Question about array values

This is a discussion on Question about array values within the C Programming forums, part of the General Programming Boards category; So I have an array (several, actually), which holds the RGB values of an image. I'm then doing some calculations ...

  1. #1
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    Question about array values

    So I have an array (several, actually), which holds the RGB values of an image. I'm then doing some calculations with each of these arrays (let's say we're doing Red. Then Red[i][j] is the intensity of "redness" at the pixel (i,j)). Red is of type int, raster is of type uint32. My calculations depend on Red[i][j] being some nice number less than 256. When I run gdb, and tell it to print Red[7], for example, I get something along the lines of
    Code:
     (int*) 0xd3ff20
    .
    Can anyone tell me why it is doing this? Thanks.
    Here is the relevant (hopefully) code
    Code:
    raster = (uint32*) _TIFFmalloc(npixels * sizeof(uint32));
        if (raster == NULL) printf("no memory");
        TIFFReadRGBAImage(tif, w, h, raster, 0); 
        for (i = 0; i<h; i++)
        {
            for (j = 0; j<w; j++)
            {
    
                z = raster[i*w + j]; 
                Red[i][j] = (z & 0xff);
                Grn[i][j] = ((z>>8) & 0xff);
                Blu[i][j] = ((z>>16) & 0xff);
    
            }
        }

  2. #2
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    Because red[7] is an array itself, and gets printed as a pointer. You do know pointers and arrays well, don't you?
    Code:
    #include <stdio.h>
    
    void J(char*a){int f,i=0,c='1';for(;a[i]!='0';++i)if(i==81){
    puts(a);return;}for(;c<='9';++c){for(f=0;f<9;++f)if(a[i-i%27+i%9
    /3*3+f/3*9+f%3]==c||a[i%9+f*9]==c||a[i-i%9+f]==c)goto e;a[i]=c;J(a);a[i]
    ='0';e:;}}int main(int c,char**v){int t=0;if(c>1){for(;v[1][
    t];++t);if(t==81){J(v[1]);return 0;}}puts("sudoku [0-9]{81}");return 1;}

  3. #3
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    Oh. I thought that Red[7] would decompose to Red[0][7]. Sorry.

  4. #4
    Super Moderator VirtualAce's Avatar
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    A speedup:

    Code:
    raster = (uint32*) _TIFFmalloc(npixels * sizeof(uint32));
        if (raster == NULL) printf("no memory");
        TIFFReadRGBAImage(tif, w, h, raster, 0); 
    
        DWORD dwOffset=0;
        DWORD dwMaxOffset=w*h;
        DWORD dwColor=0;
    
        do
        {
           dwColor=raster[dwOffset];
     
           Red[dwOffset]=(dwColor & 0xFF);
           Grn[dwOffset]=((dwColor>>8) & 0xFF);
           Blu[dwOffset]=((dwColor>>16) & 0xFF);
    
           dwOffset++;
        } while (dwOffset<dwMaxOffset);
    One offset. One multiply. Linear array traversal.

  5. #5
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    1) Use descriptive variable names, expecially if you have other people read your code.

    2) For values less than 256 you can use unsigned char.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

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