, baa,…….} Exactly a, defined over = {a, b} {ab, ba, abb, bba,……... } Question: What is Lexical Analyzer? Answer: The first phase of the compiler is the lexical analyzer, also known as the scanner, which recognizes the basic language units, called tokens. The exact characters in a token is called its lexeme. Tokens are classified by token types, e.g. identifiers, constant literals, strings, operators, punctuation marks, and key words. Different types of tokens may have their own semantic attributes (or values) which must be extracted and stored in the symbol table. The lexical analyzer may perform semantic actions to extract such values and insert them in the symbol table. Question: What is accepting string language? Answer: The strings which follow rules for the language are accepted in language. Let u and v be strings. Then uv denotes the string obtained by concatenating u with v, that is, uv is the string obtained by appending the sequence of symbols of v to that of u. For example if u = aab and v = bbab, then uv = aabbbab. Note that vu = bbabaab uv. We are going to use first few symbols of English alphabet such as a and b to denote symbols of an alphabet and those toward the end such as u and v for strings. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Question: What is transition table? Answer: A complete transition table contains one column for each character. To save space, table compression may be used. Only non-error entries are explicitly represented in the table, using hashing, indirection or linked structures. Tabular representation of a function that takes two arguments and returns a value 0 1 q0 q2 q0 *q1 q1 q1 q2 q2 q1 Rows correspond to states Columns correspond to inputs Entries correspond to next states The start state is marked with an arrow The accepting states are marked with a star Question: What does it means by the transition? Answer: Transition means which letter, after being read, is transfer from which place to which place. It is necessary to show transition of every letter from each and every state. Question: What is Null? Answer: is a string having no letter in it. e.g ( A box having no object in it). Let us observe that if the alphabet has no letters, then its closure is the language with the null string as its only word, because is always a word in a Kleen closure. Symbolically, we write if = {the empty set} then * = { }, This is not the same as if S = { } then S* = { } which is also true but for a different reason, that is =. Example If L is any language, then L = L=L If string concatenates with any string S, it does not cause any change in the string S, if we use in any string then it generates some result that is below here for both side then string is aa = aa b for left and for right then string is baa = baa for left and b for right then string is aab = abb http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Question: What is the difference between Regular Languages and Non Regular Languages? Answer: The language determined by a regular expression is regular otherwise non regular. Question: What is NFA? Answer: Nondeterminism plays a key role in the theory of computing. A nondeterministic finite state automaton is one in which the current state of the machine and the current input do not uniquely determine the next state. This just means that a number of subsequent states (zero or more) are possible next states of the automaton at every step of a computation. Of course, nondeterminism is not realistic, because in real life, computers must be deterministic. Still, we can simulate nondeterminism with deterministic programs. Furthermore, as a mathematical tool for understanding computability, nondeterminism is invaluable. As with deterministic finite state automata, a nondeterministic finite state automaton has five components. a set of states a finite input alphabet from which input strings can be constructed a transition function that describes how the automaton changes states as it processes an input string a single designatedstarting state a set of accepting states The only difference lies in the transition function, which can now target subsets of the states of the automaton rather than a single next state for each state, input pair. Question: What is a main difference between NFA and FA? Answer: Finite Automata (FA) A finite automaton with unique transitions from each state. There are no choices There is only 1 letter of the alphabet per transition (the label on the edges in the graph is limited to 1) transitions are not allowed. No implied trap states. That is, if the letter of alphabet has n letters in it, every state will have n edges coming out of it. If the letters are not part of a valid word, then the edges will go into a special state, called the trap states. Trap states are the NO states. Nondeterministic Finite Automaton (NFA) Has the freedom to do various different moves when in a state and seeing some input This is modeled mathematically as The ability to be in various states at once Accepting a string whenever at least one of those states is accepting. