pascal - Call by Reference and Call by Value Result -

well, there debate on below code between me , friend. we're bit confused output produce. can clarify call-by-reference , call-by value result below piece of code?

program params;     var i: integer;     a: array[1..2] of integer;      procedure p(x,y: integer);     begin         x := x + 1;         := + 1;         y := y + 1;     end;  begin     a[1] := 1;     a[2] := 2;     := 1;     p( a[i],a[i] );     output( a[1],a[2] ); end. 

the resulting output of program in case parameters transmitted procedure p value-result , reference.

call value

x , y in p local variables initialized actual parameters, while i global variable, call p( a[i],a[i] ) equivalent to:

x := 1  /* value of a[i] */ y := 1  /* value of a[i] */  x := 2  /* x + 1 */ := 2  /* + 1 */ y := 2  /* y + 1 */ 

and @ end values 1, 2 printed since values of a[1], a[2] weren't changed.

call reference

both x , y in p alias a[1] , (again) a[1] (since i = 1 when procedure called), call equivalent to:

a[1] := 2  /* a[1] + 1 */    := 2  /* + 1 */ a[1] := 3  /* a[1] + 1 */ 

and @ end values 3, 2 printed.

call name

call name equivalent call reference when simple variables passed parameters, different when pass expression denotes memory location, subscript. in case actual parameter re-evaluated each time encountered. in case, effect of call of p( a[i],a[i] ):

a[1] := 2  /* since = 1, result equal a[1] + 1 */    := 2  /* + 1 */ a[2] := 3  /* since 2, result equal a[2] + 1 */ 

and @ end values 2, 3 printed. in practice implementation calls anonymous function (a “thunk”), each time must evaluate parameter.

call value result

just complete discussion, here case value-result parameter passing, in x , y initialized @ beginning of procedure execution values of actual parameters, and, @ end of execution of procedure, copied original variables addresses:

x := 1       /* value of a[i] */ y := 1       /* value of a[i] */  x := 2       /* x + 1 */ := 2       /* + 1 */ y := 2       /* y + 1 */  a[1] := 1    /* value of x copied a[1] */ a[1] := 2    /* value of y copied a[1] (not a[2]!) */ 

and @ end values 2, 2 printed.

for discussion of different ways of passing parameters, see instance this.