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Subroutines are small blocks of code in a modular program designed to perform a particular task. Since a subroutine is in itself a small program, it can contain any of the sequence, selection and iteration constructs.

In the following example, the algorithm asks the user to enter two numbers. It then adds them together and, if the total is over 10, it runs the ‘CountDown’ subroutine. If the total is not over 10, it will run the ‘CountUp’ subroutine.

Here is the same algorithm written using pseudocode:

declare CountDown
                        while total <> 0
                            output total
                            total = total – 1
                        repeat
                    end Subroutine
                    declare CountUp
                    i is integer
                        set i = 0
                        while i <> total
                            output i
                            i = i + 1
                        repeat
                    end Subroutine

                    num1 is integer
                    num2 is integer
                    total is integer
                    input “Enter first number”, num1
                    input “Enter second number”, num2
                    total = num1 + num2
                    if total > 10
                        call CountDown
                    else
                        call CountUp
                    end if