3187:Passing Multidimensional Arrays as Parameters
KEYWORDS: Array, multi-dimensional, type, function, Pascal, parameter AREA: Ap
Passing an array to a procedure or function is straight
forward and behaves as expected. However, passing a
multi-dimensional array to a function or procedure is
not handled in the same way. Consider MyArray to be
defined as:
var
MyArray: array[1..3, 1..5] of double;
And you want to pass it to a procedure called
DoSomeThing() defined as:
procedure DoSomeThing(MyArray: array of double);
begin
showmessage( floattostr( MyArray[1 , 1]));
end;
One might think a simple statement like
DoSomeThing(MyArray); would do the trick.
Unfortunately, this is not the case.
The statement DoSomeThing(MyArray); will not compile.
The compiler sees the two data structures involved
as different types - so it will not allow the
statement. The DoSomeThing() procedure is
expecting an array of doubles, but the example is
passing a multi-dimensional array.
Delphi handles multi-dimensional arrays as user
defined type, so there is no syntax to tell a procedure
that its parameter(s) are multi-dimensional arrays -
without declaring a type. Creating a type, and using
this type as the parameter is the correct method to pass
a multi-dimensional array. We could just pass a pointer
to the array, but inside the function, we need to typecast
that pointer. What type to cast it as is the next issue.
You would have to have the type defined, or declared
identically in 2 places. This method just doesn't make
sense.
By defining a type, we can change the process to this:
type
TMyArray = array[1..3, 1..5] of double;
var
MyArray : TMyArray;
procedure DoSomeThing(MyArray: TMyArray);
begin
showmessage( floattostr( MyArray[1 , 1]));
end;
Now the actual call looks as we expected:
DoSomeThing(MyArray);
If you want to use the method of passing a pointer, your
function might look like this:
type
PMyArray = ^TMyArray;
TMyArray = array[1..3, 1..5] of double;
var
MyArray : TMyArray;
procedure DoSomeThing(MyArray: PMyArray);
begin
showmessage ( floattostr( (MyArray[2,3]) ));
end;
Note, under 32 bit version Delphi, you do not need to
dereference the MyArray variable inside DoSomeThing().
Under older versions you might have to refer to
MyArray as MyArray^.
If you want to pass just a generic pointer, you may not
be able to use it directly. You can declare a local
variable, and use it. Again, you may need to cast the
local variable for older versions of PASCAL. Of course
this method does offer more flexibility in data usage.
procedure DoSomeThing(MyArray: pointer);
var
t : ^TMyArray;
begin
t := MyArray;
ShowMessage ( FloatToStr( t[2,3]) );
end;
Regardless, both calls that use a pointer method, will
look something like:
MyArray[2, 3] := 5.6;
DoSomeThing(@MyArray);
TI
LiveJournal
PhotoFile