This will give you a logical mask describing the location of elements within the closed interval [50 70]. You can call it what you want.
A = randi([40 120],100,1);
idx_logical = (A>=50) & (A<=70);
While logical indexing is often the more efficient approach, sometimes you might want the actual indices. If you want the linear indices of those locations:
idx_linear = find(idx_logical);
