[Jprogramming] How to get array with indices of not-empty elements in sparse array?

['Sergey Kamenev' via Programming]

Hi!

    ] d=: (?. 3 4$2) * ?. 3 4$100
0 55 79  0
0 39  0 57
0  0  0  0

    ] s=: $. d                  convert d to sparse and assign to s
0 1 | 55
0 2 | 79                       the display of s gives the indices of the
1 1 | 39                       “non-zero” cells and the corresponding values
1 3 | 57

How I can get dense array with indices of not-empty elements?
0 1
0 2
1 1
1 3

Is there a solution without iterating over an dense array?

Nice day!
Sergey