COMP7004 - Systems Scripting

Lecture 11: Python Flow Control

Dr. Vincent Emeakaroha

vincent.emeakaroha@mtu.ie

Semester 2, 2026

Lecture Goals

• Flow control concept

• Boolean operators

• Comparison operators

• Elements of flow control

• Flow control statements

Flow Control Concept

Start

End

Is raining?

Is raining? Have umbrella?

Wait a while

Go outside

Yes No

Yes

Boolean Values

• Boolean data type has only two value

• True

• False

• Named after a mathematician

• George Boole

• Mathematic Professor in UCC

• Values must be written starting with capital T and F

• No quotes necessary as for strings

Boolean Operators

• There are three types of Boolean operators

• AND

• OR

• NOT

• Similar to other operators

• They evaluate down to a single Boolean value

Boolean Operator Truth Table

Expression

Evaluates

To…

True and True

True

and False

False

False and True

False

False and False

False

Expression

Evaluates

To…

True or True

True

True or False

True

False or True

True

False or False

False

Expression

Evaluates

To…

Not True

False

Not False

True

AND

NOT

Comparison Operators

• These values evaluates to True or False

Operator

Meaning

Equal to

Not equal

Less than

Greater than

Less than or equal

Greater

than or equal to

Comparison Operator Example

>>> 42 == 42

True

>>> 42 == 99

False

>>> 2 != 3

True

>>> 3 != 3

False

>>> 'hello' == 'hello'

True

>>> 'hello' == 'Hello'

False

Mixing Boolean and Comparison

Operators

• Boolean operators operate only on Boolean values

• True and False

• Comparison operators are not

• Boolean values

• But evaluates down to Boolean values

• Example combination

>>> (4 < 5) and (5 < 6)

True

>>> (4 < 5) and (9 < 6)

False

Elements of Flow Control

• Two important elements of flow control statement are:

1. Conditions

• Boolean expressions

• Evaluates down to True or False

• Decides which action to take

2. Block of Code

• Lines of codes that can be grouped together

• Begins with an indentation increase

• Can contain other blocks

• Ends when indentation decreases to zero or to a containing block

indentation

if Statement

• Syntax

if <expression>:

• Characteristics

• The if keyword

• A conditional expression

• A colon

• Starting on the next line, an indented block of code

• Example

• if.py

if.py

# if statement example

print('who are you?')

name = input()

if name == 'Alice':

print('Hi, Alice.')

else Statements

• An if clause can optionall y be followed by an else

statement

• Only executed when the if condition is False

• Characteristics

• The else keyword

• A colon

• Starting on the next line and indented block of code

• Example

• else.py

else.py

# else statement example

print('who are you?')

name = input()

if name == 'Alice':

print('Hi, Alice.')

else:

print('Hello, Stranger.')

elif statement

• A statement that can follow an if statement.

• Full pronouncement is “else if”.

• Allows multiple execution option rather than only if and else clause.

• Provides another condition to be checked when the previous if result to

False.

• If one conditional expression evaluates to True, it gets executed and the rest

ignored.

• No guaranteed execution of particular expression.

• Characteristics

• The elif keyword.

• A conditional expression that evaluates to True or False.

• A colon

• Starting on the next line an indented block of code.

• Example

• elif.py

elif.py

# elif statement example

print('who are you?')

name = input()

print('What is your age?')

age = int(input())

if name == 'Alice':

print('Hi, Alice.')

elif age < 12:

print('You are not Alice, Kiddo.')

elif age > 2000:

print('Unlike you, Alice is not a Vampire.')

elif age > 100:

print('You are not Alice, grannie.')

elif2.py

• elif statement with guaranteed execution of particular expression.

• By adding an else statement

# elif statement example with guaranteed execution

print('who are you?')

name = input()

print('What is your age?')

age = int(input())

if name == 'Alice':

print('Hi, Alice.')

elif age < 12:

print('You are not Alice, Kiddo.')

else:

print('You are neither Alice nor a little kid.')

Match - Case Statement

• Only available from Python 3.10 version.