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Question: How to obtain 9's complement? Answer: The (r – 1)’s Complement Given a positive number N is base r with an integer part of n digits and a fraction part of m digits, the (r-1)’s complements of N is defined as rn –r-m – N. Some numerical examples follow: The 9’s complement of (52520)10 is (105 – 1 – 52520) = 99999 – 52520 = 47479. No fraction part, so 10-m = 100 = 1. The 9’s complement of (0.3267)10 is (1 – 10-4 – 0.3267) = 0.9999 – 0.3267 = 0.6732 No integer part, so 10n = 100 = 1. The 9’s complement of (25.639)10 is (102 – 10-3 – 25.639) = 99.9999 - 25.63967 = 74.360 Question: What is DELAY box? Answer: It is a component which held input for some time and then forwards it just a holder. Question: What is the difference between pumping Lemma 1 and pumping Lemma 2? Answer: Infact PLI & PLII are same (A way to recognize Non Regular language). The only difference is in PLII we take care about the substring x & y that length (x) + length (y) less than or equal no. of state of machine. This is because through PLI palindrome (that is Non Regular) is proved to be regular and through PLII this problem is fixed. Question: What is pumping lemma? And what is history? Answer: A theorem to check validity (Regularity) of an infinite language should not be used with finite languages. Whenever an infinite is regular then there must be a loop (circuit) because without a loop means infinite no. of states that is not possible practically. (Machine can have finite states only) Question: What is the difference between semi word and word? Answer: A word is complete combinations of terminals only e.g. abba or ab or a or null string. Semiword: A semiword is a string of terminals (may be none) concatenated with exactly one nonterminal on the right i.e. a semiword, in general, is of the following form (terminal)(terminal)… (terminal)(nonterminal) Question: What is the difference between derivation tree and total tree? Answer: A Derivation tree is the one that shows how to derive any specific word of the language described by CFG but Total Language Tree shows all words of the http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Language described by CFG on it. Question: How to identify a production by it, ambiguity will be removed? Answer: It is a matter of practice that one can know how to remove ambiguity from it, only practice makes you efficient enough to do it in less time. Question: Difference between Nullable and Null production? How to make CFG for Nullable and for Null? Answer: The production of the form nonterminal ® L is said to be null production. Example: Consider the following CFG S ® aA|bB|L, A ® aa|L, B ® aS Here S ® L and A ® L are null productions. A production is called nullable production if it is of the form N ®L or there is a derivation that starts at N and leads to L i.e. N1 ® N2, N2 ® N3, N3 ® N4, …, Nn ® L, where N, N1, N2, …, Nn are non terminals. Example: Consider the following CFG S ® XY, X ® Zb, Y ® bW Z ® AB, W ® Z, A ® aA|bA|L B ®Ba|Bb|L. Here A ® L and B ® L are null productions, while Z ® AB, W ® Z are nullable productions. Method: Delete all the Null productions and add new productions e.g. Consider the following productions of a certain CFG X ® aNbNa, N ® L, delete the production N ® L and using the production X ® aNbNa, add the following new productions X ® aNba, X ® abNa and X ® aba Thus the new CFG will contain the following productions X ® Nba|abNa|aba|aNbNa Note: It is to be noted that X ® aNbNa will still be included in the new CFG. Method: Consider the following CFG S ® XY, X ® Zb, Y ® bW Z ® AB, W ® Z, A ® aA|bA|L B ®Ba|Bb|L. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Here A ® L and B ® L are null productions, while Z ® AB, W ® Z are nullable productions. The new CFG after, applying the method, will be S ® XY X ® Zb|b Y ® bW|b Z ® AB|A|B W®Z A ® aA|a|bA|b B ®Ba|a|Bb|b Note: While adding new productions all Nullable productions should be handled with care. All Nullable productions will be used to add new productions, but only the Null production will be deleted. Question: Is it possible to make CFG for infix and postfix expression’s using derivation tree? Answer: Derivation tree is only used to derive words of language that is described by a CFG. Yes, we can create CFG for languages infix expressions, postfix expressions. Question: What are the uses of push down automata in computing? Answer: PDA is just an enhancement in FAs. i.e Memory is attached with machine that recognizes some language. FA is basic structure for most advanced electronic machines such as computer etc. Question: What is difference between PUSH DOWN STACK and PUSH DOWN STORE? Answer: No difference at all. Both terms are used to describe memory structure attached with FAs to store some characters in it. Question: How to accommodate NULL string if it is part of language during converting from CFG to CNF and building FA's? Answer: When we convert CFG to CNF and Null is a part of language then null string is not part of language in CNF. This is the only change a language gets in CNF. When we draw an FA for a CFG, there is no change in language and simply draws FA that accepts the language of CFG. Question: How to accommodate NULL string if it is part of language during converting from CFG to CNF and in building PDA? Answer: When we convert CFG to CNF and Null is a part of language then null string is not part of language in CNF. This is the only change a language gets in CNF. When we draw an PDA for a CFG, there is no change in language and simply draws PDA that