Initializing an array

[stoffman]

I need to Initialize an array of unknown size with the index as the value of each element. The obvious solution is to use a loop:

Code: Pascal  [Select][+][-]

1. for i:=0 to length(arr)-1 do
2.    arr[i]:=i;

I wonder if there is a faster option?

Thanks

[ojz0r]

Not an answer to your question but use "for i := low(arr) to high(arr) do", then you wont run into trouble if the array doesnt start at 0.

Edit: On the other hand, a dynamic array always starts at 0 so in that case its known...Yet i think the syntax looks clearer

[BeniBela]

AVX would probably be the fastest way

[jamie]

you need an intrinsic binary increment fill function which I don't see in the list of functions in the system unit?

if you are doing this many times, then I would suggest you could map a reference array to its largest ever size and then use the MOVE function each time, whereas this operation uses a more efficient way to transfer memory.

 Also, I don't know how to write such a code with a inline assembler but it wouldn't be cross platform.

 using a String copy ASM feature to move the counter in as it goes alone.

 I guess it depends on how serious you are on speed.

[MathMan]

I need to Initialize an array of unknown size with the index as the value of each element. The obvious solution is to use a loop:

Code: Pascal  [Select][+][-]

1. for i:=0 to length(arr)-1 do
2.    arr[i]:=i;

I wonder if there is a faster option?

Thanks

The fastest way would be to not do it at all.

To be less obtuse - if you have a fixed line of modifications you apply to the array afterwards then simply integrate the setting into the first of these calculations. So, instead of

Code: Pascal  [Select][+][-]

1. for i:=Low(arr) to High(arr) do
2.    arr[i]:=i;
3. ModifyStep1(arr);
4. ModifyStep2(arr);
5.  

you change 'ModifyStep1' in a way that it integrates arr:=i. You can play around with this approach - maybe you need two variants of ModifyStep1, one integrating the setting, one without, etc. Can't comment more specific here, as I don't know your specific requirements.

Cheers,
MathMan

[jamie]

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. Label One, Two;
3. Var
4.   A:Array of Integer;
5.   P:PInteger;
6.   C,M:Integer;
7. begin
8.   SetLength(A, 1000);
9.   // Start of  incremental fill code.
10.   M:= Length(A);
11.   C := 0;
12.   P := @A[0];
13. One:      //loop here.
14.   Inc(C);
15.   IF C >= M Then
16.     Goto two;
17.   P^:=C;
18.   Inc(P);
19.   Goto One;
20. Two:
21. end;                
22.  

If you look at the ASM code on that loop, its about as close as it gets without going into ASM.