Conditionals

Conditionals allow a program to make decisions based on certain conditions.

They are one of the most important building blocks in programming because they control the flow of execution.

Basic syntax

In Python, the most common conditional statement is if.

age = 20

if age >= 18:
    print("You are an adult")

Let’s break this down:

• if starts the conditional statement
• age >= 18 is a condition (a boolean expression)
• the colon : marks the start of a code block
• the indented line is executed only if the condition is True

A condition is an expression that evaluates to either True or False.

Boolean expressions

A boolean expression is any expression that results in a boolean value (True or False).

Examples:

x = 10

print(x > 5)   # True
print(x == 10) # True
print(x < 3)   # False

These expressions are commonly used inside if statements to control the flow of the program.

You can think of them as questions your program asks:

• Is x greater than 5?
• Is x equal to 10?

Depending on the answer, the program decides what to do next.

Using else

You can define an alternative path using else. The code block inside the else statement will only run if the initial condition isn't met.

age = 16

if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")

Using elif

Alternatively, if you need to check multiple conditions, you can use elif:

score = 85

if score >= 90:
    print("Excellent")
elif score >= 70:
    print("Good")
else:
    print("Needs improvement")

Comparison operators

Conditionals often use comparison operators:

• == equal to
• != not equal to
• > greater than
• < less than
• >= greater than or equal to
• <= less than or equal to

Logical operators

You can combine conditions using logical operators:

age = 20
has_id = True

if age >= 18 and has_id:
    print("Access granted")

• and – both conditions must be true
• or – at least one condition must be true
• not – reverses a condition

Example

temperature = 30

if temperature > 25:
    print("It is hot outside")
else:
    print("It is not very hot")

Exercise

Now let's practice! Write a program that:

• asks the user for a number
• prints whether the number is positive, negative, or zero
• if the number is positive, it should also print if it's even or odd

tip

To check if a number is even or add, you can evaluate the following expression: n % 2 == 0. If this expression is true, then the number is even! Otherwise, it's odd.

Notes

info

• Indentation is required in Python and defines code blocks
• Be careful to use == when comparing values, not =
• Keep conditions simple and readable