Introduction to Programming with Python.pdf

Programming basics   code or source code: The sequence of instructions in a program. syntax: The set of legal structures and commands that can be used in a particular programming language.  output: The messages printed to the user by a program.  console: The text box onto which output is p...

Programming basics   code or source code: The sequence of instructions in a program. syntax: The set of legal structures and commands that can be used in a particular programming language.  output: The messages printed to the user by a program.  console: The text box onto which output is printed.  Some source code editors pop up the console as an external window, and others contain their own console window. 2 Compiling and interpreting  Many languages require you to compile (translate) your program into a form that the machine understands. compile source code Hello.java  byte code Hello.class execute output Python is instead directly interpreted into machine instructions. interpret source code Hello.py output 3 The Python Interpreter Python is an interpreted language • The interpreter provides an interactive environment to play with the language • Results of expressions are printed on the screen • >>> 3 + 7 10 >>> 3 < 15 True >>> 'print me' 'print me' >>> print 'print me' print me >>> Expressions  expression: A data value or set of operations to compute a value. Examples:  Arithmetic operators we will use:     1 + 4 * 3 42 + - * / % ** addition, subtraction/negation, multiplication, division modulus, a.k.a. remainder exponentiation precedence: Order in which operations are computed.  * / % ** have a higher precedence than + 1 + 3 * 4 is 13  Parentheses can be used to force a certain order of evaluation. (1 + 3) * 4 is 16 5 Integer division  When we divide integers with / , the quotient is also an integer. 3 4 ) 14 12 2  More examples:     52 27 ) 1425 135 75 54 21 35 / 5 is 7 84 / 10 is 8 156 / 100 is 1 The % operator computes the remainder from a division of integers. 3 4 ) 14 12 2 43 5 ) 218 20 18 15 3 6 Real numbers  Python can also manipulate real numbers.   -15.9997 42.0 2.143e17 The operators + - * / % ** ( ) all work for real numbers.    Examples: 6.022 The / produces an exact answer: 15.0 / 2.0 is 7.5 The same rules of precedence also apply to real numbers: Evaluate ( ) before * / % before + - When integers and reals are mixed, the result is a real number.  Example: 1 / 2.0 is 0.5  The conversion occurs on a per-operator basis.      7 / 3 * 1.2 + 3 / 2 2 * 1.2 + 3 / 2 2.4 + 3 / 2 2.4 + 1 3.4 7 Math commands   Python has useful commands (or called functions) for performing calculations. Command name Description abs(value) absolute value ceil(value) rounds up cos(value) cosine, in radians floor(value) rounds down log(value) logarithm, base e log10(value) logarithm, base 10 max(value1, value2) larger of two values min(value1, value2) smaller of two values round(value) nearest whole number sin(value) sine, in radians sqrt(value) square root Constant Description e 2.7182818... pi 3.1415926... To use many of these commands, you must write the following at the top of your Python program: from math import * 8 Numbers: Floating Point int(x) converts x to an integer  float(x) converts x to a floating point  The interpreter shows a lot of digits  >>> 1.23232 1.2323200000000001 >>> print 1.23232 1.23232 >>> 1.3E7 13000000.0 >>> int(2.0) 2 >>> float(2) 2.0 Variables  variable: A named piece of memory that can store a value.  Usage:     Compute an expression's result, store that result into a variable, and use that variable later in the program. assignment statement: Stores a value into a variable.  Syntax: name = value  Examples: x   5 x = 5 gpa = 3.14 gpa 3.14 A variable that has been given a value can be used in expressions. x + 4 is 9 Exercise: Evaluate the quadratic equation for a given a, b, and c. 10 Example >>> x = 7 >>> x 7 >>> x+7 14 >>> x = 'hello' >>> x 'hello' >>> print  print : Produces text output on the console.  