CSC1015F – Python Chapter2

1. CSC1015F – Python Chapter2 Michelle Kuttel mkuttel@cs.uct.ac.za

2. Programming languages • A program is a sequence of instructions telling a computer what to do. • Human languages still don’t work for this • Ambiguous and imprecise • E.g. “They are hunting dogs.” • Programming languages express computations in an exact and unambiguous way • Precise SYNTAX (form) and SEMANTICS (meaning)

3. Origins of the Python language • Created by Dutch computer programmer Guido van Rossum in the early 1990’s • Benevolent Dictator for Life (BDFL) • Intellectual property now owned by the Python Software Foundation (PSF) • We are using Python Version 3 • Free and well documented • runs everywhere • Clean syntax

4. Integrated Development Environment (IDE) • Graphical interface with menus and windows, much like a word processor • We recommend Wingware 101 • free, scaled-down Python IDE designed for use in teaching introductory programming classes. • It omits many features found in Wing IDE Professional and makes simplifications appropriate for beginners. • you are welcome to use any other tools – e.g. IDLE • even a separate text editor and interpreter if you wish to simply do things the hard way • Michelle likes to do this sometimes…

5. Python expressions Programs are made up of commands (or statements) that a computer executes or evaluates. One kind of statement is an expression statement, or expression.

6. Elements of programs:Expressions • Expressions are fragments of code that produce or calculate new data values, such as • Literals • e.g. 3.9, “hello”, 1 • Variables • e.g. x • Simple expressions combined with operators, e.g: • 5+2 • x**2 • “bat”+”man” • e.g. mathematical expressions like: • >>> 4 + 3 • will be evaluated as 7.

7. Python interactive shell The >>> part is called a prompt, because it prompts us to type something.

8. Elements of programs: Variables and assigment x=3 x+4 • x is a variable • A variable is used to name a value so that we can refer to it later an assignment statement assigns a value to a variable

9. Elements of programs:Assignment statements • One of the most important kinds of statement in Python. e.g.: x=5 x=x+1 • Basic form: <variable> = <expr> 5 x 6 x

10. Elements of programs: Names • We name: • Modules • Functions • Variables • Examples of valid names: • X • y • happy • dazed_and_Confused • Dazed_and_Confused2

11. Elements of programs: Names • BUT you can’t use Python keywords (reserved words) as names. These are: 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

12. Checkpoint height = 1.69 weight = 71.5 weight/(height*height) From this Python code, give an example of: • a variable • an assignment statement • a literal

13. Comments • And text on a line after the ‘#” symbol will be ignored by the Python interpreter: # algorithm to calculate BMI height = 1.69 weight = 71.5 weight/(height*height) #calculates BMI • It is essential to include comments to explain your code to others (and your future self!)

14. Elements of programs:Output statements • The print function will display text to the screen • e.g. >>> print("Hello") >>> Hello • e.g. # algorithm to calculate BMI height = 1.69 weight = 71.5 BMI = weight/(height*height) #calculates BMI print("Body mass index = ",BMI)

15. Checkpoint # file BobBill.py Bob="Bob" Bill="Bill" Bob=Bill Bill=Bill*3 print(Bob,Bill) From this code, give an example of: • a variable • a literal • an expression • an assignment statement

16. Checkpoint: What is the output of this code? # file BobBill.py Bob="Bob" Bill="Bill" Bob=Bill Bill=Bill*3 print(Bob,Bill)

17. Elements of programs:More about print • print is a built in function with a precise set of rules for its syntax and semantics • e.g. two forms: print(<expr>,<expr>,…,<expr>) print() • By default, print puts a single blank space between the displayed values

18. More about “print()” function • To print a series of values separated by spaces: print("The values are", x, y, z) • To suppress or change the line ending, use the end=ending keyword argument. e.g.: print("The values are", x, y, z, end=‘’) • To change the separator character between items, use the sep=sepchr keyword argument. e.g. print("The values are", x, y, z,sep=’*’)

19. Escape sequences Escape sequences are special characters that represent non-printing characters • tabs, newlines e.g: \a - bell (beep) \b – backspace \n – newline \t – tab \’ – single quote \” – double quote \v - vertical tab

20. Checkpoint: What is the exact output of this code? x=20 y=30 z=40 print(x) print(y,'\n',z)

21. Checkpoint: What is the exact output of this code? x=20 y=30 z=40 print("The values are", x, y, z, end='!!!',sep='!***')

22. Elements of programs:Input statements • The purpose of an input statement is to get some information from the user and store it into a variable. • In Python, input is accomplished using an assignment statement combined with a special expression called input. This template shows the standard form: <variable> = input(<prompt>) Here, prompt is usually some text asking the user for input e.g. weather = input(“What is the weather like today?”)

23. Elements of programs:Input statements • the input statement treats whatever the user types as an expression to be evaluated. e.g. n = input("Please enter a number: ") personA = input(“What is your name?”)

