Algorithm help?

This is a discussion on Algorithm help? within the C Programming forums, part of the General Programming Boards category; Hello, What line of codes would work to make the following derivation(with the exact order): int y[i][j]; y[0][0]=0 y[1][0]=0+4 y[2][0]=0+16 ...

  1. #1
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    Algorithm help?

    Hello,

    What line of codes would work to make the following derivation(with the exact order):

    int y[i][j];

    y[0][0]=0

    y[1][0]=0+4

    y[2][0]=0+16
    y[2][1]=0+4+16

    y[3][0]=0+64
    y[3][1]=0+4+64
    y[3][2]=0+16+64
    y[3][3]=0+4+16+64

    y[4][0]=0+256
    y[4][1]=0+4+256
    y[4][2]=0+16+256
    y[4][3]=0+4+16+256
    y[4][4]=0+64+256
    y[4][5]=0+4+64+256
    y[4][6]=0+16+64+256
    y[4][7]=0+4+16+64+256

    ...
    ..
    .

    See the pattern? it keep adding itself for every increase in 'i'.

    Help?
    Last edited by Zul56; 02-21-2013 at 09:29 PM.

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    Are you sure that's correct? Why do you have two sections that are y[3][ ]?

    Frankly, I don't (yet) see a pattern, but maybe it's because your sample isn't quite right. If you see the pattern of "adding itself for every increase in 'i'", then what is stopping you from coding it?

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    Ahh, just saw the pattern, though I still think there's an error in your sample. Jumping from 0 to 4 to 16 to 32 to 64 does not seem to follow a pattern though. If it's powers of 2, what happened to 2 and 8? If it's powers of 4, why is 32 in there?

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    edited.

    Sorry for the error in copying..

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    So powers of 4 it is. Okay, so that gets you your "big" number, i.e. the 0, 4, 16, 64, 256.

    Now, try to express each line in terms of i, j and previous lines (except the first one, y[0][0]). For example
    Code:
    y[0][0] = 0
    
    y[1][0] = y[0][0] + 4**1
    
    y[2][0] = y[0][0] + 4**2
    y[2][1] = y[1][0] + 4**2
    ...
    That should help you figure out the algorithm.

  6. #6
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    This is what I have, and it doesn;t work, I'm struggling to get what I want. Some help please?

    Code:
            int a,b,c,d;
            int a_max=4;
            int y[50][50];
            y[0][0]=0;
            
            //algorithm
            for(a=0;a<a_max;a++)
            for(b=0;b<pow(pow(4,a),0.5);b++)
            {            
                for(c=0;c<2;c++)
                for(d=0;d<2;d++) 
                {
                    y[a][b]=pow(4,a+1) + y[c][d];         
                }
            }
    
            for(a=0;a<a_max;a++)
            for(b=0;b<pow(pow(4,a),0.5);b++)
            {
                printf("y[a][b]=y[%d][%d]=%d",a,b,y[a][b]);
                getchar();
            }
    Last edited by Zul56; 02-21-2013 at 11:08 PM.

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    nevermind. solved. thanks!

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    Less loops. No pow.
    Code:
    #define ROWS 5
    int i, j, lim, t, f, y[ROWS][1 << ROWS - 2];
    int k1 = 0x2AAAAAAc;
    int k2 = 0x55555554;
    
    memset(y, 0, sizeof(y));
    
    for (i = 1; i < ROWS; i++) {
        lim = 1 << i - 1;
        f = 1 << i * 2;
        for (t = j = 0; j < lim; j++) {
            y[i][j] = t + f;
            printf("[%d][%d] %d\n", i, j, y[i][j]);
            t = t + k1 & k2;
            }
        printf("\n");
        }
    [1][0] 4

    [2][0] 16
    [2][1] 20

    [3][0] 64
    [3][1] 68
    [3][2] 80
    [3][3] 84

    [4][0] 256
    [4][1] 260
    [4][2] 272
    [4][3] 276
    [4][4] 320
    [4][5] 324
    [4][6] 336
    [4][7] 340
    Last edited by nonoob; 02-22-2013 at 01:23 PM.

  9. #9
    Algorithm Dissector iMalc's Avatar
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    It's simply bit-scrambling. The results have the same number of bits set as the values 0 to 15 do, it's just that they are different bits. All you need to do is to swap the bits around.
    My homepage
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    Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"

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