accepts the language of CFG. Question: What is Push down Automata? Answer: Pushdown Automaton (PDA), consists of the following http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ An alphabet S of input letters. An input TAPE with infinite many locations in one direction. Initially the input string is placed in it starting from first cell; the remaining part of the TAPE is empty. An alphabet G of STACK characters. A pushdown STACK which is initially empty, with infinite many locations in one direction. Initially the STACK contains blanks. One START state with only one out-edge and no in-edge. Two halt states i.e. ACCEPT and REJECT states, with in-edges and no out- edges. A PUSH state that introduces characters onto the top of the STACK. A POP state that reads the top character of the STACK (may contain more than one out-edges with same label). A READ state that reads the next unused letter from the TAPE (may contain more than one out-edges with same label). Question: Why we study Automata? Answer: Automata theory is the study of abstract computing devices, or "machines". This topic goes back to the days before digital computers and describes what is possible to compute using an abstract machine. These ideas directly apply to creating compilers, programming languages, and designing applications. They also provide a formal framework to analyze new types of computing devices, e.g. biocomputers or quantum computers. Finally, the course should help to turn you into mathematically mature computer scientists capable of precise and formal reasoning. More precisely, we'll focus primarily on the following topics. Don't worry about what all the terms mean yet, we'll cover the definitions as we go: 1. Finite state automata: Deterministic and non-deterministic finite state machines; regular expressions and languages. Techniques for identifying and describing regular languages; techniques for showing that a language is not regular. Properties of such languages. 2. Context-free languages: Context-free grammars, parse trees, derivations and ambiguity. Relation to pushdown automata. Properties of such languages and techniques for showing that a language is not context-free. 3. Turing Machines: Basic definitions and relation to the notion of an algorithm or program. Power of Turing Machines. 4. Undecidability: Recursive and recursively enumerable languages. Universal Turing Machines. Limitations on our ability to compute; undecidable problems. 5. Computational Complexity: Decidable problems for which no sufficient algorithms are known. Polynomial time computability. The notion of NP-completeness and problem reductions. Examples of hard problems. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Question: What is valid and invalid alphabets explain with example? Answer: Example 1 If s=abc is a string defined over = {a,b,c} then Rev(s) or sr = cba = {a,b} s=abbaa Rev(s)=aabba When more then letter in the alphabet you have to be quite careful that don’t reverse the symbols however you write the letter from right to left. Example 2 = {B, aB, bab, d} s=BaBbabBd Rev(s)=dBbabaBB Example 3 = {ab, b, aa} s=abbaa Rev(s)=aabab 1= {B, aB, bab, d} is valid alphabet as there is no letter in 1 that lies in start of any other letter means all the tokens of any word (string) will be unique. Whereas in 2= {B, Ba, bab, d} letter B lies in start of letter Ba. This makes it difficult to decide which token to select at some point if B occurs in any string. Question: Why we use Capital Letters for Languages. Is it possible to combine two languages together like EVEN-EVEN & EQUAL, and so on? Answer: We use capital letter for our convenient and yes you can combine two languages. Question: What are the rules to form WORDS in languages developed by Automata? Are strings not following any rule? Answer: Rules are different for different languages. e.g ={a,b} The language L of strings of even length, defined over = {a,b}, can be written as Valid for even length L= {aa, bb, aabb, bbaa, baab, abba …..} Invalid for even length L= {a, b, aaa, bbb, aba, bab, …….} The language L of strings of odd length, defined over = {a,b}, can be written as Valid for odd length L1= {a, b, aaa, bbb, aba, bab, …….} Invalid for odd length L= {aa, bb, aabb, bbaa, baab, abba …..} Strings cannot follow any rule. Question: What are graphs of palindromes of length 2n and length 2n-1? Answer: Palindromes of even length are always symmetric about the middle line and http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ palindromes of odd length are always symmetric about middle letter. If we try to create palindromes then middle elements (2 in even palindromes & 1 in odd palindrome) does not cause any change in no. of palindromes Defining the language PALINDROME, of length 2n and 2n-1 defined over S = {a,b} e.g if we take n= 2 for 2n Length (2n) = 4 and string can be written as {aaaa, abba, baab, bbbb} And if we take n = 2 for 2n-1 Length (2n-1) = 3 and string can be written as {aaa, aba, bab, bbb} Question: How can we write a RE for a given number of words? Answer: In example let us consider a finite language L that contains all the strings of a’s and b’s of length three exactly: L = {aaa aab aba abb baa bab bba bbb} The first letter of each word in L is either an a or a b. The second letter of each word in L is either an a or a b. The