Programming concepts - AQAProcedures and functions of subroutines

There are two types of subroutine:

• procedures
• functions

Procedures

A procedure is a subroutine that performs a specific task. When the task is complete, the subroutine ends and the main program continues from where it left off. For example, a procedure may be written to reset all the values of an array to zero, or to clear a screen.

A procedure is created using the following pseudo-code syntax:

SUBROUTINE identifier (parameters) procedure code
ENDSUBROUTINE

This procedure would clear the screen by printing x blank lines:

SUBROUTINE clear_screen(lines) FOR count <- 1 TO lines OUTPUT " " ENDFOR
ENDSUBROUTINE

A procedure is run by calling it. To call it, a programmer uses the procedure name and includes any parameters (values) that the procedure needs, for example:

clear_screen(5)

This would print five blank lines on the screen.

Functions

A function works in the same way as a procedure, except that it manipulates data and returns a result back to the main program.

For example, a function might be written to turn Fahrenheit into Celsius:

SUBROUTINE f_TO_c(temperature_in_f) temperature_in_c ← (temperature_in_f –32) * 5/9 RETURN temperature_in_c
ENDSUBROUTINE

A function is run by calling it. To call it, a programmer uses the function's identifier, the value to be passed into the function, and a variable for the function to return a value into, for example:

celsius ← f_to_c(32)

This would result in the value of Celsius being zero.

Note: AQA pseudo-code does not use a different keyword to show the difference between a procedure and a function. Instead, it adds a RETURN statement to show that a subroutine is a function.