Procedures

A PROCEDURE is a named block

• A procedure is a named set of steps you run with CALL.
• Define it once, then call it as many times as you like.

PROCEDURE Greet
    OUTPUT "Hello!"
ENDPROCEDURE

CALL Greet
CALL Greet          // → Hello! (twice)

Parameters pass data in

• Put a parameter in brackets to send a value in.

PROCEDURE Greet(Name : STRING)
    OUTPUT "Hi, ", Name
ENDPROCEDURE

CALL Greet("Ada")   // → Hi, Ada

A parameter is a copy

• A parameter is a copy of the value you passed in.
• Changing it inside the procedure does not change the caller's variable.
• To get an answer back, use a FUNCTION and RETURN it (next lesson).

FUNCTION Double(N : INTEGER) RETURNS INTEGER
    RETURN N * 2
ENDFUNCTION

DECLARE X : INTEGER
X ← 5
OUTPUT Double(X)    // → 10

Local and global

• A variable declared in the main program is global. A procedure can read it and change it.
• A variable declared inside the procedure is local. It hides a global of the same name, and it disappears when the call ends.
• A parameter is local too. It is a copy, so changing the parameter does not change the caller's variable.

DECLARE Total : INTEGER
Total ← 10
PROCEDURE ShowLocal
    DECLARE Total : INTEGER
    Total ← 3
    OUTPUT Total            // → 3, the local one
ENDPROCEDURE
CALL ShowLocal
OUTPUT Total                // → 10, the global one

Watch out

• You must write CALL before a procedure's name — but never before a function's.
• A procedure cannot hand a value back; that is what a FUNCTION is for.
• (BYREF makes a parameter the real variable rather than a copy. It is in the AS 9618 guide, not in the IGCSE 0478 syllabus for 2026–2028, so do not use it in Paper 2.)

Common mistakes

• PROCEDURE Name(...) ... ENDPROCEDURE; run it with CALL name(...).
• A procedure does a job but returns no value.

Now you try

• Define a procedure, then CALL it.
• Press Check answer to test it.