Summing arrays within a cell array.

I've got a cell array which consists of 501 4D arrays, I want to sum all of these together and output a single 4D array. I've tried using sum and plus but get the error saying too many input arguments.

[Jan]

Please post the code, instead of telling what it should do, and a copy of the complete error message, instead of a short rephrasing. Thanks.

Answer 1

[Jan]

What about a simple loop?

S = 0;
for k = 1:numel(C)
  S = S + C{k};
end

You can check if a vectorized code is faster:

S = sum(cat(5, C{:}), 5);

While it does not have the overhead for the loop, it requires a large temporary array.

[mathman]

The loop worked! The vectorized code gave the error:

Error using cat
Requested 81x81x1x200901 (9.8GB) array exceeds maximum array size preference. Creation of arrays greater
than this limit may take a long time and cause MATLAB to become unresponsive. See array size limit or
preference panel for more information.

[Jan]

This is a nice example for a problem, which does not profit from vectorization.

[mathman]

Sorry, I just want to make sure I'm understanding the loop correctly. So S would initially be 0. It would then be added to the first cell array C{1}. This would change the value of S to equal to C{1} which would then be added to C{2} and so on?

[Jan]

@mathman: Only the variable on the left side of the assignment operator "=" is modified. This means, that S is 0 initially. Then in the first iteration, S is replaced by S + C{1}. In the 2nd iteration, this new S is replaced by S + C{2}, and so on. This is equivalent to (assuming that C has 5 elements):

S = C{1} + C{2} + C{3} + C{4} + C{5}

You can simply try if:

C = {1,3,4,7,8}
S = 0;
for k = 1:numel(C)
  fprintf('Loop %d\n', k);
  disp(C{k})
  disp(S)
  S = S + C{k};
  disp(C{k})
  disp(S)
end

The debugger helps also to find out, what's going on: https://www.mathworks.com/help/matlab/matlab_prog/debugging-process-and-features.html