Issue converting Matlab code to Python non-broadcastable output

Question

I have the following code in Matlab

a= a + b(c,:);

where 'a' is a 4524x3 matrix, 'b' is 1131x3 and 'c' 4524x1.

In Python I have

a[:]+= b[c, :]

Where I'm getting 'a' as a 4524x4524x3 matrix. Why does Python create this extra dimension instead of sum the values?

What are the exact dimensions of c? What happens when you do c.shape in the command prompt? Do you get (4524,) or (4524,1)? — rayryeng
– rayryeng, Commented Jun 5, 2015 at 17:59

rayryeng · Accepted Answer · 2015-06-05 18:14:42Z

4

Try doing this instead:

a[:] += b[c.ravel(), :]

What's happening is that c is considered as a two-dimensional matrix rather than a single 1D array, which is why the unnecessary broadcasting is happening. You are basically trying to index the matrix with a 2D array when you need it to be 1D.

answered Jun 5, 2015 at 18:14

rayryeng

105k22 gold badges201 silver badges206 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Issue converting Matlab code to Python non-broadcastable output

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related