fillchar for packed array of bit: strange result

[winni]

Hi!

Given is

Code: Pascal  [Select][+][-]

1. TYPE
2. TBit = 0..1;
3. TBigSet =   packed array of TBit;      
4.  
5. VAR
6. BS  : TBigSet;
7. ....
8. SetLength(BS,256);
9. writeln (sizeOf (BS))
10.  

The result is 8 - as exspected.

But now it's getting strange:

Code: Pascal  [Select][+][-]

1. fillchar(BS[0],sizeof(BS),1);
2.  

This fills 8 bits and not 8 byte as it should!

Is this a bug or am I thinking wrong?

I am curious.

Winni

[jamie]

Shouldn't you be using LENGTH and not SizeOf which is a compile time object size, more like a pointer to the object..

if you did this in 64bit then I guess it would explain a little.

Just my observations..

[Kays]

Code: Pascal  [Select][+][-]

1. fillchar(BS[0],sizeof(BS),1);

BS is a dynamic array. sizeOf a dynamic array variable returns the size of a pointer, not the dynamically allocated array data. You actually wanted to write

Code: Pascal  [Select][+][-]

1. fillChar(BS[0], length(BS) * sizeOf(BS[0]), 1);

but then I am wondering why you aren’t simply using a for loop and a plain := assignment.

[winni]

Thanx Kays!

II did not know the pointer stuff. Now it's clear.

but then I am wondering why you aren’t simply using a for loop and a plain := assignment.

Because I use fillchar since UCSD times to speed up initialization .....

btw.: Are elements of a  dynamic array initialized with zeros or not?
Hard to find an answer in the docs.

Winni

[howardpc]

btw.: Are elements of a  dynamic array initialized with zeros or not?
Hard to find an answer in the docs.

AFAIK all such compiler-managed types are initialised with zeros.

[winni]

btw.: Are elements of a  dynamic array initialized with zeros or not?
Hard to find an answer in the docs.

AFAIK all such compiler-managed types are initialised with zeros.

Thanx.

Confirmed with test:

For i := 1 to 1 000 000 do
* setLength(BS,1024*1024);
* check if only zeros
* fill with ones
* setLength to zero

Not one error. It seems that the initialization fills with zeros.

Winni
Set

[PascalDragon]

A few notes:

• packed is ignored for arrays (no matter if dynamic or not)
• bitpacked (what you probably wanted) is not allowed for dynamic arrays
• As Kays wrote, SizeOf returns the size of the pointer to the dynamic array while Length returns the number of elements

In essence the data of your TBigSet will always be a multiple of a Byte. For really accessing bits in the array you need to do this manually using bit operators.

[winni]

A few notes:

...

• bitpacked (what you probably wanted) is not allowed for dynamic arrays
...

In essence the data of your TBigSet will always be a multiple of a Byte. For really accessing bits in the array you need to do this manually using bit operators.

Hi

Thanx  for this info!

So you have to fool the compiler:

Code: Pascal  [Select][+][-]

1. TYPE
2.    TBit = 0..1;
3.    Tpack = Record case boolean of
4.             true : (D: Dword);
5.             false: (b: bitPacked array[0..31] of TBit);
6.             end;
7.    TBigset = Array of Tpack;
8.    ....
9.   procedure conv (v: DWord; var d: dword; var b : byte); inline;
10.   var tmp: dword;
11.   begin
12.   divmod(v, 32,d,tmp);
13.   b := tmp;
14.   end;
15.  
16.  
17.   procedure ChangeBit (var BS : TBigset; bit: dword; BitVal : TBit);
18.   var d: dword;
19.       b : byte;
20.   begin
21.    conv(bit,d,b);
22.    BS[d].b[b] := BitVal;
23.    end;
24.  
25.  
26.  

   

A little bit more complicated but it works.

Winni

[PascalDragon]

So you have to fool the compiler:

I wouldn't say that this is fooling the compiler as you're simply using language features the way they're supposed to be used... 

[winni]

I wouldn't say that this is fooling the compiler as you're simply using language features the way they're supposed to be used... 

Hi

I just wanted to remark that it is not very comfortable that bitpacked is not allowed for dynamic arrays.

Winni

[jamie]

I believe that is what Tbits class is for. Although I find it a little overboard with how it's coded. I use something of my own instead.