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3.1. Basic Conditionals

Every program needs to make some decisions during its executions. These decisions are typically made on the basis of the user input. To make such decisions, we use conditional statements.

In this article, we'll understand the basic if and else statements used for making decisions on the basis of logical expressions (conditions).

if statement

Let us understand this with an example:

age = int(input('Enter your age: '))

if age >= 18:
    print('You are over 18.')
    print('You can drive.')

Enter your age: 23
You are over 18.
You can drive.

Enter your age: 15

As you can see, when we give a number greater than or equal to 18, the program produces an output showing that the user can drive.

age > 18 is a logical expression and when it evaluates to True, the indented lines are executed. Logical expressions are covered in detail in a later topic.

else statement

However, if the number is less than 18, the program simply exits without any message which seems a little odd. To fix this, we can include an else block:

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age = int(input('Enter your age: '))

if age >= 18:
    print('You are over 18.')
    print('You can drive.')
else:
    print('You are under 18.')
    print('You cannot drive yet.')

Enter your age: 23
You are over 18.
You can drive.

Enter your age: 15
You are under 18.
You cannot drive yet.

The else block is executed when the if is not executed.

Indentation

The lines after if statement are indented (leading spaces before the line of code) to indicate that they are executed only when the if condition is satisfied.

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age = int(input('Enter your age: '))

if age >= 18:
    print('You are over 18.') # (1)!
    print('You can drive.')
else:
    print('You are under 18.')
    print('You cannot drive yet.')
  1. Highlighted lines indicate the code block executed when expression age >= 18 evaluates to True.

Similarly, the lines after else statement are indented to indicate that they are executed only when else is reached (i.e. when condition is not satisfied.)

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age = int(input('Enter your age: '))

if age >= 18:
    print('You are over 18.')
    print('You can drive.')
else:
    print('You are under 18.') # (1)!
    print('You cannot drive yet.')
  1. Highlighted lines indicate the code block executed when expression age >= 18 evaluates to False.

In Python, the indentation (i.e. the leading spaces before the code) in both clause bodies is significant. The next page discusses the concept of indentation in detail.

The concept of conditionals is continued in 3.4. Conditionals (Continued)