Programming – Pascal: Essential skills 6

 Programming – Pascal: Essential skills 6

 

Learning Outcomes:

• Implement parameters passing in manipulating sub-programs.
• The sub-programs are called by two parameters passing methods: call by value and call by reference.

  

Call-by-value/Call-by-reference

 

When using procedure or function, parameters may be passed into the called procedure / function. The original parameters may or may not be affected depending on call-by-value or call-by-reference.

 

Call-by-value:

var                 x, y : integer;      (* global variables *)                 procedure add (a, b : integer);    (* a and b have their own memory address *) begin                 a := a + 1;                 b := b + 1;                 write (a+b) end;   begin                 x := 1;                 y := 2;                 add (x, y);            (* the use of this procedure does NOT affect the  value of x and y *)                 write (x + y) end.

Output: 53

 

Call-by-reference:

(the only difference from the program above is the var in the procedure heading)

var                 x, y : integer;      (* global variables *)                 procedure add (var a, b : integer);             (* a and b uses the memory address of x and y *) begin                 a := a + 1;             (* x is now 2 *)                 b := b + 1;            (* y is now 3 *)                 write (a+b) end;   begin                 x := 1;                 y := 2;                 add (x, y);            (* the use of this procedure AFFECTS the values of x and y *)                 write (x + y) end.

Output: 55

 

Procedure and function

While a procedure is a subprogram which is quite similar to a usual program (see above), a function always returns a single result.

 

Procedure:

procedure add (a, b : integer); …

 

Function:

function add (a, b : integer) : integer; …

Having the extra part (: integer), it means this function is going to return an integer when called.

 

var                 x, y, sum : integer;   function add (a, b : integer) : integer; begin                 add := a + b end;   begin                 x := 1;                 y := 2;                 sum := add (x, y);              (* similar to using a built-in function *)                 write (sum) end.

Output: 3

 

You can use call-by value or call-by-reference when using functions depending on the purposes of the function / program.

 

Relevant past paper:

DSE ICT Elect B(SP-2017): SP 3d. PP 3a-d. 2012 3ac, 4ai.

CE CIT Elect A(2005-2011): 2009 4bii,iii.