Consider the following:

    variable X = [0.0:10.0:0.01];
    variable A = sin (X);
    variable I = where (A < 0.0);
    A[I] = cos (X) [I];

Here the variable X has been assigned an array of doubles whose elements range from 0.0 through 10.0 in increments of 0.01. The second statement assigns A to an array whose elements are the sin of the elements of X. The third statement uses the where function to get the indices of the elements of A that are less than 0.0. Finally, the last statement substitutes into A the cos of the elements of X at the positions of A where the corresponding sin is less than 0. The end result is that the elements of A are a mixture of sines and cosines.