"Matrix dimensions must agree" despite using .*

 Matrix dimensions must agree.

Error in nnCostFunction (line 74) delta1_h = (d2).*(X1_tr);

d2 and X1_tr are the two vectors wich have sizes unsuitable for matrix multiplication, but my goal is element-wise multiplication and somehow Matlab fails to do it. Can you please help me with this problem?

In the console I've tried element-wise multiplication of [1 2 3] and [2; 3; 4; 5]. It was alright. Wonder what may be wrong here.

Thank you.

Answer 1

[Star Strider]

It’s called ‘implicit expansion’ and is new in R2016b.

The ‘traditional’ (and my preferred) way to do this is to use the bsxfun function to achieve the same result:

a = [1 2 3];
b = [2; 3; 4; 5];

q1 = bsxfun(@times, a, b);
q2 = bsxfun(@times, b, a);

You will get the same result regardless of the way the operators are entered.