Lecture1_22-23.pptx

Lecture 1: Intro. to Programming with Python* *Partially based on Chapter1 and 2 of Think Python: How to Think Like a Computer Scientist (see Reading List) Problem Solving & Programs Values, Types and Variables Assignment Statements Arithmetic Operators, Order of Operations Program Commen...

Lecture 1: Intro. to Programming with Python* *Partially based on Chapter1 and 2 of Think Python: How to Think Like a Computer Scientist (see Reading List) Problem Solving & Programs Values, Types and Variables Assignment Statements Arithmetic Operators, Order of Operations Program Comments Introduction to Strings print(), str() 1 Problem Solving & Programs Problem solving means the ability to formulate problems, think creatively about solutions, and express a solution clearly and accurately. Problem solving is an important skill for a computer scientist and learning to program will be an opportunity to practice problem-solving skills. 2  Problem Solving & Programs (Cont’d) A Program - sequence of instructions that specifies how to perform a computation. Details look different in different languages, but basic instructions appear in most languages.  input: Get data from the keyboard/file/database etc.  output: Display on the screen, save to file/database etc.  math: Perform basic mathematical operations.  conditional execution: Check for condition and run appropriate code.  repetition: Perform some action repeatedly Programming - process of breaking a complex task into smaller & smaller subtasks until simple enough to be performed with one of the above. 3  Values and Types Programs work with values. These values belong to different types:  2 is an integer - whole numbers (4, 99, 0, -99)  42.0 is a floating-point number with decimal points (3.5, 42.1)  'Hello World!' is a string (allows single/double quotes “Hello”)  True or False is a boolean variable Python supports integers and floating-point numbers. There is no type declaration to distinguish them; Python tells them apart by the presence or absence of a decimal point. Question - How does Python tell the difference between a string and boolean variable if the type is not declared? 4  Variables (Cont’d) Different languages have different naming conventions for variables (e.g., camelCase). For Python it is recommended to use lowercase with multiple words separated with underscores: your_name airspeed_of_unladen_swallow Cannot be a Python 3 reserved keyword: 'False', 'None', 'True', 'and', 'as', 'assert', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield' 5  Variables A name that refers to a value. The value stored in a variable can be accessed or updated later. Programmers should choose descriptive variable names. Should follow the following variable names rules  Must begin with a letter (a - z, A - Z) or underscore (_).  Other characters can be letters, numbers or _  Variable names are case sensitive. These are different variables: case_sensitive, CASE_SENSITIVE, Case_Sensitive  Must not have spaces inside them (e.g., 'running total' not allowed). 6 Self-Check 1 Which of these are illegal variable names? a. the cost b. 2_much c. much2 d. *star e. more@ f. class g. the_cost 7 Assignment Statements An assignment statement creates a variable and gives it a value: message = 'Something completely different' n = 17 pi = 3.141592653589793 The first assigns a string to a variable named message; The second assigns the integer 17 to variable n; The third assigns the (approximate) value of π to variable pi. 8  A statement A statement is a unit of code that has an effect: n = 17 print(n) The first line is an assignment statement that gives a value to n. The second line is a print statement that displays the value of n. You can change the value stored in a variable by entering another assignment statement. n = 6 The previous value 17 is replaced, or overwritten with the value 6. 9 Self-Check 2 Set variables a = 1 and b = 2. Then write the instructions to swap them so the value in a ends up in b and the value in b ends up in a. 10  Arithmetic Operators Operat Operation Examples or + Addition b=a+a - Subtraction newTotal = price – discount * Multiplication total = cost * vat / Division 8/3 # Python 3 - integer / integer -> 2.6666666666666665 float % Modulus - Returns remainder result = 16 % 5 ** Exponent answer = 4**2 (4 to power of 2) // Floor division (only integer 8 // 3 #2 Note: If a program tries to divide by zero, the program is part) terminated/produces error Python 3 - integer // integer -> 11  Program Comments As programs get more complicated add comments to your programs to explain what the program is doing. Python uses # symbol. Everything from the # to the end of the line is ignored. percentage = (minute * 100) / 60 # hour elapsed Comments can appear on a line by itself. # compute the percentage of the hour that has elapsed percentage = (minute * 100) / 60 Comments most useful when they document non-obvious code features. Redundant comment: v = 5 # assign 5 to v 12  Self-Check Exercise 3 3. What will be the values in the variables at the end of each program sequences? On paper draw a box for each variable, and show the changing values (cross out old values & write new values). x=3 y=5 z=6 x = y + z + x # value of x? ________ ---------------------------------------------------------------------------------------------- a=2 b=3 a=b+a b = a + a # value of b?