[was_found, idx] = ismember([26 4 28], B1);
will assign to two variables: was_found and idx. was_found will be true for each element of [26 4 28] that was located somewhere in B1, and will be false for any element that was not found in B1. idx will be 0 for any element that was not found, and otherwise will be the index of the "first" location of the value in B1. idx will be a "linear index"