CSC 1015 F Python Chapter 2 Michelle Kuttel

CSC 1015 F – Python Chapter 2 Michelle Kuttel mkuttel@cs. uct. ac. za

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

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

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 4 Michelle likes to do this sometimes…

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. 5

Elements of programs: Expressions are fragments of code that produce or calculate new data values, such as Literals Variables e. g. x Simple expressions combined with operators, e. g: e. g. 3. 9, “hello”, 1 5+2 x**2 “bat”+”man” e. g. mathematical expressions like: 6 >>> 4 + 3 will be evaluated as 7.

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

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 8

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

Elements of programs: Names We name: Modules Functions Variables Examples of valid names: 10 X y happy dazed_and_Confused Dazed_and_Confused 2

Elements of programs: Names BUT you can’t use Python keywords (reserved words) as names. These are: False None 11 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

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 12

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!) 13

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) 14

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

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

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

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=’*’) 18

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 19

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

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

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? ”) 22

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: ") person. A = input(“What is your name? ”) 23

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

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 25 eval(“ 20”) -> 20

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) 26

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: greet 3. py person = “Bob” three. Times = person*3 print("Hello", three. Times) print("Oops. . ”) 27

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 28

Simultaneous assignment Can assign several values at once: person. A, person. B, person. C =“Tom”, “Dick”, “Harry” x, y = 10, 20 sum, diff = x+y, x-y x, y = y, x 29

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

Python first function def hello(): print(“Hello”) print(“I love CSC 1015 F”) Defines a new function called hello Indentations show that the lines below hello belong to the function Invoked like this: hello() 31

Python second function def hello 2(): name = input("What is your name? ") print("Hellooooo", name*3, end='!!!', sep='…') 32

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) 33

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

Checkpoint: What is the exact output of this code? # function checkpoint One. Two. Three. py def A(word 1, word 2, word 3): print(word 3, word 2, word 1, sep='. . . ', end='STOP') A("green", "orange", "red") 35

Checkpoint: What is the exact output of this code? # function checkpoint One. Two. Three. py def A(word 1, word 2, word 3): print(word 3, word 2, word 1, sep='. . . ', end='STOP') print(“Testing…”) x, y, z="green", "orange", "red" A(x, y, z) A(z*2, y*2, x*2) 36

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: 37 Input, Process, Output (IPO) Ask user for input in miles, convert to kilometers, output result by displaying it on the screen

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

Algorithms and 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 39

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() 40

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) 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() 42

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 43

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 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 output statement 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() Calls the main method automatically when this module is run 46

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

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