_________ 13  Self-Check Exercise 4 cost = 5 factor = 7 factor = factor * 3 factor = factor + cost # value of factor? _________ ------------------------------------------------------------------------- --------- cost = 10 vat = 17.5 total = cost * vat # value of total? _________ 14 Self-Check Exercise 5 a=2 b=3 c = a**b # value of c? _________ ------------------------------------------------------------------------------------- --------- num_1 = 21 num_2 = 10 num_3 = num_1/num_2 # value of num_3? _________ ------------------------------------------------------------------------------------- -------- num_4 = num_1//num_2 # value of num_4? _________ 15  Self-Check Exercises 6 & 7 6. Write the code to put 4 into a variable called 'item1'. Then put 6 into a variable called 'item2'. Then write an instruction to add the two variable values together and put the answer into a variable ‘item3‘. 7. A meal costs £56. Write the code to set 56 into a variable. Then multiply whatever is in the variable by 1/10 to work out the 10% tip (store the answer in a variable). 16 Order of Operations When an expression contains more than one operator, the order of evaluation depends on the order of operations. Acronym PEMDAS - useful to remember rules: Parentheses have the highest precedence & can force evaluation in the order required: 2 * (3-1) is 4 You can use parentheses to make an expression easier to read (doesn’t change result): (minute * 100) / 60 17  Order of Operations (Cont’d) Exponentiation has the next highest precedence: 1 + 2**3 # is 9, not 27 2 * 3**2 # is 18, not 36. Multiplication and Division have higher precedence than Addition and Subtraction: 2 * 3 - 1 # is 5, not 4 6 + 4 / 2 # is 8.0, not 5.0. Operators with the same precedence are evaluated left to right. degrees / 2 * pi # division first and then multiply by pi 18 Self-Check Exercise 8 Using the acronym PEMDAS, what is the value of the following expressions? 2**1+1 # result is 3 or 4? 3*1**3 # result is 3 or 27? 16 - 2 * 5 // 3 + 1 # result is 14, 24, 3 or 13.667? 19 Introduction to Strings A string is a sequence of characters. Python allows single ('...') or double quotes ("...") to surround strings. 20 String Operations In general, you can’t perform mathematical operations on strings: '2'-'1' # illegal Two exceptions follow.  The + operator performs string concatenation. E.g., first = 'throat' second = 'warbler' third = first + second # throatwarbler  The * operator performs repetition on strings. 'Spam' * 3: # SpamSpamSpam 21  Strings – Introduction. If a single quote is a part of the string place string in double quotes. Double quoted strings can contain single quotes inside them: "Bruce's beard" # not 'Bruce's beard' Single quoted strings can have double quotes inside them: 'She said "Hi!"' # not "She said "Hi!" " Using escape sequence (\") or (\'): 'Bruce\'s beard' 22 Strings – Introduction Printing strings over multiple lines using triple-quotes: hello = '''This is one line. Another line.''' print(hello) Using escape sequence (\n) - printing strings over multiple lines: print('This is one line.\n Another line.') 23 Common escape sequences: Sequence Meaning \\ literal backslash \' single quote \" double quote \n Newline \t Tab 24  Lecture Self-Check Question Assign a variable question with the value: Where’s the lecture room? 25 Built-in Functions A function is a piece of code written to carry out a specified task. To use an existing built-in function, you need to know its name, inputs and outputs. print() function - sends content to the screen Python is case sensitive. Use print(), rather than Print() or PRINT(). print() # empty line print('Hello') # Hello print(42) # 42 greeting = 'Hello' # Assign a string to variable print(greeting) # print variable value 26 print() function Multiple objects separated by commas print separated by a space: print('dog', 'cat', 42) # dog cat 42 To suppress printing of a new line, use end=' ' : print('Dog', end='') print('Cat') # DogCat 27 print() with string concatenation product = 'mask' price = 99 print("The product: " + product + "costs" + str(price) + "pence") String concatenation requires strings! Convert price with str(). The product: maskcosts99pence The + operation on strings adds no extra space between strings. print("The product: " + product + " costs " + str(price) + " pence") 28 print() with commas (reminder) product = 'mask' price = 99 print("The product:", product, "costs", price, "pence") The 5 arguments passed to print will be converted to strings and with a space between: The product: mask costs 99 pence 29 Lecture 1 Problem Solving & Programs Values, Types and Variables Assignment Statements Arithmetic Operators, Order of Operations (PEMDAS) Program Comments # Introduction to Strings print(), str() 30