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Chapter 4. Decision Making

Table of Contents:

Introduction

Decision Making (Also known as conditional branching) is the process of running certain part of code whenever the condition satisfies. For example, If we want to allow the player if he/she is 18 years or older only then, we need to create condition that handles the game with respect to the age.

We use if, elif, and else conditions to perform conditional branching.

The if condition

  • It is written with if keyword
  • code blocks that should run whenever the condition matches are indented.
  • We need to un-indent the code block to represent the end of a code block.

Syntax:

if <contition>:
    statements that need to run when the condition fulfills
    ...

end of the code block

Example 1: Condition is satisfied

a = 20
b = 25
if a < b:
    print(f'{a} is less than {b}')
print('I am outside of the code block')

'''
out:
20 is less than  25
I am outside of the code block
'''

Example 2: Condition is not satisfied

a = 20
b = 25
if a > b:
    print(f'{a} is greater than {b}')  # this code block does not run
print('I am outside of the code block')

'''
out:
I am outside of the code block
'''

The if else condition

The if else condition is similar to the if condition but has another code block that runs when the condition is not satisfied.

Syntax:

if <condition>:
    code block that runs only when condition is satisfied
else:
    code block that runs if the condition is not satisfied

Example:

x = 5
if x % 2 == 0:
    # this code block runs only when x is assigned even numbers
    print(f'{x} is even number')
else:
    # this code block runs only when x is assigned odd numbers
    print(f'{x} is odd number')

The if elif else condition

The if elif else condition adds more flexibility to branching providing more than one conditions.

Syntax:

if < condition 1 >:
    # code block that runs only when condition satisfies
elif < condition 2 >:
    # code block that runs if the condition 1 satisfies and condition 2 satisfies
    # ...
elif < condition n >:
    # ...
else:
    # code block that runs only if all of above condition does not satisfy

example:

num = 12

if num % 2 == 0 and num % 3 == 0:
    print("Divisible by both 2 and 3")

elif num % 2 == 0:
    print("Divisible by only 2")

elif num % 3 == 0:
    print("Divisible by only 3")

else:
    print("Not divisible by both 2 and 3")

Note: In case of if elif conditions, if the first condition satisfies, it never goes to check another condition so we need to make sure we're using it properly to get the expected output

The match Condtion

The match keyword is a new concept introduced in python version 3.10. It can be used in pattern matching. A match statement is similar to if-else statement but with great flexibility to match single and multiple patterns.

The match block is made up of match and case statements. A match statement feeds the input whereas case matches the input with the specific pattern.

Syntax:

match statement:
    case a:
        < body
        when
        case
        a
        matches >
    case b:
        < body
        when
        case
        b
        matches >
    case _:
        < fallback
        statements >
        < body
        when
        all
        of
        above
        does
        not match >

Example:

number = int(input('Enter a number: '))

match number:
    case 1:
        print("The first natural number.")

    case 7:
        print("The lucky number.")

    case _:
        print('What is it\'s significance?')

We can do more with match statements. Example, instead of comparing one value with it, we can also compare wildcards.

The match statement gives more flexibility in matching patterns since it enables us to match the following:

  • single value
  • multiple values
  • wildcards
  • multiple conditions

Example: matching multiple values with wildcards

(x, y) = (5, 0)
match (x, y):
    case (0, 0):
        print("The point lies in the origin")
    case (_, 0):
        print("The point lies in the x-axis")
    case (0, _):
        print("The point lies in the y-axis")
    case (_, _):
        print("The point lies somewhere between")

In the above statement, x, y are matched against different conditions where _ is used as a wildcard.

To know more about match statement, you can check the documentation at https://docs.python.org/3/reference/compound_stmts.html#the-match-statement.

Ternary operators

Ternary operators are also known as single statement conditional branching and are used whenever we have conditions that has only one statement or assignment operation.

Example:

if x < 7:
    size = 'small'
elif x >= 7 and x < 1:
    size = 'medium'
else:
    size = 'large'

The Above statement can be converted into single statement conditional branching using the following code:

size = 'small' if x < 7 else ('medium' if (7 <= x < 10) else 'large')

Nested Conditions

Nested conditions are conditions that exist within another condition.

number = 20
if number < 10:
    if number % 2 == 0:
        print('The number is even and less than 10')
    else:
        print('The number is odd and less than 10')
else:
    if number % 3 == 0:
        print('The number is divisible by 3 and greater than 10')
    else:
        print('The number is not divisible by 3 and greater than 10')