• Used to execute statements based on specific pattern values. Often used in place of an if-elif-else

statement when there are large number of conditions, especially patterns to match.

• Pattern value can be a number, string, an expression, iterable, class instance, etc.

• Subject value can be any data type

• a _: is used to accept any value not matched with list of values

match subject:

case <pattern1>:

action 1

case <pattern2>:

action 2

case _:

action for a no match

• One can combine several literals in a single pattern using |

• Case <pattern1> | <pattern2> | <pattern3>:

action

Match.py

while Loop Statement

• Provides the ability to execute a code over and over again

• As long as the conditional expression is true.

• Characteristics

• The while keyword

• A conditional expression that evaluates to True or False

• A colon

• Starting on the next line, an indented block of code (called the

while clause).

• Similar to if statement

• But the difference is that at the end of a while clause,

execution jumps back to the beginning of the while loop.

while.py

# example while loop

count = 0

while count < 5:

print('Hello, World.')

count = count + 1

Annoying while Loop

• In while loops

• Poor constructed conditional expression can have annoying

consequences.

• Example:

• while2.py

# example annoying while loop

name = '’

while name != 'your name':

print('Please type your name.')

name = input()

print('Thank you!')

break Statements

• A shortcut from getting out of a while loop

• When execution reaches a break statement

• It immediately exits the while loop

• Characteristics

• The break keyword

• Example

• break.py

break.py

# example break statement

while True:

print('Please type your name.')

name = input()

if name == 'your name':

break

print('Thank you!')

continue Statement

• Provides the possibility of skipping a loop cycle

• When continue statement is reached, it terminates the cycle

immediately and jump back to the beginning of loop to

continue execution.

• Same behaviour as when the execution reaches end of a

loop cycle

• Characteristics

• The continue keyword

• Example

• continue.py

continue.py

# example continue statement

while True:

print('Who are you?')

name = input()

if name != 'Vincent':

continue

print('Hello, Vincent. What is the password? (It is a fish.)')

password = input()

if password == 'swordfish':

break

print('Access granted.')

for Loop Statement

• Provides the ability to execute a block of code

• Only a certain number of times

• Key difference to while loop

• Characteristics

• The for keyword

• A variable name

• The in keyword

• A call to the range()function with up to three integer values passed

to it

• A colon

• Starting on the next line, an indented block of code (called the for

clause)

for.py

# example for loop statement

print('My name is')

for i in range(5):

print('Vincent Five Times (' + str(i) + ')')

for and while Loops

• One can use while loop instead of for loop

• Difference is that for loop is more concise

• Example

• forEquivalent.py

# example for loop statement using while

print('My name is')

i = 0

while i < 5:

print('Vincent Five Times (' + str(i) + ')')

i = i + 1

Starting, Stopping and Stepping

Arguments to range()

• The range() function can accept up to three comma separated values

• First value represents the starting point

• Second value represents the stopping point

• Third value represents the stepping

• Example:

for i in range(0, 6, 2):

Print(i)

• Output:

Importing Module

• Python can call a set of built-in functions

• print(), input(), str(), etc.

• Default knowledge

• There are standard library modules

• Contains a set of related functions

• Can be embedded in your programs

• Extends basic Python

• Linked knowledge

• Example

• Math module: has mathematical related functions

• Random module: has random number related functions

Import Statement

• Before the functions in a module can be used

• Module must be imported using an import statement

• Characteristics

• The import keyword

• The name of the module

• Optionally, more module names separated by commas

• Example:

• import.py

import.py

# import statement example

import random

for i in range(5):

print(random.randint(1, 10))

Ending Program Early

• Usually program terminates when

• Execution reaches the bottom of instruction

• Error situation

• Programs can be stopped at any stage by

• Calling the sys.exit() function

• A function contained in sys module

• Example

• exit.py

exit.py

# exit function example

import sys

while True:

print('Type exit to exit.')

response = input()

if response == 'exit':

sys.exit()

print('You typed '+ response + '.')

Thank You!