How to copy array?

Question

I have such a basic problem in Delphi,I can't solve it.

My Code:

Note:DataR is local in the methods below,but usually it's a class var.Just for the concept it's local.

class procedure TCelebrity.BeginRead(var input:Array of byte);
var DataR:Array of byte;
begin
  VirtualFree(@DataRead,High(DataRead),MEM_RELEASE);
  SetLength(DataR,Length(input));
  Move(input,DataR,Length(input));
end;

This compiles,but after Move() is executed DataR = nil.

Second try:

class procedure TCelebrity.BeginRead(var input:Array of byte);
var DataR:Array of byte;
begin
  VirtualFree(@DataRead,High(DataRead),MEM_RELEASE);
  SetLength(DataR,Length(input));
  DataR := Copy(input,0,Length(input));
end;

This doesn't compile at all.Error at the third line(DataR := Copy(input....) saying "Incompatible types".

Where's the problem? They are all Array of byte!

Possible duplicate of Is there way of copying the whole array into another array? (Other than using a For-loop) — JensG
– JensG, Commented Jan 24, 2019 at 12:55

test · Accepted Answer · 2009-07-13 13:29:19Z

9

try this

type
  TByteDynArray = array of Byte;

function CopyData(const Input:array of Byte):TByteDynArray;
begin
  SetLength(Result, Length(Input));
  Move(input[0], Result[0], Length(Input));
end;

answered Jul 13, 2009 at 13:29

test

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4 Comments

Rob Kennedy Over a year ago

A correct answer. but to make it a good answer, you should explain why your changes are significant. Otherwise, it's just a magical incantation.

Richard A Over a year ago

Correct me if I'm wrong, but it might be worth noting that this only works as written for array of Byte. If the array was anything different, say array of Integer, then you'd need Move(input[0], Result[0], Length(Input) * SizeOf(Integer)). Just saying.

dns Over a year ago

@Rob Kennedy: Move() in delphi is similar to memcpy()/memmove() in C

Rob Kennedy Over a year ago

I know that, @Dns. So what?

Argalatyr · Accepted Answer · 2009-07-29 01:15:18Z

8

Why not use FOR?

SetLength(DataR,Length(input));
for i:=Low(input) to High(input) do
  DataR[i]:=input[i];

BTW: if you want to have arrays passing as parameter, you should declare them as a type, eg:

type
  TMyArray = array of byte;

and use TMyArray as parameters type.

Edit: I was notified about i lower value. In my original post it was for i:=0, but i:=Low(input) is safer and more pure.

edited Jul 29, 2009 at 1:15

Argalatyr

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answered Jul 13, 2009 at 12:44

smok1

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5 Comments

Ivan Prodanov Over a year ago

I have one question.The VirtualFree always returns false so how do I free the mem currently used with DataR? Note DataR is not local,but class var in the original code.Thank you in advance for the answer!

smok1 Over a year ago

No, you do not have free simple arrays. Such arrays are freed automatically (for example when your class is being destroyed). Note: if you have an array of references to objects, you have to free those objects manually, because only array will be freed by Delphi.

smok1 Over a year ago

Also note, that SetLenght reallocates the whole array, so avoid intensive usage of this procedure. When you make the array longer with SetLength, the current content remains intact, however, when you make it shorter, some of the content will be lost.

Richard A Over a year ago

In answer to 'Why not use FOR?'. My only reason would be if there was a real performance problem. If you were doing this a serious number of times, then Move would be faster, I reckon.

dns Over a year ago

use Move() it's similar to memcpy()/memmove() in C. Never use FOR to copy array

Roee Adler · Accepted Answer · 2009-07-13 12:53:26Z

2

Try:

class procedure TCelebrity.BeginRead(var input:Array of byte);
var DataR:Array of byte;
begin
  VirtualFree(@DataRead,High(DataRead),MEM_RELEASE);
  SetLength(DataR,Length(input));
  Move(input[0],DataR,Length(input));
end;

answered Jul 13, 2009 at 12:53

Roee Adler

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1 Comment

JensG Over a year ago

The VirtualFree() call seems a bit superfluous.

Alexander · Accepted Answer · 2012-11-21 01:54:22Z

1

Move procedure will not move a section of memory. It copies Count bytes. Hereby you will get two different identical arrays: Input and DataR.

procedure CopyData(const Input: Array of Byte);
var 
    DataR: Array of Byte;
begin
    SetLength(DataR, Length(Input));
    Move(Input[0], DataR[0], SizeOf(Byte)*Length(Input));
end;

P.s. with static arrays you can use SizeOf(Input) instead of SizeOf(Byte)*Length(Input). Instead of Byte could be other data type.

edited Nov 21, 2012 at 1:54

answered Nov 20, 2012 at 21:03

Alexander

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1 Comment

TmTron Over a year ago

why not use SizeOf(Input[0]) instead of a hardcoded data-type?

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