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# Introduction to Computing Using Python

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1. Python Introduction to Computing Using Python • Python is an interactive language. • Java or C++: compile, run • Also, a main function or method • Python: type expressions directly into interactive shell (or load them from a file) • Wherever you want your program to start, type this into the interactive shell (or in a file that is loaded into the shell)

2. Python Download and run Introduction to Computing Using Python • http://www.python.org/download • Downloads the Python language, plus an interactive shell called idle • To run: type idle into Search programs and files • You can type Python expressions directly into the idle window • Or, you can open a file window by clicking New fileunder the File menu, or Ctrl-o • To load Python code from a file, hit F5 key

3. Algebraic expressions Introduction to Computing Using Python >>> 2 + 3 5 >>> 7 - 5 2 >>> 2*(3+1) 8 >>> 5/2 2.5 >>> 5//2 2 >>> 14//3 4 >>> 14%3 2 >>> 2**3 8 >>> min(3, 2, 4) 2 >>> 2 + 3 5 >>> 7 - 5 2 >>> 2*(3+1) 8 >>> 5/2 2.5 >>> 5//2 2 >>> 14//3 4 >>> 14%3 2 >>> 2 + 3 5 >>> 7 - 5 2 >>> 2*(3+1) 8 >>> 5/2 2.5 >>> 5//2 2 >>> 14//3 4 >>> 14%3 2 >>> 2**3 8 >>> abs(-3.2) 3.2 >>> 2 + 3 5 >>> 7 - 5 2 >>> 2*(3+1) 8 >>> 5/2 2.5 >>> 5//2 2 >>> 14//3 4 >>> 14%3 2 >>> 2**3 8 >>> abs(-3.2) 3.2 >>> min(23,41,15,24) 15 >>> 2 + 3 5 >>> 7 - 5 2 >>> 2*(3+1) 8 >>> 5/2 2.5 >>> 5//2 2 >>> 14//3 4 >>> 14%3 2 >>> 2**3 8 >>> abs(-3.2) 3.2 >>> min(23,41,15,24) 15 >>> max(23,41,15,24) 41 >>> 2 + 3 5 >>> 7 - 5 2 >>> 2*(3+1) 8 >>> 5/2 2.5 >>> 2 + 3 5 >>> 7 - 5 2 >>> 2*(3+1) 8 >>> 2 + 3 5 >>> 2 + 3 5 >>> 7 - 5 2 >>> 2 + 3 5 >>> 7 - 5 2 >>> 2*(3+1) 8 >>> 5/2 2.5 >>> 5//2 2 The Python interactive shell can be used to evaluate algebraic expressions 14//3 is the quotient when 14 is divided by 3 and 14%3 is the remainder 2**3 is 2 to the 3rd power • abs(), min(), and max() are functions • abs() takes a number as input and returns its absolute value • min() (resp., max()) take an arbitrary number of inputs and return the “smallest” (resp., “largest”) among them

4. Operator precedence Introduction to Computing Using Python In algrabraic expressions that use more than one operator, Python must have some way to determine the order in which operators are applied. For example: >>> 2 + 3 * 5 (is it 25 or 17?) It’s 17, because * has higher precedence than + In other words * is computed first You can always override precedence by using parentheses

5. Functions Introduction to Computing Using Python A function is a sequence of simple statementsthat is given a name Many functions are built in to the Python language Examples: abs, min, max When you call a function, you must place parentheses after it; e.g., abs(-4) NOT abs -4

6. Functions Introduction to Computing Using Python Functions are often passed parameters which are pieces of information a function needs to run Example: >>> abs(-3) 3 Different functions take different numbers of parameters. For example:

7. Functions Introduction to Computing Using Python abs takes one parameter >>> abs(-1) 1 min takes one or more parameters >>> min(2,1) 1 >>> min(4, 2, 6) 2 Some take no parameters • >>> import time • >>> time.time() • 1377628104.592236

8. Comments Introduction to Computing Using Python When you write non-trivial code, it is important to comment it. Comment symbol is # Anything on a line that is after # will be ignored by the computer

9. Comments Introduction to Computing Using Python Comments are used to make code more understandable for people (including you) Example: x = 3.141 * radius * radius # compute the area of a circle Don’t overcomment: y = 3 # set the value of y to 3 Even the first example of a comment might not be necessary

10. Boolean expressions Introduction to Computing Using Python In addition to algebraic expressions, Python can evaluate Boolean expressions • Boolean expressions evaluate to • True or False • Boolean expressions often involve arithmetic comparison operators • <, >, ==, !=, <=, and >= >>> 2 < 3 True >>> 2 > 3 False >>> 2 == 3 False >>> 2 != 3 True >>> 2 <= 3 True >>> 2 >= 3 False >>> 2+4 == 2*(9/3) True • In an expression containing algebraic and • comparison operators: • Algebraic operators are evaluated first • Comparison operators are evaluated next