Syntax: print "Message" print Expression  Prints the given text message or expression value on the console, and moves the cursor down to the next line. print Item1, Item2, ..., ItemN   Prints several messages and/or expressions on the same line. Examples: print "Hello, world!" age = 45 print "You have", 65 - age, "years until retirement" Output: Hello, world! You have 20 years until retirement 12 Example: print Statement Elements separated by commas print with a space between them •A comma at the end of the statement (print ‘hello’,) will not print a newline character • >>> print 'hello' hello >>> print 'hello', 'there' hello there input  input : Reads a number from user input.   You can assign (store) the result of input into a variable. Example: age = input("How old are you? ") print "Your age is", age print "You have", 65 - age, "years until retirement" Output: How old are you? 53 Your age is 53 You have 12 years until retirement  Exercise: Write a Python program that prompts the user for his/her amount of money, then reports how many Nintendo Wiis the person can afford, and how much more money he/she will need to afford an additional Wii. 14 Input: Example print "What's your name?" name = raw_input("> ") print "What year were you born?" birthyear = int(raw_input("> ")) print "Hi “, name, “!”, “You are “, 2016 – birthyear % python input.py What's your name? > Michael What year were you born? >1980 Hi Michael! You are 31 Repetition (loops) and Selection (if/else) 16 The for loop  for loop: Repeats a set of statements over a group of values.  Syntax: for variableName in groupOfValues: statements     We indent the statements to be repeated with tabs or spaces. variableName gives a name to each value, so you can refer to it in the statements. groupOfValues can be a range of integers, specified with the range function. Example: for x in range(1, 6): print x, "squared is", x * x Output: 1 squared 2 squared 3 squared 4 squared 5 squared is is is is is 1 4 9 16 25 17 range  The range function specifies a range of integers:   - the integers between start (inclusive) and stop (exclusive) It can also accept a third value specifying the change between values.   range(start, stop) range(start, stop, step) - the integers between start (inclusive) and stop (exclusive) by step Example: for x in range(5, 0, -1): print x print "Blastoff!" Output: 5 4 3 2 1 Blastoff!  Exercise: How would we print the "99 Bottles of Beer" song? 18 Cumulative loops  Some loops incrementally compute a value that is initialized outside the loop. This is sometimes called a cumulative sum. sum = 0 for i in range(1, 11): sum = sum + (i * i) print "sum of first 10 squares is", sum Output: sum of first 10 squares is 385  Exercise: Write a Python program that computes the factorial of an integer. 19 if  if statement: Executes a group of statements only if a certain condition is true. Otherwise, the statements are skipped.   Syntax: if condition: statements Example: gpa = 3.4 if gpa > 2.0: print "Your application is accepted." 20 if/else  if/else statement: Executes one block of statements if a certain condition is True, and a second block of statements if it is False.   Syntax: if condition: statements else: statements Example: gpa = 1.4 if gpa > 2.0: print "Welcome to Mars University!" else: print "Your application is denied."  Multiple conditions can be chained with elif ("else if"): if condition: statements elif condition: statements else: statements 21 Example of If Statements import math x = 30 if x <= 15 : y = x + 15 elif x <= 30 : y = x + 30 else : y=x print ‘y = ‘, print math.sin(y) In file ifstatement.py >>> import ifstatement y = 0.999911860107 >>> In interpreter while  while loop: Executes a group of statements as long as a condition is True.   good for indefinite loops (repeat an unknown number of times) Syntax: while condition: statements  Example: number = 1 while number < 200: print number, number = number * 2  Output: 1 2 4 8 16 32 64 128 23 While Loops x=1 while x < 10 : print x x=x+1  In whileloop.py >>> import whileloop 1 2 3 4 5 6 7 8 9 >>>  In interpreter Logic  Many logical expressions use relational operators: Operator   Meaning Example Result == equals 1 + 1 == 2 True != does not equal 3.2 != 2.5 True < less than 10 < 5 False > greater than 10 > 5 True <= less than or equal to 126 <= 100 False >= greater than or equal to 5.0 >= 5.0 True Logical expressions can be combined with logical operators: Operator Example Result and 9 != 6 and 2 < 3 True or 2 == 3 or -1 < 5 True not not 7 > 0 False Exercise: Write code to display and count the factors of a number. 