Question

Sorry for the beginners matlab question…

function [ A B C ] = crunch(i)
  A = i^2;
  B = 2*A;
  C = A+B;
end;

vals = zeros(5,3);
  for i=1:5
    vals(i,:) = crunch(i);
  endfor;
disp(vals);

This is not the result I expected.

vals =
 1 1 1
 4 4 4
 9 9 9
 ..... etc

if I instead explicitiy place A, B & C in a row vector and return that, then everything is fine.

function retval = crunch(i)
  A = i^2;
  B = 2*A;
  C = A+B;
  retval = [ A B C ];
end;

ans = 
  1    2    3
  4    8   12
  9   18   27
 16   32   48
 25   50   75

What’s going wrong here?

Answer 1

MATLAB is generally reluctant to give you multiple return values unless you explicitly ask for them. So in the first version, when you do:

vals(i,:) = crunch(i);

What MATLAB does is take just the first return value from crunch(i), then broadcast that to all the elements of vals(i,:). It’s like doing:

return_val = crunch(i);
vals(i,:) = return_val;

If you explicitly ask MATLAB for multiple return values, you get the desired behavior. As @igon notes, this version will fix the behavior:

[vals(i,1), vals(i,2), vals(i,3)] = crunch(i);

Or, I think if vals were a cell array, vals{i, :} = crunch(i); would work too, but that doesn’t make sense in this case — using a regular (not cell) array seems like the right approach in your code.