11. Logical operators Introduction to Computing Using Python >>> 2<3 and 3<4 True >>> 4==5 and 3<4 False >>> False and True False >>> True and True True >>> 4==5 or 3<4 True >>> False or True True >>> False or False False >>> not(3<4) False >>> not(True) False >>> not(False) True >>> 4+1==5 or 4-1<4 True Logical operators can also be used in Boolean expressions - and, or, not • In a an expression containing algebraic, comparison, and Boolean operators: • Algebraic operators are evaluated first • Comparison operators are evaluated next • Boolean operators are evaluated last

12. Exercise Introduction to Computing Using Python >>> 25 - 21 4 >>> 14.99 + 27.95 + 19.83 62.769999999999996 >>> 20*15 300 >>> 2**10 1024 >>> min(3, 1, 8, -2, 5, -3, 0) -3 >>> 3 == 4-2 False >>> 17//5 == 3 True >>> 17%5 == 3 False >>> 284%2 == 0 True >>> 284%2 == 0 and 284%3 == 0 False >>> 284%2 == 0 or 284%3 == 0 True • Translate the following into Python algebraic or Boolean expressions and then evaluate them: • The year that was 25 years ago (use -) • The total of 14.99, 27.95, and 19.83 • The area of a rectangle of length 20 and width 15 • 2 to the 10th power • The minimum of 3, 1, 8, -2, 5, -3, and 0 • 3 equals 4-2 • The value of 17//5 is 3 • The value of 17%5 is 3 • 284 is even • 284 is even and 284 is divisible by 3 • 284 is even or 284 is divisible by 3

13. Variables and assignments Introduction to Computing Using Python >>> x = 3>>> x3 >>> 4*x12>>> y Traceback (most recent call last): File "<pyshell#18>", line 1, in <module> y NameError: name 'y' is not defined >>> y = 5*x 15 >>> y 15 Just as in algebra, a value can be assigned to a variable, such as x When variable x appears inside an expression, it evaluates to its assigned value A variable (name) does not exist until it is assigned The assignment statement has the format <expression>is evaluated first, and the resulting value is assigned to variable <variable> <variable> = <expression>

14. Naming rules Introduction to Computing Using Python • Variable names can contain these characters: • a through z • A through Z • the underscore character _ • digits 0 through 9 >>> My_x2 = 21 >>> My_x2 21 >>> My_x2 = 21 >>> My_x2 21 >>> 2x = 22 SyntaxError: invalid syntax >>> new_temp = 23 >>> newTemp = 23 >>> >>> My_x2 = 21 >>> My_x2 21 >>> 2x = 22 SyntaxError: invalid syntax >>> new_temp = 23 >>> newTemp = 23 >>> counter = 0 >>> temp = 1 >>> price = 2 >>> age = 3 >>> My_x2 = 21 >>> My_x2 21 >>> 2x = 22 SyntaxError: invalid syntax >>> • Names cannot start with a digit • For a multiple-word name, use • either the underscore as the delimiter • or camelCasecapitalization Short and meaningful names are ideal

15. Strings Introduction to Computing Using Python >>> 'Hello, World!' 'Hello, World!' >>> >>> 'Hello, World!' 'Hello, World!' >>> s = 'rock' >>> t = 'climbing' >>> In addition to number and Boolean values, Python supports string values "Hello, World!" 'Hello, World!' • A string value is represented as a sequence of characters enclosed within quotes • A string value can be assigned to a variable • String values can be manipulated using string operators and functions

16. String operators Introduction to Computing Using Python >>> 'Hello, World!' 'Hello, World!' >>> s = 'rock' >>> t = 'climbing' >>> s == 'rock' True >>> s != t True >>> s < t False >>> s > t True >>> s + t 'rockclimbing' >>> s + ' ' + t 'rock climbing' >>> 5 * s 'rockrockrockrockrock' >>> 30 * '_' '______________________________' >>> 'o' in s True >>> 'o' in t False >>> 'bi' in t True >>> len(t) 8 To view all operators, use the help() tool >> help(str) Help on class str in module builtins: class str(object) | str(string[, encoding[, errors]]) -> str ...

