The LOC function finds nonzero elements of a matrix. It creates a row vector, where is the number of nonzero elements in the argument matrix. Missing values are treated as zeros. The values in the resulting row vector are the locations of the nonzero elements in the argument (in row-major order).

For example, consider the following statements:

a = {1 0 2 3 0};
b = loc(a);
print b;

Because the first, third, and fourth elements of a are nonzero, these statements result in the row vector shown in Figure 23.172:

If every element of the argument vector is 0, the result is empty; that is, b has zero rows and zero columns.

The LOC function is useful for subscripting parts of a matrix that satisfy some condition. For example, the following statements create a matrix y that contains the rows of x that have a positive element in the diagonal of x:

x = {1  1  0,
     0 -2  2,
     0  0  3};
y = x[loc(vecdiag(x)>0), ];
print y;