24. Checkpoint: What is the exact output of this code? name = input("What is your name? ") print("Hellooooo", name*3, end='!!!',sep='…')

25. Elements of programs:Input statements and numbers the input expression is evaluated as a string (text) • this must be converted into a number if you are going to do math • text (strings) and numbers are handled differently by the computer – more about this later! • eval() is a Python function that converts a string into a number • eval(“20”) -> 20

26. Elements of programs:Input statements and numbers Useful example using eval… # algorithm to calculate BMI, using input height = input("Type in your height: ") weight = input("Type in your weight: ") height=eval(height) #change a string into a number weight=eval(weight) #change a string into a number BMI = weight/(height*height) #calculates BMI print("Body mass index = ",BMI)

27. Saving programs to file • If you type them in directly, programs definition are lost when you quit the shell • So, we usually type programs into a separate file, called a module, or script #file: greet3.py person = “Bob” threeTimes = person*3 print("Hello", threeTimes) print("Oops.... ”)

28. Where to use comments • Brief description, author, date at top of function. • Brief description of purpose of each method (if more than one). • Short explanation of non-obvious parts of code within methods. #My very first program #Author: Michelle Kuttel #date 21/2/2012

29. Simultaneous assignment • Can assign several values at once: personA, personB, personC =“Tom”, “Dick”, “Harry” x,y = 10,20 sum, diff = x+y, x-y x,y = y,x

30. Checkpoint: What is the output of this code? x,y = 10,20 x,y = y,x print(x,y)

31. Python first function def hello(): print(“Hello”) print(“I love CSC1015F”) • Defines a new function called hello • Indentations show that the lines below hello belong to the function • Invoked like this: hello()

32. Python second function def hello2(): name = input("What is your name? ") print("Hellooooo", name*3, end='!!!',sep='…')

33. Parameters • Functions can have changeable parameters (or arguments) that are placed within parenthesis. E.g. def greet(person): print(“hello”, person) print(“How are you?”) n = input(”What is your name?”) greet(n)

34. Checkpoint def greet(person): print(“hello”, person) print(“How are you?”) n = input(”What is your name?”) greet(n) From this code, give an example of: • a variable • a literal • an expression • an assignment statement • a parameter • a function

35. Checkpoint: What is the exact output of this code? # function checkpoint OneTwoThree.py def A(word1, word2, word3): print(word3,word2,word1,sep='...',end='STOP') A("green","orange","red")

36. Checkpoint: What is the exact output of this code? # function checkpoint OneTwoThree.py def A(word1, word2, word3): print(word3,word2,word1,sep='...',end='STOP') print(“Testing…”) x,y,z="green","orange","red" A(x,y,z) A(z*2,y*2,x*2)

37. Example program: Miles-to-kilometres converter • You’re visiting the USA and distances are all quoted in miles • I mile = 1.61 km • Write a program to do this conversion • Algorithm conforms to a standard pattern: • Input, Process, Output (IPO) • Ask user for input in miles, convert to kilometers, output result by displaying it on the screen

38. Algorithms and Pseudocode • Pseudocode is just precise English that describes what a program does • Communicate the algorithm without all the details e.g.

39. Algorithms and Pseudocode • Pseudocode is just precise English that describes what a program does • Communicates the algorithm without all the details e.g. Input the distance in miles (call it miles) Calculate kilometres as miles x 1.61 Output kilometres The next step is to convert this into a Python program

40. Miles.py #miles.py # A program to convert miles to kilometres # by: Michelle Kuttel def main(): miles = eval(input("What is the distance in miles? ")) kilometres = miles* 1.61 print("The distance in kilometres is",kilometres,"km.") main()

41. Breakdown of Miles.py #miles.py # A program to convert miles to kilometres # by: Michelle Kuttel def main(): miles = eval(input("What is the distance in miles? ")) kilometres = miles* 1.61 print("The distance in kilometres is",kilometres,"km.") main() • Defines a function called main (which has a special meaning)

42. Breakdown of Miles.py #miles.py # A program to convert miles to kilometres # by: Michelle Kuttel def main(): miles = eval(input("What is the distance in miles? ")) kilometres = miles* 1.61 print("The distance in kilometres is",kilometres,"km.") main()

43. Breakdown of Miles.py miles = eval(input("What is the distance in miles? ")) • eval() is a Python function that converts a string into a number • eval(“20”) -> 20 • for now, think of a string as a word or sentence (more on strings later) • the input statement treats whatever the user types as an expression to be evaluated

44. Breakdown of Miles.py #miles.py # A program to convert miles to kilometres # by: Michelle Kuttel def main(): miles = eval(input("What is the distance in miles? ")) kilometres = miles* 1.61 print("The distance in kilometres is",kilometres,"km.") main() • An expression which assigns a value to the variable kilometres using the multiplication operator

45. Breakdown of Miles.py #miles.py # A program to convert miles to kilometres # by: Michelle Kuttel def main(): miles = eval(input("What is the distance in miles? ")) kilometres = miles* 1.61 print("The distance in kilometres is",kilometres,"km.") • main() • An output statement

46. Breakdown of Miles.py #miles.py # A program to convert miles to kilometres # by: Michelle Kuttel def main(): miles = eval(input("What is the distance in miles? ")) kilometres = miles* 1.61 print("The distance in kilometres is",kilometres,"km.") main() • Calls the main method automatically when this module is run

47. More Python statements in future lectures… We also also have statements to: • Perform selections • Iterate (loop)

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