25 Loop Control Statements break Jumps out of the closest enclosing loop continue Jumps to the top of the closest enclosing loop pass Does nothing, empty statement placeholder More Examples For Loops  Similar to perl for loops, iterating through a list of values for x in [1,7,13,2]: print x forloop1.py forloop2.py %python forloop1.py 1 7 13 2 for x in range(5) : print x % python forloop2.py 0 1 2 3 4 range(N) generates a list of numbers [0,1, …, n-1] More Data Types 28 Everything is an object Everything means everything, including functions and classes (more on this later!)  Data type is a property of the object and not of the variable  >>> x = 7 >>> x 7 >>> x = 'hello' >>> x 'hello' >>> Numbers: Integers Integer – the equivalent of a C long  Long Integer – an unbounded integer value.  >>> 132224 132224 >>> 132323 ** 2 17509376329L >>> Numbers: Floating Point int(x) converts x to an integer  float(x) converts x to a floating point  The interpreter shows a lot of digits  >>> 1.23232 1.2323200000000001 >>> print 1.23232 1.23232 >>> 1.3E7 13000000.0 >>> int(2.0) 2 >>> float(2) 2.0 Numbers: Complex Built into Python  Same operations are supported as integer and float  >>> x >>> y >>> x (3+1j) >>> x (2-3j) = 3 + 2j = -1j +y *y String Literals  + is overloaded to do concatenation >>> x = 'hello' >>> x = x + ' there' >>> x 'hello there' String Literals  Can use single or double quotes, and three double quotes for a multi-line string >>> 'I am a string' 'I am a string' >>> "So am I!" 'So am I!' Substrings and Methods >>> s = '012345' >>> s[3] '3' >>> s[1:4] '123' >>> s[2:] '2345' >>> s[:4] '0123' >>> s[-2] '4' len(String) – returns the number of characters in the String • str(Object) – returns a String representation of the Object • >>> len(x) 6 >>> str(10.3) '10.3' String Formatting Similar to C’s printf  <formatted string> % <elements to insert>  Can usually just use %s for everything, it will convert the object to its String representation.  >>> "One, %d, three" % 2 'One, 2, three' >>> "%d, two, %s" % (1,3) '1, two, 3' >>> "%s two %s" % (1, 'three') '1 two three' >>> Types for Data Collection List, Set, and Dictionary   List Ordered  Unordered list Pairs of values Lists Ordered collection of data  Data can be of different types  Lists are mutable  Issues with shared references and mutability  Same subset operations as Strings  >>> x = [1,'hello', (3 + 2j)] >>> x [1, 'hello', (3+2j)] >>> x[2] (3+2j) >>> x[0:2] [1, 'hello'] List Functions         list.append(x)  Add item at the end of the list. list.insert(i,x)  Insert item at a given position.  Similar to a[i:i]=[x] list.remove(x)  Removes first item from the list with value x list.pop(i)  Remove item at position I and return it. If no index I is given then remove the first item in the list. list.index(x)  Return the index in the list of the first item with value x. list.count(x)  Return the number of time x appears in the list list.sort()  Sorts items in the list in ascending order list.reverse()  Reverses items in the list Lists: Modifying Content x[i] = a reassigns the ith element to the value a  Since x and y point to the same list object, both are changed  The method append also modifies the list  >>> x = [1,2,3] >>> y = x >>> x[1] = 15 >>> x [1, 15, 3] >>> y [1, 15, 3] >>> x.append(12) >>> y [1, 15, 3, 12] Lists: Modifying Contents The method append modifies the list and returns None  List addition (+) returns a new list  >>> x = [1,2,3] >>> y = x >>> z = x.append(12) >>> z == None True >>> y [1, 2, 3, 12] >>> x = x + [9,10] >>> x [1, 2, 3, 12, 9, 10] >>> y [1, 2, 3, 12] >>> Using Lists as Stacks You can use a list as a stack >>> a = ["a", "b", "c“,”d”] >>> a ['a', 'b', 'c', 'd'] >>> a.append("e") >>> a ['a', 'b', 'c', 'd', 'e'] >>> a.pop() 'e' >>> a.pop() 'd' >>> a = ["a", "b", "c"] >>>  Tuples Tuples are immutable versions of lists  One strange point is the format to make a tuple with one element: ‘,’ is needed to differentiate from the mathematical expression (2)  >>> x = (1,2,3) >>> x[1:] (2, 3) >>> y = (2,) >>> y (2,) >>> Sets A set is another python data structure that is an unordered collection with no duplicates. >>> setA=set(["a","b","c","d"]) >>> setB=set(["c","d","e","f"]) >>> "a" in setA True >>> "a" in setB False  Sets >>> setA - setB {'a', 'b'} >>> setA | setB {'a', 'c', 'b', 'e', 'd', 'f'} >>> setA & setB {'c', 'd'} >>> setA ^ setB {'a', 'b', 'e', 'f'} >>> Dictionaries   A set of key-value pairs Dictionaries are mutable >>> d= {‘one’ : 1, 'two' : 2, ‘three’ : 3} >>> d[‘three’] 3 Dictionaries: Add/Modify  Entries can be changed by assigning to that entry >>> d {1: 'hello', 'two': 42, 'blah': [1, 2, 3]} >>> d['two'] = 99 >>> d {1: 'hello', 'two': 99, 'blah': [1, 2, 3]} • Assigning to a key that does not exist adds an entry >>> d[7] = 'new entry' >>> d {1: 'hello', 7: 'new entry', 'two': 99, 'blah': [1, 2, 3]} Dictionaries: Deleting Elements  The del method deletes an element from a dictionary >>> d {1: 'hello', 2: 'there', 10: 'world'} >>> del(d[2]) >>> d {1: 'hello', 10: 'world'} Iterating over a dictionary >>>address={'Wayne': 'Young 678', 'John': 'Oakwood 345', 'Mary': 'Kingston 564'} >>>for k in address.keys(): print(k,":", address[k]) Wayne : Young 678 John : Oakwood 345 Mary : Kingston 564 >>> >>> for k in sorted(address.keys()): print(k,":", address[k]) John : Oakwood 345 Mary : Kingston 564 Wayne : Young 678 >>> Copying Dictionaries and Lists The built-in list function will copy a list  The dictionary has a method called copy  >>> >>> >>> >>> [22] >>> [1] l1 = [1] l2 = list(l1) l1[0] = 22 l1 l2 >>> d = {1 : 10} >>> d2 = d.copy() >>> d[1] = 22 >>> d {1: 22} >>> d2 {1: 10} Data Type Summary Integers: 2323, 3234L Floating Point: 32.3, 3.1E2 Complex: 3 + 2j, 1j Lists: l = [ 1,2,3] Tuples: t = (1,2,3) Dictionaries: d = {‘hello’ : ‘there’, 2 : 15} Lists, Tuples, and Dictionaries can store any type (including other lists, tuples, and dictionaries!)  Only lists and dictionaries are mutable  All variables are references  Functions 52 Function Basics def max(x,y) : if x < y : return x else : return y functionbasics.py >>> import functionbasics >>> max(3,5) 5 >>> max('hello', 'there') 'there' >>> max(3, 'hello') 'hello' Functions are objects Can be assigned to a variable  Can be passed as a parameter  Can be returned from a function • Functions are treated like any other variable in Python, the def statement simply assigns a function to a variable  Function names are like any variable Functions are objects  The same reference rules hold for them as for other objects  >>> x = 10 >>> x 10 >>> def x () : ... print 'hello' >>> x <function x at 0x619f0> >>> x() hello >>> x = 'blah' >>> x 'blah' Functions as Parameters def foo(f, a) : return f(a) >>> from funcasparam import * >>> foo(bar, 3) 9 def bar(x) : return x * x funcasparam.py Note that the function foo takes two parameters and applies the first as a function with the second as its parameter  Higher-Order Functions map(func,seq) – for all i, applies func(seq[i]) and returns the corresponding sequence of the calculated results.  def double(x): return 2*x highorder.py >>> from highorder import * >>> lst = range(10) >>> lst [0,1,2,3,4,5,6,7,8,9] >>> map(double,lst) [0,2,4,6,8,10,12,14,16,18] Higher-Order Functions filter(boolfunc,seq) – returns a sequence containing all those items in seq for which boolfunc is True.  def even(x): return ((x%2 == 0) highorder.py >>> from highorder import * >>> lst = range(10) >>> lst [0,1,2,3,4,5,6,7,8,9] >>> filter(even,lst) [0,2,4,6,8] Higher-Order Functions reduce(func,seq) – applies func to the items of seq, from left to right, two-at-time, to reduce the seq to a single value.  >>> from highorder import * >>> lst = [‘h’,’e’,’l’,’l’,’o’] >>> reduce(plus,lst) ‘hello’  def plus(x,y): return (x + y) highorder.py Functions Inside Functions  Since they are like any other object, you can have functions inside functions def foo (x,y) : def bar (z) : return z * 2 return bar(x) + y funcinfunc.py >>> from funcinfunc import * >>> foo(2,3) 7 Functions Returning Functions def foo (x) : def bar(y) : return x + y return bar # main f = foo(3) print f print f(2) funcreturnfunc.py % python funcreturnfunc.py <function bar at 0x612b0> 5 Parameters: Defaults Parameters can be assigned default values  They are overridden if a parameter is given for them  The type of the default doesn’t limit the type of a parameter  >>> ... ... >>> 3 >>> 10 >>> hello  def foo(x = 3) : print x foo() foo(10) foo('hello') Parameters: Named Call by name  Any positional arguments must come before named ones in a call  >>> def foo (a,b,c) : ... print a, b, c ... >>> foo(c = 10, a = 2, b = 14) 2 14 10 >>> foo(3, c = 2, b = 19) 3 19 2 Anonymous Functions A lambda expression returns a function object  The body can only be a simple expression, not complex statements  >>> >>> 5 >>> >>> 16 f = lambda x,y : x + y f(2,3) lst = ['one', lambda x : x * x, 3] lst[1](4) Modules The highest level structure of Python  Each file with the py suffix is a module  Each module has its own namespace  Modules: Imports import mymodule from mymodule import x from mymodule import * Brings all elements of mymodule in, but must refer to as mymodule.<elem> Imports x from mymodule right into this namespace Imports all elements of mymodule into this namespace Text and File Processing 67 Strings  string: A sequence of text characters in a program.  Strings start and end with quotation mark " or apostrophe ' characters.  Examples: "hello" "This is a string" "This, too, is a string.   It can be very long!" A string may not span across multiple lines or contain a " character. "This is not a legal String." "This is not a "legal" String either." A string can represent characters by preceding them with a backslash. tab character new line character quotation mark character backslash character  \t \n \" \\  Example:    "Hello\tthere\nHow are you?" 68 Indexes  Characters in a string are numbered with indexes starting at 0:   Example: name = "P. Diddy" index 0 1 character P . 2 3 4 5 6 7 D i d d y Accessing an individual character of a string: variableName [ index ]  Example: print name, "starts with", name[0] Output: P. Diddy starts with P 69 String properties     len(string) - number of characters in a string str.lower(string) str.upper(string) (including spaces) - lowercase version of a string - uppercase version of a string Example: name = "Martin Douglas Stepp" length = len(name) big_name = str.upper(name) print big_name, "has", length, "characters" Output: MARTIN DOUGLAS STEPP has 20 characters 70 raw_input  raw_input : Reads a string of text from user input.  Example: name = raw_input("Howdy, pardner. What's yer name? ") print name, "... what a silly name!" Output: Howdy, pardner. What's yer name? Paris Hilton Paris Hilton ... what a silly name! 71 Text processing  text processing: Examining, editing, formatting text.   often uses loops that examine the characters of a string one by one A for loop can examine each character in a string in sequence.  Example: for c in "booyah": print c Output: b o o y a h 72 Strings and numbers  ord(text)     Example: ord("a") is 97, ord("b") is 98, ... Characters map to numbers using standardized mappings such as ASCII and Unicode. chr(number)  - converts a string into a number. - converts a number into a string. Example: chr(99) is "c" Exercise: Write a program that performs a rotation cypher.  e.g. "Attack" when rotated by 1 becomes "buubdl" 73 File processing  Many programs handle data, which often comes from files.  Reading the entire contents of a file: variableName = open("filename").read() Example: file_text = open("bankaccount.txt").read() 74