17. Exercise Introduction to Computing Using Python >>> s1 'good' >>> s2 'bad' >>> s3 'silly' >>> >>> s1 'good' >>> s2 'bad' >>> s3 'silly' >>> 'll' in s3 True >>> ' ' not in s1 True >>> s1 + s2 + s3 'goodbadsilly’ >>> ' ' in s1 + s2 + s3 False >>> 10*s3 'sillysillysillysillysillysillysillysillysillysilly' >>> len(s1+s2+s3) 12 >>> • Write Python expressions involving strings s1, s2, and s3 that correspond to: • 'll' appears in s3 • the blank space does not appear in s1 • the concatenation of s1, s2, and s3 • the blank space appears in the concatenation of s1, s2, and s3 • the concatenation of 10 copies of s3 • the total number of characters in the concatenation of s1, s2, and s3

18. Index and indexing operator Introduction to Computing Using Python • The index of an item in a sequence is its position with respect to the first item • The first item has index 0, • The second has index 1, • The third has index 2, … • The index of an item in a sequence is its position with respect to the first item • The first item has index 0, • The second has index 1, • The index of an item in a sequence is its position with respect to the first item • The index of an item in a sequence is its position with respect to the first item • The first item has index 0, The indexing operator []takes a nonnegative index i and returns a string consisting of the single character at index i 'A pple' s = 0 1 2 3 4 'A' s = >>> s = 'Apple' >>> s 'A' >>> s 'p' >>> s 'e' 'p' s = 'p' s = 'l' s = 'e' s =

19. Negative index Introduction to Computing Using Python • A negative index is used to specify a position with respect to the “end” • The last item has index -1, • The second to last item has index -2, • The third to last item has index -3, … -5 -4 -3 -2 -1 'A pple' s = 0 1 2 3 4 'e' s[-1] = 'l' >>> s = 'Apple' >>> s[-1] 'e' >>> s[-2] 'l' >>> s[-5] 'A' s[-2] = 'A' s[-5] =

20. Exercise Introduction to Computing Using Python • String s is defined to be • 'abcdefgh' • Write expressions using s and the indexing operator [] that return the following strings: • 'a' • 'c' • 'h' • 'f' >>> s = 'abcdefgh' >>> >>> s = 'abcdefgh' >>> s 'a' >>> s 'c' >>> s 'h' >>> s[-1] 'h' >>> s[-3] 'f' >>>

21. Lists Introduction to Computing Using Python In addition to number, Boolean, and string values, Python supports lists ['ant', 'bat', 'cod', 'dog', 'elk'] [0, 1, 'two', 'three', [4, 'five']] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] A comma-separated sequence of items enclosed within square brackets The items can be numbers, strings, and even other lists >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk'] >>> lst = [0, 1, 'two', 'three', [4, 'five']] >>> nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>> >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk'] >>> lst = [0, 1, 'two', 'three', [4, 'five']] >>> >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk’] >>>

22. List operators and functions Introduction to Computing Using Python >>> lst = [1, 2, 3] >>> lstB = [0, 4] >>> 4 in lst False >>> 4 not in lst True >>> lst + lstB [1, 2, 3, 0, 4] >>> 2*lst [1, 2, 3, 1, 2, 3] >>> lst 1 >>> lst 2 >>> lst[-1] 3 >>> len(lst) 3 >>> min(lst) 1 >>> max(lst) 3 >>> sum(lst) 6 >>> help(list ... Like strings, lists can be manipulated with operators and functions

23. Lists are mutable, strings are not Introduction to Computing Using Python Lists can be modified; they are said to be mutable pets = ['ant', 'bat', 'cod', 'dog', 'elk'] pets = ['ant', 'bat', 'cow', 'dog', 'elk'] Strings can’t be modified; they are said to be immutable pet = 'cod' >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk'] >>> pets = 'cow' >>> pets ['ant', 'bat', 'cow', 'dog', 'elk'] >>> pet = 'cod' >>> >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk'] >>> >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk'] >>> pets = 'cow' >>> pets ['ant', 'bat', 'cow', 'dog', 'elk'] >>> >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk'] >>> pets = 'cow' >>> pets ['ant', 'bat', 'cow', 'dog', 'elk'] >>> pet = 'cod' >>> pet = 'w' Traceback (most recent call last): File "<pyshell#155>", line 1, in <module> pet = 'w' TypeError: 'str' object does not support item assignment >>> The elements can be numbers, strings, and even other lists >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk'] >>> lst = [0, 1, 'two', 'three', [4, 'five']] >>> >>> pets = ['ant', 'bat', 'cod', 'dog', 'elk’] >>>

24. List methods Introduction to Computing Using Python len()and sum() are examples of functions that can be called with a list input argument; they can also be called on other type of input argument(s) >>> lst = [1, 2, 3] >>> len(lst) 3 >>> sum(lst) 6 >>> lst.append(7) >>> lst [1, 2, 3, 7] >>> lst.remove(3) >>> lst [1, 2, 7] There are also functions that are called on a list; such functions are called list methods (note the different syntax). Two of these methods are append and remove >>> lst = [1, 2, 3] >>> len(lst) 3 >>> sum(lst) 6 >>> ` Later we will discuss in more detail what the difference is between a method and a function

