Question

Say I have a 3x4x5x6 java double array a that I unroll into an ArrayList b of length 360 in the following way:

for (int i = 0; i<a.length; i++){
    for (int j = 0; j<a[0].length; j++){
        for (int k = 0; k<a[0][0].length; k++){
            for (int m = 0; m<a[0][0][0].length; m++){
                b.add(a[i][j][k][m]);
            }
        }
    }
}

Given the index of b, is there an easy way to find the corresponding 4-tuple of indices in a?

Answer 1

Assuming that

• b is the index on the monodimensional array
• i,j,k,m are the four resulting indices on the multidimensional array
• si,sj,sk,sm are the size of any dimension

you can use basic math, it should be something like

• m = b % sm
• k = (b / sm) % sk
• j = (b / (sm*sk)) % sj
• i = b / (sm*sk*sj)

Basically you increment every index by one for every size of the contained arrays (by multiplying sizes) and you wrap it on its dimension.