Non-recursive merge sort

float a[50000000],b[50000000];
void mergesort (long num)
{
    int rght, wid, rend;
    int i,j,m,t;

    for (int k=1; k < num; k *= 2 ) {
        for (int left=0; left+k < num; left += k*2 ) {
            rght = left + k;
            rend = rght + k;
            if (rend > num) rend = num;
            m = left; i = left; j = rght;
            while (i < rght && j < rend) {
                if (a[i] <= a[j]) {
                    b[m] = a[i]; i++;
                } else {
                    b[m] = a[j]; j++;
                }
                m++;
            }
            while (i < rght) {
                b[m]=a[i];
                i++; m++;
            }
            while (j < rend) {
                b[m]=a[j];
                j++; m++;
            }
            for (m=left; m < rend; m++) {
                a[m] = b[m];
            }
        }
    }
}