third letter of each word in L is either an a or a b. So, we may write L = language ((a+b)(a+b)(a+b)) Or L = language ((a+b)3) If we want to define the set of all seven-letter strings of a’s and b’s, we could write (a+b)7. In general, if we want to refer to the set of all possible strings of a’s and b’s of any length whatsoever, we could write (a+b)* This is the set of all possible strings of letters from the alphabet ∑ = {a, b} including the null string. This is a very important expression and we shall use it often. Again, this expression represents a language. If we choose that * stands for 5, then (a+b)* gives (a+b)5 = (a+b)(a+b)(a+b)(a+b)(a+b) We now have to make five more choices: either a or b for the first letter, either a or b for second letter, and so on. 21. Please explain that in some expression having more then one plus sign the resulting string is only one from them or it can be more then one? For example if there is an expression: a +b + c, with out any small bracket between them the resultant string is "a or b or c" OR it cans both "a or b and c”,"a and b or c" The resulting string is only one from them. Whenever we put + signs between n words that means one option (only) out of all available. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ Question: One Language generates many REs? Answer: Sometimes we generate many Regular Expressions (RE) for one language these REs are called Equal RE. Example: Consider the following regular expressions r1= (a + b)* (aa + bb) r2= (a + b)*aa + ( a + b)*bb then both regular expressions define the language of strings ending in aa or bb. This RE (aa+bb) is separated by +. Whenever RE is separated by +, two possibilities occur 1. Before + part 2. After + part (a+b)*aa comes before + part, means RE generate all strings that end with aa. (a+b)*bb comes after + part this means RE generate all strings that end with bb. In short we can say r2 generates the language of strings ending either aa or bb this is equal to r1. Question: Rules for determining RE for a given language defined on a set? Answer: The following rules define the language associated with any regular expression: Rule 1: The language associated with the regular expression that is just a single letter is that one-letter word alone and the language associated with null is just {null}, a one-word language. Rule 2: If r1 is a regular expression associated with the language L1 and r2 is regular expression associated with the language L2, then: The regular expression (r1)(r2) is associated with the product L1L2 that is the language L1 times L2 language (r1r2) = L1L2 The regular expression r1 + r2 is associated with the language formed by the union of the sets L1 and L2: language (r1 + r2) = L1 + L2 The language associated with the regular expression (r1)* is L1*, the Kleene closure of the set L1 as a set of words: language(r1*) = L1* Once again, this collection of rules proves recursively that there is some language associated with every regular expression. As we build up a regular expression from the rules, we simultaneously are building up the corresponding language. Question: What is EVEN - EVEN LANGUAGE? Answer: Even-Even means count of a's is even and cout of b's is also even. Even + Even = Even (Proved) So we can divide any string excluding which is also in Even-Even in substrings of length 2 each. It gives us following combinations aa, bb makes no change in string status ab, ba create disorder in string status http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ so (aa+bb+(ab+ba)(aa+bb)*(ab+ba))*. Question: What is tokenizing string? Answer: Tokenize a string means make its valid units. Question: What is whole star? Answer: Whole star of any RE means all possible combinations of that RE including. Question: How to use + operator in Automata? Answer: Plus Operation is same as Kleene Star Closure except that it does not generate (null string), automatically. Question: How to know what is RE? Answer: It is a matter of practice that you come to know what does an RE represent. You may start with simpler REs and later you will be able to recognize different REs gradually. Question: Why we use null string in FA? Answer: If null string is part of our language then we have to handle it in FA, its not compulsion. Question: What is tree structure? Answer: A tree is a connected undirected graph with no simple circuits. Since a tree cannot have a simple circuit, a tree cannot contain multiple edges or loops. Therefore any tree must be a simple graph. Question: What is difference between (a, b) & (a + b)? Answer: (a, b) & (a + b) are same and both represents either a or b. Question: Difference between FA & TG? Answer: Finite Automata (FA) · A finite automaton with unique transitions from each state. · There are no choices · There is only 1 letter of the alphabet per transition (the label on the edges in the graph is limited to 1) · transitions are not allowed. · No implied trap states. That is, if the letter of alphabet has n letters in it, every state will have n edges coming out of it. If the letters are not part of a valid word, then the edges will go into a special state, called the trap states. Trap states are the NO states. Transition Graphs (TG) http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ ·TG are generalizations of FA. · Can change state