Selection in programmingUsing ELSE IF to provide more choices

In programming, selection is implemented using IF statements.

Using IF and ELSE gives two possible choices (paths) that a program can follow. However, sometimes more than two choices are wanted. To do this, the statement ELSE IF is used.

This simple algorithm prints out a different message depending on how old you are. Using IF, ELSE and ELSE IF, the steps are:

• Ask how old you are
• IF you are 70 or older, say “You are aged to perfection!”
• ELSE IF you are exactly 50, say “Wow, you are half a century old!”
• ELSE say “You are a spring chicken!”

As a flow diagram, the algorithm would look like this:

Python uses the statement elif, which stands for ‘ELSE IF.’

This flow diagram would be implemented in Python (3.x) as:

age = int(input("How old are you?"))
if age >= 70: print("You are aged to perfection!")
elif age == 50: print("Wow, you are half a century old!")
else: print("You are a spring chicken!")

The program examines the value of age. If the inputted age is 70 or over, the program prints a particular message. If the inputted age is exactly 50, the program prints a different message. Otherwise (else) it prints a third message. Using elif allowed us to include a third choice in the program.

We can keep on adding elif statements to give us as many choices as we need:

age = int(input("How old are you?"))
if age >= 70: print("You are aged to perfection!")
elif age == 50: print("Wow, you are half a century old!")
elif age >= 18: print("You are an adult.")
else: print("You are a spring chicken!")

When using an ‘if...else if’ statement, the program will stop checking as soon as it finds a positive answer. It is therefore very important to get the ‘if’ and ‘else’ conditions in the right order to make the program as efficient as possible.