CSC1015F – Python Chapter2

CSC1015F – Python Chapter2

Michelle Kuttel

[email protected]

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)2

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

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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…

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Python expressionsPrograms are made up of commands (or

statements) that a computer executes or evaluates.

One kind of statement is an expression statement, or expression.

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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.

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Python interactive shellThe >>> part is called a prompt, because it

prompts us to type something.

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Elements of programs: Variables and assigment

x=3x+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

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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>

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5x

6x

Elements of programs: Names We name:

Modules Functions Variables

Examples of valid names: X y happy dazed_and_Confused Dazed_and_Confused2

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Elements of programs: Names

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Checkpointheight = 1.69weight = 71.5weight/(height*height)

From this Python code, give an example of: a variable an assignment statement a literal

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Comments And text on a line after the ‘#” symbol will be

ignored by the Python interpreter:

# algorithm to calculate BMIheight = 1.69weight = 71.5weight/(height*height) #calculates BMI

It is essential to include comments to explain your code to others (and your future self!)

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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)

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Checkpoint# file BobBill.pyBob="Bob"Bill="Bill"Bob=BillBill=Bill*3print(Bob,Bill)

From this code, give an example of: a variable a literal an expression an assignment statement15

Checkpoint: What is the output of this code?

# file BobBill.pyBob="Bob"Bill="Bill"Bob=BillBill=Bill*3print(Bob,Bill)

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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

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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=’*’)

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Escape sequencesEscape 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 tab19

Checkpoint: What is the exact output of this code?

x=20y=30z=40print(x)print(y,'\n',z)

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Checkpoint: What is the exact output of this code?

x=20y=30z=40print("The values are", x, y, z, end='!!!',sep='!***')

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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 inpute.g.

weather = input(“What is the weather like today?”)

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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?”)

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Checkpoint: What is the exact output of this code?

name = input("What is your name? ")

print("Hellooooo", name*3, end='!!!',sep='…')

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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

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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)

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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.pyperson = “Bob”

threeTimes = person*3print("Hello", threeTimes)print("Oops.... ”)

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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

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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

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Checkpoint: What is the output of this code?

x,y = 10,20

x,y = y,x

print(x,y)

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Python first functiondef 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()

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Python second functiondef hello2():

name = input("What is your name? ")

print("Hellooooo", name*3, end='!!!',sep='…')

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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)

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Checkpoint

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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")

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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)

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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

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Algorithms and Pseudocode Pseudocode is just precise English that

describes what a program does Communicate the algorithm without all the

details e.g.

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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 1.61Output kilometres

The next step is to convert this into a Python program39

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()

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Breakdown of Miles.pymiles = 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 evaluated43

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 operator44

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

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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 run46

More Python statements in future lectures…

We also also have statements to:

Perform selections

Iterate (loop)

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This work is licensed under a Creative Commons Attribution-ShareAlike 2.5 South Africa License.

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