WHERE

WHERE(x) produces a variate listing the units of x that are logically true, i.e. non-zero (synonym WHICH). If all the units are false, it produces a variate of length one, containing a missing value. If x is restricted or qualified (by $[]), WHERE looks only in the defined subset of units for true values. However, the unit numbers in the result give their positions in the full vector x, not the subset.