Florian - 6 months ago 81

Pascal Question

I have to build a prime factorization program with pascal using a function or procedure.

I think I am on a pretty good path, but my problem at the moment is, that it seems to be impossible to assign a dynamic relay as the ouput of a function/procedure. And I have no real clue what I could use or do instead (except a string, but that feels just not right at all).

`PROCEDURE PrimFac(n: INTEGER; VAR res: array of INTEGER);`

VAR

divisor, a, counter: INTEGER;

b: array of INTEGER;

BEGIN

divisor := 2;

a := n;

counter := 0;

WHILE divisor <= n DIV 2 DO BEGIN

IF a MOD divisor = 0 THEN BEGIN

a := a DIV divisor;

counter := counter + 1;

SetLength(b, counter);

b[counter] := divisor;

END

ELSE

divisor := divisor + 1;

END;

res := b

END;

BEGIN

WriteLn(PrimFac(210));

END.

Any help or hint would be highly appreciated. (:

Thank you very much in advance

-Florian

Answer

I see this is FreePascal, which is quite similar to Delphi.

Instead of using an *open array parameter* (which should not be confused with a *dynamic array*, despite the similar syntax), you should pre-define a type to return:

```
type
TIntegerDynArray = array of Integer;
function PrimFac(n: Integer): TIntegerDynArray;
...
SetLength(Result, counter);
Result[counter - 1] := divisor;
...
```

FWIW, reallocating a dynamic array each time you want to add an element is generally not a good idea. Better to keep the results in a temporary `TList`

(if possible, a generic `TList`

) and then, at the end, to turn that into an array of the desired length, and then to get rid of the temporary list again, IOW something like (untested):

```
uses
fgl;
type
TIntegerDynArray = array of Integer;
TIntegerList = specialize TFPGList<Integer>;
function PrimFac(N: Integer): TIntegerDynArray;
var
Divisor, A, I: Integer;
L: TIntegerList;
begin
A := N;
L := TIntegerList.Create;
try
{ Find divisors and add each to list. }
for Divisor := 2 to N div 2 do
begin
{ Use "while" so a divisor can be found multiple times, e.g. }
{ using "if": 1000 --> 2 4 5 25 (but 4 = 2*2, and 25 = 5*5) }
{ using "while": 1000 --> 2 2 2 5 5 5 }
while A mod Divisor = 0 do
begin
A := A div Divisor;
L.Add(Divisor);
end;
end;
{ Copy list to result array. }
SetLength(Result, L.Count);
for I := 0 to L.Count - 1 do
begin
Result[I] := L[I];
end;
finally
L.Free;
end;
end;
```

Note that your algorithm could do with a few extra checks (for 0, for 1, etc.), but that is up to you. I merely answered how to return the values.

If you want to print the list, then do something like:

```
{ Find divisors and print each one. }
for Divisor := 2 to N div 2 do
begin
while A mod Divisor = 0 do
begin
A := A div Divisor;
L.Add(Divisor);
Write(Divisor, ' ');
end;
end;
Writeln;
```

This lists all numbers separated by spaces and performs a final newline when it is finished. If you want more sophisticated output, use your imagination.