How to find index of value based on value from a different variable?

• I have variable X that contains ~50 names.
• I have variable Y that contains a directory of 200 files, of which 50 match the name of variable X.

I would like to look into the Y variable, and for each name that matches a name within variable X, I would like to return that matche's index within Y variable.

• In other words, I would like to know the location of each duplicate's name in variable Y, for future use.

I tried extracting the names from variable Y into a new variable, so that I can then use a find function. But I am having trouble.

% create Y2 that stores only the names from directory Y
for tmp = 1:length(Y)
    Y2(tmp)= string(Y(tmp).name);
end
% however, this returns only NaN values and I'm not sure why

%% use ? function to compare list of names in X to Y2
%% For each match in name, get Y2's index position. 

Answer 1

[Image Analyst]

Try ismember():

[~, index] = ismember(Y, X); % Find what index Y is in the list of the 50 names that comprise X.