without an input. · Can read more than one letter at a time. · Can have a regular expression as a edge label. · Can have more then one start state. We are not bound in TG. We are only given a freedom or relation which we are not forced to use. We may or may not use it on will. In fact we enjoy freedom of staying at multiple places at one time in TGs while reading any letter strings which provides us a facility that any one available path it leads us to final state, word is accepted. Question: Difference between (a+b)+ and (a+b)*? Answer: (a + b)+ means, we repeat RE a + b infinite (any) no. of times but minimum once, whereas (a + b)* means we may repeat a + b any no. of times even zero times. () are only used to clear (distinguish) the one RE from some other RE. Question: What is length of string? Answer: The length of a string indicates how many symbols are in that string. For example, the string 0101 using the binary alphabet has a length of 4. The standard notation for a string w is to use |w|. For example: Length of string: |0101| is 4. Length of string: |0010| = 4, |aa| = 2, | |= 0 Question: After looking the on diagram how can we say it is TG or is FA? Answer: Every FA is also a TG but not every TG is FA. In every FA, every state shows transition of all letters of given alphabet but in any TG it is not must. In TG, we may or may not show all letters transition according to requirement. We can also show transitions on reading any strings in TGs but it is not possible in FAs. Question: Differentiate between FA, TG and GTG? Answer: Every FA is also a TG but not every TG is FA. In every FA, every state shows transition of all letters of given alphabet but in any TG it is not must. In TG, we may or may not show all letters transition according to requirement. We can also show transitions on reading any strings in TGs but it is not possible in FAs. In GTG Directed edges connecting some pair of states labeled with regular expression. It may be noted that in GTG, the labels of transition edges are corresponding regular expressions. In TG we write strings and in GTG we are bound to write RE. Question: Difference between even clumps and odd clumps? Answer: Letters may be a's (for example) in even count at one place. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ e.g. aab = valid baa = valid aba = invalid abab = invalid Question: Difference between containing and consisting? Answer: Containing something (e.g. R1) means R1 is there (must), whatever other things are, we don’t care. Consisting something (e.g. R1) means only R1 is there all other things should be rejected. Question: Define the main formula of Regular expressions? Define the back ground of regular expression? Answer: Regular expressions are a notation that you can think of similar to a programming language. In fact, regular expressions are quite fundamental in some programming languages like perl and applications like grep or lex. Regular expressions are similar to NFA and end up describing the same things we can express with a finite automaton. However, regular expressions are declarative in what strings are accepted, while automata are machines that accept strings. We use Regular expressions for defining the languages. Say that R is a regular expression if R is: 1. a for some a in the alphabet , standing for the language {a} 2. , standing for the language { } 3. R1+R2 where R1 and R2 are regular expressions, and + signifies union 4. R1R2 where R1 and R2 are regular expressions and this signifies concatenation 5. R* where R is a regular expression and signifies closure 6. (R) where R is a regular expression, then a parenthesized R is also a regular expression A set of strings from an alphabet. The set may be empty, finite or infinite. The building blocks of regular languages are symbols, concatenation of symbols to make strings (words), set union of strings and Kleene closure (denoted as *, also called the Kleene star, it should be typed as a superscript but this is plain text.) Informally, we use a syntax for regular expressions. Using sigma as the set {0, 1} (an alphabet of two symbols) 01110 is a string starting with the symbol 0 and then concatenating 1, then 1, then 1, and finally concatenating 0. No punctuation is used between symbols or strings that are concatenated. (01+10) is a union of the two strings 01 and 10. The set {01, 10} (00+11)* is the Kleene closure of the union of 0 concatenated with 0 and 1 concatenated with 1. The Kleene closure (star) is defined as the concatenation of none, one, two, or http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ any countable number strings it applies to. Examples of Kleene star: 1* is the set of strings { , 1, 11, 111, 1111, 11111, etc. } This set is infinite. (1100)* is the set of strings { , 1100, 11001100, 110011001100, etc. } (00+11)* is the set of strings { , 00, 11, 0000, 0011, 1100, 1111, 000000, 000011, 001100, etc. } Note how the union ( + symbol) allows all possible choices of ordering when used with the Kleene star. (0+1)* is all possible strings of zeros and ones, often written as * where = {0, 1}(0+1)* (00+11) is all strings of zeros and ones that end with either 00 or 11. Note that concatenation does not have an operator symbol. (w)+ is a shorthand for (w)(w)* w is any string or expression and the superscript plus, + , means one or more