Procedures, functions and maintainable programs

Computer Science · Programming. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A subroutine is a named block of code that does one job. You write it once and call it whenever it is needed. A procedure carries out a task and does not return a value. A function carries out a task and always returns one value to the place where it was called.

A parameter is a value passed into a subroutine when it is called. A local variable is declared inside a subroutine and exists only there. A global variable is declared in the main program and can be used anywhere.

Library routines are ready-made functions. A maintainable program is one that another programmer can read, understand and change easily.

Rules to remember

PROCEDURE DrawLine(Size : INTEGER) ... ENDPROCEDURE. Call it with CALL DrawLine(5).
FUNCTION Area(L : INTEGER, W : INTEGER) RETURNS INTEGER ... RETURN L * W ... ENDFUNCTION. Use it in an expression: X ← Area(5, 3).
Local variable: declared inside a subroutine, usable only there. Global variable: declared outside all subroutines, usable anywhere in the program.
MOD(x, y) gives the remainder and DIV(x, y) gives the whole-number quotient: MOD(14, 4) = 2, DIV(14, 4) = 3.
ROUND(x, places) rounds x to the given number of decimal places: ROUND(6.283, 2) = 6.28.
RANDOM() returns a random number between 0 and 1 inclusive.
Maintainable program: meaningful identifiers, comments, procedures and functions, and consistent indentation.

Common mistake

A function is not called with CALL. Its returned value must be used, for example assigned to a variable or output. CALL is only for procedures.

Worked example

What is output by this program?
FUNCTION Area(L : INTEGER, W : INTEGER) RETURNS INTEGER
RETURN L * W
ENDFUNCTION

OUTPUT Area(3, 4) − Area(2, 5)

Area(3, 4): L = 3 and W = 4, so it returns 3 * 4 = 12.
Area(2, 5): L = 2 and W = 5, so it returns 2 * 5 = 10.
The two returned values are subtracted: 12 − 10 = 2.

Answer: 2

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