Procedures and BYREF

CALL runs a procedure

• A procedure is a named block. It does a job and returns no value.
• Define it with PROCEDURE … ENDPROCEDURE, and run it with CALL.
• Parentheses are written even when there are no parameters.

PROCEDURE Greet()
  OUTPUT "Hi"
ENDPROCEDURE
CALL Greet()

By value is the default

• If no mode has been specified, parameters start by value: the procedure gets a copy.
• Changing the copy does not change the caller's variable.
• BYVAL explicitly selects a copy. At the start of a parameter list it is the default; after BYREF, write BYVAL to switch back.

PROCEDURE Show(Number : INTEGER)
  Number ← Number + 1
  OUTPUT Number
ENDPROCEDURE
DECLARE FirstNumber : INTEGER
FirstNumber ← 5
CALL Show(FirstNumber)
OUTPUT FirstNumber

BYREF passes the real variable

• BYREF means the parameter is the caller's variable. A change is still there after CALL.
• A mode keyword continues across later parameters until another mode keyword changes it. BYREF FirstValue : INTEGER, SecondValue : INTEGER makes both parameters references.
• Do not pass a parameter to a function by reference. Use BYREF on a procedure.

PROCEDURE Swap(BYREF FirstValue : INTEGER, BYREF SecondValue : INTEGER)
  DECLARE Temp : INTEGER
  Temp ← FirstValue
  FirstValue ← SecondValue
  SecondValue ← Temp
ENDPROCEDURE
DECLARE FirstNumber : INTEGER
DECLARE SecondNumber : INTEGER
FirstNumber ← 1
SecondNumber ← 2
CALL Swap(FirstNumber, SecondNumber)
OUTPUT FirstNumber, ",", SecondNumber

Common mistakes

• CALL before a procedure. Never CALL a function.
• IGCSE 0478 has no BYREF. On that paper every parameter is a copy.

Now you try

• Write a procedure that swaps two integers.

Independent transfer

The final task has no supplied algorithm. Plan its inputs, processing and outputs, then solve it before opening support. Check more than the sample case.