Interesting way to copy dynamic arrays.

[pyscripter 586]

I am sure this is well known but I recently discovered a new way to copy dynamic arrays:

var
  A, B : TArray<Integer>;
...
  B = A;
  SetLength(B, Length(B));
...

Test procedure:

procedure Test();
var
  A, B : TArray<Integer>;
begin
  A := [1 , 2, 3];
  B := A;
  SetLength(B, Length(B));
  B[1] := 12;

  Assert(A[1] = 2);
  Assert(B[1] = 12);
end;

Of course Copy is simpler, but in my use case I did not want to create a copy unless it was necessary e.g.

In one part of the code

//  create A

SetLength(A, L);

...

 

In another part of the code

B = A

// do stuff with B or store it as an object field

 

If A is not recreated you save the copying operation.  Not a big deal...

 

[corneliusdavid 143]

47 minutes ago, pyscripter said:

SetLength(B, Length(B));

Wouldn't you want to set B's length to Length(A)?

[Anders Melander 1644]

5 minutes ago, corneliusdavid said:

Wouldn't you want to set B's length to Length(A)?

B := A makes them the same array. SetLength just makes B unique (and contains an implicit copy).

 

I don't get what advantage assignment followed by SetLength gives you over a simple Copy. With Copy the intent is explicit.

[David Heffernan 2303]

14 minutes ago, corneliusdavid said:

Wouldn't you want to set B's length to Length(A)?

Why would that be different? 

[pyscripter 586]

1 minute ago, Anders Melander said:

I don't get what advantage assignment followed by SetLength gives you over a simple Copy.

In the use case I described you avoid copying until (if) it is needed.

 

2 minutes ago, Anders Melander said:

With Copy the intent is explicit.

Agree.

[corneliusdavid 143]

Just now, David Heffernan said:

Why would that be different?

That's just as good a question. So either the assignment statement simply copies all elements it knows about in array A without setting the length of B (which seems like an incomplete copy) or setting the length is not needed.

[Uwe Raabe 1891]

The assignment just sets the pointer and increments the reference count.

[Anders Melander 1644]

Just now, pyscripter said:

In the use case I described you avoid copying until (if) it is needed.

I still don't get it.

Just start with the assignment and use B:= Copy(B) when you need it to be unique....  Ah, I think I get it now. You don't want to keep track of the arrays reference count.

So it's like UniqueString for dynamic arrays? Does nothing if the reference count is already 1, otherwise makes a unique copy. Right?

[Anders Melander 1644]

5 minutes ago, corneliusdavid said:

So either the assignment statement simply copies all elements it knows about in array A without setting the length of B (which seems like an incomplete copy) or setting the length is not needed.

Read what I wrote again.

[corneliusdavid 143]

I just tested this in Delphi 11:

program ArrayCopy;
{$APPTYPE CONSOLE}
{$R *.res}
uses
  System.SysUtils;

procedure ShowArray(AnArray: TArray<Integer>);
begin
  Writeln('Length: ' + Length(AnArray).ToString);
  for var i := 0 to Length(AnArray) - 1 do
    Writeln('[' + i.ToString + '] = ' + AnArray[i].ToString);
end;

begin
  var a, b: TArray<Integer>;

  a := [1, 2, 3];
  ShowArray(a);

  b := a;
  b[1] := 12;
  ShowArray(b);

  Readln;
end.

And the output shows SetLength() is not necessary:

[pyscripter 586]

9 minutes ago, Anders Melander said:

Just start with the assignment and use B:= Copy(B) when you need it to be unique....  Ah, I think I get it now. You don't want to keep track of the arrays reference count.

So it's like UniqueString for dynamic arrays? Does nothing if the reference count is already 1, otherwise makes a unique copy. Right?

B=A does not cost much

B = Copy(A, 0)  moves memory and has a cost.

 

In my use case A may be recreated (maybe not).  If it is not recreated then I save the moving of memory.  If it is recreated B becomes a unique copy after SetLength(A,...)

Edited December 6, 2021 by pyscripter

[corneliusdavid 143]

1 minute ago, pyscripter said:

If it is recreated B becomes unique after SetLength(A,...)

Isn't B already unique? See the output of my test program--those are distinct arrays.

[pyscripter 586]

1 minute ago, corneliusdavid said:

just tested this in Delphi 11:

Try to output a again at the end of your routine.

[corneliusdavid 143]

Ah! Yes, I see it now!  Calling SetLength() is necessary to make it unique!

[Anders Melander 1644]

4 minutes ago, pyscripter said:

B=A does not cost much

B = Copy(A, 0)  moves memory and has a cost.

 

In my use case A may be recreated (maybe not).  If it is not recreated then I save the moving of memory.  If it is recreated B becomes a unique copy after SetLength(A,...)

A simple "yes" would have been enough 🙂

[Rollo62 502]

I am just considering what could be the use cases.

 

How would it behave if you put B into a const parameter, passing it into a procedure.

Is the SetLength( AParamOfB, SizeOf( AParamOfB ); behaviour then still be maintained ?

 

This could maybe make sense, to have several different functions, while some need a unique copy, some don't.

 

Edited December 7, 2021 by Rollo62

[Attila Kovacs 617]

I'm not sure that is well known or frequently needed, I think I saw it first here (maybe, dunno):

https://stackoverflow.com/questions/19466038/finding-common-elements-in-two-arrays/19524788#19524788

 

 

[David Heffernan 2303]

What would be nice would be if Emba could add a MakeUnique(arr) so that you could write code like with with clear intent

[Anders Melander 1644]

There's DynArrayUnique but it's undocumented and not really fit for direct use:

 

procedure DynArrayUnique(var A: Pointer; typeInfo: Pointer);
begin
  if (A <> nil) and (PDynArrayRec(PByte(A) - SizeOf(TDynArrayRec))^.RefCnt > 1) then
    DynArrayCopy(A, A, typeInfo);
end;

 

[Anders Melander 1644]

2 hours ago, Attila Kovacs said:

I'm not sure that is well known

It's in the documentation... So probably not 🙂