Subset of a matrix

Hi,

This question is a bit tricky to explain with words, so I'll write the matlab code I would like...

Starting with : x = 1:5; y = (-5:5)'; iPos = y > 0;

I would like to write something like :

z = [y*x](iPos);

Which would be the equivalent of

z = y*x; z = z(iPos);

without creating the z matrix. I do it with Python but I can't figure out the way to do it in Matlab... Is it possible ?

Thanks

Answer 1

[Guillaume]

Did you mean

z(:, iPos)

as it's a bit unusual to use a logical index as a linear index when it's smaller than the number of elements in the matrix.

To answer your question, unfortunately, you can't index into temporaries in matlab, be it the result of an expression as in your example, or the return value of function. You always have to create a variable to store the result and there is no trick that can work around that.

However, in your case, you could do

   z = y(iPos) * x(1);

Or if you meant to get all rows:

z = y(iPos) * x;

[Stéphane]

Thanks for your answer.

I actually have large 2D matrices and I apply different operations depending on conditions in both direction, so the way I found to do it is to apply binary masks.

By the way, I'm using these temporaries because I think it saves memory when using large matrices but I'm not sure about that...

Am I correct ?

[Guillaume]

Can you show a more realistic example?

Note that even if matlab allowed

z = [y*x](iPos);

it would still use the same amount of memory.