Logical operations of multi dimensional matrices

Hi,

I am looking for a solution to transform an algorithm with for loop to a faster solution by logical operations.

I have an M(mxmxn) matrix and an n length v vector. So, the M matrix has the same size in the 3rd dimension as the length of the vector. I would like to compare the elements of the M matrix to the value of the vector as follows: M(:,:,i)==v(i)

The output should be an mxmxn size logical.

I know, it is a basic task with a for loop, but I would like to solve it with logical operations to save some computation time.

I would appreciate any suggestion. Thanks.

[Torsten]

Comparing doubles for equality is never a good idea because of possible floating point errors.

[bzsfarkas]

Sorry, my bad. I should check M(:,:,i)>v(i), not equality.

Answer 1

[the cyclist]

If you do

M == reshape(v,1,1,3)

then MATLAB will implicitly expand the vector across dimension 1 and 2, to do the equality check.

[bzsfarkas]

Thank you very much. It looks good.

Won't it use more memory due to the expansion? Later, I would like to expand to 4th dimension and running similar calculations. If reshaping results in a big-size array consuming too much memory, I should try to find more effective way.

Anyway, a high-five goes for you.

[the cyclist]

I have to admit that I was thinking more about code "elegance" in response to what you were trying to achieve, more so than speed (which you had mentioned) or memory management.

I also have to admit that off the top of my head, I don't know if, under the hood, MATLAB actually expands the vector, or if this is only a notational convenience.

My reshaping solution is faster than a for loop under a pretty wide range of array sizes I just tried, but it was not always faster. You may need to do your own testing for optimization.