MATLAB: Comparing two cell arrays of strings of different sizes

Question

Dear all,

I'm trying to compare two cell arrays of strings, a lot like the following:

A = [X_ABCDE;X_BCDEA;X_BCED]
B = [X_A;X_B]

and the resulting array should be:

index = [1;2;2]

as I have a matrix D = [B, C] with C containing the cells I want to end up with.

I wanted to do it using a for-loop and a nested for-loop, but, as A is 8000×1 and D is 2000×2, this will take forever. I tried using strcmp and ismember, but these only work when the cells (or strings) are identical. strfind doesn't work either, because when I use strfind I need to have a for-loop over array A, and strfind only works if the smallest of both is a string.

for iCell = 1:length(A)
    index = strfind(A(iCell), cell2mat(D(iCell,1)));
end

A string in B is ALWAYS shorter than in A and a string in B can occur more than once in A. I hope you can help me out 🙂

Answer 1

Use strncmp (not strcmp) to make the code simpler. Loop over the smaller cell array B, and pass the larger cell array A whole to strncmp:

A = {'X_ABCDE';'X_BCDEA';'X_BCED'}
B = {'X_A';'X_B'}
%
X = zeros(size(A));
for k = 1:numel(B)
    X(strncmp(A,B{k},3)) = k;
end

produces:

>> X
X =
   1
   2
   2

Do not use arrayfun or cellfun if you want fast code: they will be slower than a for-loop.