If I understand the quesiton, you want to find indices in A where the values in B are found when they are in A.
You can use the second output of ismember for this, it'll return a 0 for items that aren't found:
A = {'Mary'; 'had'; 'a'; 'little'; 'lamb'};
B = {'Mary'; 'lamb'; 'ary'};
[~,idx] = ismember(B,A)
Worth pointing out that B might be in A more than once, ismember will return the first place it's found (the documentation says this is 'Generally' true, though I'm not sure I can think of a case where it's not):
A = {'Mary'; 'had'; 'a'; 'little'; 'Mary'};
B = {'Mary';'little';'sheep'};
[~,idx] = ismember(B,A)
