Using `bsxfun` for non-numeric data

Question

Is there an equivalent to bsxfun for non-numeric data?

For example, I want to compare all pairs of strings stored in two cell-arrays:

>> a = {'aa', 'bb', 'cc'};
>> b = {'dd', 'aa'};
>> bsxfun( @strcmp, a, b' ); % not working for cells :-(

Answer 1

I'm afraid there's no such equivalent for cell-arrays :-(

As far as I can see, you can either:

1. Follow Oleg's suggestion and use loops
2. Use existing implementations such as mAryCellFcn or csxfun from the File Exchange.

3. Roll your own function. For example, here's a variant of Robert's idea that works for inputs of any dimensions (under the restrictions of bsxfun, of course) and an arbitrary binary function func:

   function C = mybsxfun(func, A, B)
       idx_A = reshape(1:numel(A), size(A));
       idx_B = reshape(1:numel(B), size(B));
       C = bsxfun(@(ii, jj)func(A(ii), B(jj)), idx_A, idx_B);
   

   If your function can operate on entire cell arrays element-wise, you could perform a singleton expansion on your cell arrays first, and then feed them directly to the function func:

   mask = bsxfun(@or, true(size(A)), true(size(B)));
   idx_A = bsxfun(@times, mask, reshape(1:numel(A), size(A)));
   idx_B = bsxfun(@times, mask, reshape(1:numel(B), size(B)));
   C = func(A(idx_A), B(idx_B));
   

   The latter method might be faster if func is optimized for vectorized operations on cell arrays.

Answer 2

I like Rody's solution, but you could also do a workaround like this:

ia=(1:length(a)).'; ib=1:length(b);
a=a(:);
bsxfun(@(ii,jj) strcmp(  a(ii),b(jj) )  ,ia, ib);

Answer 3

How about

[str,ia,ib] = intersect(a,b)

?

Answer 4

As the error message says, broadcasting only works for numeric types. Here are other possible alternatives:

a = {'aa', 'bb', 'cc'};
b = {'dd'; 'aa'};

%# obviously doesnt work
%#bsxfun(@strcmp, a, b)

%# do the singleton expansion ourselves
strcmp(repmat(a,size(b)), repmat(b,size(a)))

%# if you dislike REPMAT, we can use Tony's trick
strcmp(a(ones(size(b)),:), b(:,ones(size(a))))

%# we could also use CELLFUN
cell2mat(cellfun(@(s)strcmp(a,s), b, 'UniformOutput',false))