25. List methods Introduction to Computing Using Python >>> lst = [1, 2, 3] >>> lst.append(7) >>> lst.append(3) >>> lst [1, 2, 3, 7, 3] >>> lst.count(3) 2 >>> lst.remove(2) >>> lst [1, 3, 7, 3] >>> lst.reverse() >>> lst [3, 7, 3, 1] >>> lst.index(3) 0 >>> lst.sort() >>> lst [1, 3, 3, 7] >>> lst.remove(3) >>> lst [1, 3, 7] >>> lst.pop() 7 >>> lst [1, 3] Methods append(), remove(), reverse(), and sort() do not return any value; they, along with method pop(), modify list lst

26. Exercise Introduction to Computing Using Python List lstis a list of prices for an item at different online retailers • You found another retailer selling the item for \$160.00; add this price to list lst • Compute the number of retailers selling the item for \$160.00 • Find the minimum price in lst • Using c), find the index of the minimum price in list lst • Using c) remove the minimum price from list lst • Sort list lst in increasing order >>> lst = [159.99, 160.00, 205.95, 128.83, 175.49] >>> lst.append(160.00) >>> lst.count(160.00) 2 >>> min(lst) 128.83 >>> lst.index(128.83) 3 >>> lst.remove(128.83) >>> lst [159.99, 160.0, 205.95, 175.49, 160.0] >>> lst.sort() >>> lst [159.99, 160.0, 160.0, 175.49, 205.95] >>>

27. Introduction to Computing Using Python Objects and classes A class is a kind of data (int, list, str, …) An object is a particular value associated with a class (3, [1, 2, 3], ‘abc’) All values in Python are objects Unlike many other programming languages An object’s type determines what values it can have and how it can be manipulated Terminology: object X is of type int = object X belongs to class int

28. Values of number types Introduction to Computing Using Python >>> 3 3 >>> type(3) <class 'int'> >>> 3. 3.0 >>> type(3.) <class 'float'> >>> type(3.0) <class 'float'> An object’s type determines what values it can have and how it can be manipulated An object of type intcan be any integer number value (with no decimal point) Another type for numbers is called float. Any number with a decimal point is a float.

29. Input from a user Introduction to Computing Using Python Function for user input: inp(prompt) where prompt is a string Example (assume the user enters ‘hello’) >>> inp = input(‘Enter a word ’) >>>inp + ‘ was your input’ Hello was your input • The user’s input is always interpreted as a string • Example (the user enters ‘1’) • >>> x= input(‘Enter a number ’) • >>> x * 2 • 11

30. Conversion of types Introduction to Computing Using Python To change a string into an integer: int(<string>) For example: >>> int(‘3’) 3 • Example (the user enters ‘1’) • >>> x = int(input(‘Enter a number ’)) • >>> x * 2 • 2

31. Output to a user Introduction to Computing Using Python Function: print(<output>) where <output> is a string For example: >>> print(‘Hello world’) Hello world

32. Conversion of types, continued Introduction to Computing Using Python What if you want to print something that isn’t a string? The strfunction does this >>> str(1) ‘1’ For example: >>> int(‘3’) 3 >>> int(‘3’) + 2 5

33. Example using str Introduction to Computing Using Python Assume user types 3 >>> x = int(input(‘Enter an integer ’)) >>> print(x + ' plus 2 is ' + y) • Traceback (most recent call last): • File "<pyshell#58>", line 1, in <module> • print(x + ' plus 2 is ' + y) • TypeError: unsupported operand type(s) for +: 'int' and 'str' • >>> print(str(x) + ‘ plus 2 is ’ + str(y)) 3 + 2 is 5

34. Writing code in a file Introduction to Computing Using Python • If code becomes too complex, It is a pain to type • it in to the IDLE window over and over again • An alternative: write the code in a file, then hit F5 • key • Output might differ, so you might need to call print()

35. Special characters Introduction to Computing Using Python If a string contains a ‘\’, then the character after that is interpreted differently Example: ‘\n’ means a newline >>> print(‘abc\ndef’) abc def (‘\’ is an escape character within print function)

36. Special characters, continued Introduction to Computing Using Python ‘\t’: tab character >>> print(‘abc\tdef’) abcdef However: >>> ‘abc\ndef’ abc\ndef (‘\’ is an escape character within print function)

37. Homework assignment 1 Introduction to Computing Using Python See Course Online https://col.cdm.depaul.edu Choose ‘CSC-241’ Click ‘Assignments’
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