copies of w are in the set defined by this expression. Question: Are S* and S+same? Answer: No because S+ means same as Kleene Star Closure except that it does not generate (null string), automatically. So in the above example, there is no null string. Concatenation of finite letters from the alphabet is called a string. If we have S={a, bb, bab, abaab}, first we factorize the string like (a) (bb) (bab) (abaab) then concatenate to each other and make more string to its concatenation but the string abbabaabab is not in S* because the last member (ab) of the group does not belong to S, so abbabaabab is not in S. Question: What is set? Answer: Set is a collection of distinct objects. Question: How many Methods of defining the languages? Answer: 1. You can describe a language in English like statement 2. You can define a language by putting all its words in a set 3. You can define a language in a mathematical way 4. You can define a language by Recursive Definition 5. You can define a language by Regular Expression 6. You can define a language by Finite Automata 7. You can define a language by Transition Graph 8. You can define a language by Context Free Grammar Question: What is difference between Palindrome, Kleene star closure and plus operation? Answer: The language consisting of and the strings s defined over such that Rev(s)=s. http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ or A string x is a palindrome if xR=x. It is to be denoted that the words of PALINDROME are called palindromes. Reverse (w) = w Example: ={a,b}, PALINDROME={ , a, b, aa, bb, aaa, aba, bab, bbb,...} If a is a word in some language L, then reverse (a) is the same string of letters spelled backwards, called the reverse of a. e.g reverse (xxx) = xxx reverse (623) = 326 reverse (140) = 041 Given , then the Kleene Star Closure of the alphabet , denoted by *, is the collection of all strings defined over , including. Plus Operation is same as Kleene Star Closure except that it does not generate (null string), automatically. Question: What is inside language? Answer: Any language contains words in it and certain rules to validate strings for that language. Question: What is Equal RE? Answer: Sometimes we generate many REs for one language these are called Equal RE. Question: How can we make Finite Automaton from a language and a language from FA? Answer: There is not any formal procedure to design FA for a language. This ability just improves with time and practice. Question: Could we just use +, - symbols with x not with y in FA? Answer: Yes, you can use only + or – for the place of x and y, but remember when you don't write + or –, So you should write start and final at the beginning or ending state Or you should indicated by an arrow and a final state by drawing box or another circle around its circle because if you don't write how can we indicate the start and final state. Question: Explain the language L of string, defined over ∑= {0,1}, having double 0's and double 1's? Answer: Language of strings with 00 or 11 means string that have substrings 00 or 11 in them at least. Minimum words which are included in this language are 00 and 11. This language does not accept null and it also does not accept 0 or 1. Question: What are the difference between single 1 and 0 and double 1's and 0's? Answer: 1. Consider the language L of strings, defined over http://vustudents.ning.com/  Join Us on http://vustudents.ning.com/ If ={0, 1}, having words with either 0’s or 1’s without null. The language L may be expressed by RE (0 + 1)+ When you make string by above RE you have all possible combination of 0’s and 1’s except null i.e {0, 1, 00, 01, 10, 11,……….}. Minimum words which are included in this language are 0 and 1. 2. Consider the language L of strings, defined over If ={0, 1}, having double 0’s or double 1’s, The language L may be expressed by the RE (0+1)* (00 + 11) (0+1)* Double 1’s and 0’s means clumps of letter which will always come together. Minimum words which are included in this language are 00 and 11. Question: On what basis we select initial and final states? Answer: It depends on the expression given to us. Question: How we know that the given expression has how many states? Answer: There is not any formal procedure to know the number of states. This ability just improves with time and practice. Question: How will we develop the rules of transition? Answer: Transition means which letter, after being read, is transfer from which place to which place. It is necessary to show transition of every letter from each and every state. Question: Can we accept the strings going from final to initial? Answer: It is to be noted that if any state start from the final state it does not accept any string. Even it does not accept the null string, because there is no path starting from initial state and ending in final state. Question: What are the basic rules to build FA? Answer: One and only rule is to build a Finite Automata (FA) should accept all words of the language and reject all the words which are not part of the language. Any FA that ensures these above things is the right FA for the language. Note: One language can have many FA. Question: What is Dead state? Answer: The DEAD STATE is introduced to be able to make an automaton complete without altering its behavior. http://vustudents.ning.com/