Cant figure it out.

This is a discussion on Cant figure it out. within the C++ Programming forums, part of the General Programming Boards category; Is there a fast way to set 2 unsigned char arrays equal to eachother? Right now im just running a ...

  1. #1
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    Cant figure it out.

    Is there a fast way to set 2 unsigned char arrays equal to eachother?

    Right now im just running a for loop which sets x[i] = y[i] but is there a way to do


    unsigned char Xarray[4];
    unsigned char Yarray[4];

    Xarray = Yarray;

    ?

  2. #2
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    Quote Originally Posted by Coder87C
    Is there a fast way to set 2 unsigned char arrays equal to eachother?

    Right now im just running a for loop which sets x[i] = y[i] but is there a way to do


    unsigned char Xarray[4];
    unsigned char Yarray[4];

    Xarray = Yarray;

    ?
    It is more C'ish but you can use memcpy or memset.


    Mezzano

  3. #3
    #include<xErath.h> xErath's Avatar
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    memcpy(Xarray,YArray,sizeof(YArray));
    beware that if Yarray is a pointer and not an array you'll need so specify the size of the contained buffer on memcpy last paramater

  4. #4
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    Works like a charmed ANgel

  5. #5
    and the hat of wrongness Salem's Avatar
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    If you're looking for all-out speed, then unroll the loop (some compilers will do this for you)

    Code:
    Xarray[0] = Yarray[0];
    Xarray[1] = Yarray[1];
    Xarray[2] = Yarray[2];
    Xarray[3] = Yarray[3];
    The problem with say memcpy() is that it is likely to be a function call, which can get expensive in comparison if you're only copying small blocks of data.
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  6. #6
    Just Lurking Dave_Sinkula's Avatar
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    Something like this can be amusing.
    Code:
       struct block
       {
          unsigned char array[4];
       };
       *(struct block *)Xarray = *(struct block *)Yarray;
    [edit]Hmm. For a Win32 console app using BC55, it makes pretty tiny assembly code; copying all 4 characters at once via a 4-byte register:
    Code:
    	mov       eax,dword ptr [_Yarray]
    	mov       dword ptr [_Xarray],eax
    Last edited by Dave_Sinkula; 05-20-2005 at 01:58 PM.
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  7. #7
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    It might seem ridiculous for a large array, but your loop idea is fine. The CPU's little brain can only handle one variable (one element of the array) at a time. ...No matter what technique you use, the machine code has to step-thru the array.

    EDIT-
    Oh, with character arrays (null-terminated C-style strings) you can use the strcpy() or strncpy() functions in the <cstring> header to copy one character array into another. These functions do the looping for you.
    Last edited by DougDbug; 05-20-2005 at 01:58 PM.

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