Consider the following program:

program test;
var
  a: array [1..3] of Integer;
  i: Integer;
begin
  for i := 1 to 4 do
    a[i] := i;
end.

The compiler will accept this code happily, despite the fact that there's an out of bounds array index when i = 4.

Since the behavior of for loop in FPC (at least in non Delphi / TP mode) is deterministic, it should be possible to check whether the index is out of bounds or not. Isn't it? Or is there any situation where it's not?

On Mon, 25 Oct 2010 01:06:16 -0700 (PDT), leledumbo wrote about [fpc-pascal] Range checking in a for loop:

[snip]

There is no reason in the for-loop construct that "i" cannot have the value 4; it is only a problem when "i" is used as a subscript on the array "a".

Try declaring

   i : 1..3;

instead, as that range matches the array's bounds. You should then get a range check at compile time. It is also the idiomatic Pascal way of doing this.

a[i] is an expression. You can write a[(i+j)/n] there and compiler will fail to calculate whether the expression match the bound in this case.

Moreover, to create a dynamic array you usually use ^a[0..0] type and it would be sad if compiler rejected any index 0 for this case.

Anyway, if you want to be sure you are in array bounds just use Low(a) as the lower bound and High(a) as the high bound.

Best regards,
Paul Ishenin.

With current released versions, you won't get such a compile time error. I fixed that just yesterday in svn though (r16213).

Jonas

... if range checking is disabled (then you'll only get a warning). When range checking is enabled, you do get an error already.

I fixed that just yesterday in svn though (r16213).

and now you get an error under all circumstances (common sense and Delphi-compatible).

Jonas

OK, thanks. I just want to know why such a thing cannot be detected at compile time. So, a good typing practice is the solution, right? Like this:

program test;
type
  TArrayRange = 1..3;
  TArray = array[TArrayRange] of Integer;
var
  a: TArray;
  i: TArrayRange;
begin
  for i := 1 to 4 do
    a[i] := i;
end.