Selection

Simple

Selection makes use of a comparison operator to decide what to do.

Example 1 - if

Code is only run if the comparison is True.

# Initialise variable
age = 21

# Check value
if age >= 18:

    # True: display message
    print("You can go to the pub.")

Example 2 - if, else

One section of code is always run.

# Initialise variable
age = 16

# check value
if age >= 18:

    # True: display message
    print("You can go to the pub.")

# Otherwise
else:

    # False: display message
    print("You can't go to the pub.")

Example 3 - Nested if

Only the section of code for the first comparison that is True is run. There can be multiple if statements, and the else is optional.

Numerical values are compared in order: largest to smallest (see below), or smallest to largest.

# Initialise variable
score = 53

# Check value
if score >= 80:

    # True: display message
    print("Excellent score!")

else:

  # Nested if

    # Check value
    if score >= 50:

        # True: display message
        print("Well done!")

    # Otherwise
    else:

        # All False: display message
        print("Oh dear!")

Example 4 - if, elif, else

Only the section of code for the first comparison that is True is run. There can be multiple elif statements, and the else is optional.

Numerical values are compared in order: largest to smallest (see below), or smallest to largest.

# Initialise variable
score = 53

# Check value
if score < 50:

    # True: display message
    print("Oh dear!")

# Check value
elif score < 80:

    # True: display message
    print("Well done!")

# Otherwise
else:

    # All False: display message
    print("Excellent score!")