基数排序比较

Question

I have an implementation of LSD Radix Sort algorithm and was wondering how to count the number of digit comparisons during the sort procedure? I know that the algorithm is not comparison based but there is still some kind of a comparison between digits of the Integer elements the algorithm sorts. Can someone point out where the comparison is taking place?

Thanks!

LSDRadixSort:

public static void lsdRadixSort(int[] a)
{
    final int BITS = 32; // each int is 32 bits 
    final int R = 1 << BITS_PER_BYTE; // each bytes is between 0 and 255
    final int MASK = R - 1; // 0xFF
    final int w = BITS / BITS_PER_BYTE;  // each int is 4 bytes

    int n = a.length;
    int[] aux = new int[n];

    for(int d = 0; d < w; d++)
    {
        // compute frequency counts
        int[] count = new int[R+1];

        for(int i = 0; i < n; i++)
        {           
            int c = (a[i] >> BITS_PER_BYTE*d) & MASK;
            count[c + 1]++;
        }

        // compute cumulates
        for(int r = 0; r < R; r++)
        {
            count[r+1] += count[r];
        }

        //for most significant byte, 0x80-0xFF comes before 0x00-0x7F
        if(d == (w - 1))
        {
            int shift1 = count[R] - count[R/2];
            int shift2 = count[R/2];

            for(int r = 0; r < R/2; r++)
            {
                count[r] += shift1;
            }

            for(int r = (R/2); r < R; r++)
            {
                count[r] -= shift2;
            }
        }

        // move data
        for(int i = 0; i < n; i++)
        {
            int c = (a[i] >> BITS_PER_BYTE*d) & MASK;
            aux[count[c]++] = a[i];
        }

        // copy back
        for(int i = 0; i < n; i++)
        {
            a[i] = aux[i];
        }
    }
}

Answer 1

Well, as you said, there are no comparisons. The operation that comes closest to that is:

count[c + 1]++;

where c is a byte of the integer. Each integer has 4 bytes, so you do it exactly 4*n times.