Matlab: Operations along the first dimension of a 3D array

Question

I have a 3D array M(d*d,m,n). For each d*d vector of M (i.e. vectors of the first dimension), I split it into d parts and take the sum of each part to form a new vector (of size d). For example, if u is a vector along the first dimension of M, then it will be replaced by the vector v, computed by:

v = sum(reshape(u,d,d))';

For the moment I use a loop as following, but I think there should be a much faster way to do it.

N = zeros(d,m,n)
for i=1:m
    for j=1:n
        N(:,i,j) = sum(reshape(M(:,i,j),d,d))'; %//'
    end
end

Thank you so much for any suggestions!

Divakar · Accepted Answer · 2014-05-29 13:45:05Z

3

Try this -

N = reshape(sum(reshape(M,d,[])),d,m,n)

answered May 29, 2014 at 13:45

Divakar

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ASantosRibeiro · Accepted Answer · 2014-05-29 14:01:03Z

3

I may not be understanding the question correctly but is this what you are looking for?

N=squeeze(sum(reshape(M,[d,d,size(M,2),size(M,3)])))

edited May 29, 2014 at 14:01

answered May 29, 2014 at 13:51

ASantosRibeiro

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6 Comments

matheburg Over a year ago

Btw: Both answers are similar fast :)

Divakar Over a year ago

@matheburg Seems like you are doing benchmarks around here, which is great, keep 'em coming! :)

matheburg Over a year ago

Thanks :) In my view it is really important. The only reason I want to vectorize anything is the speed-up, isn't it? I am usually implementing huge simulations where speed and memory optimization is really important. Today I have a day off ;-)

Divakar Over a year ago

@matheburg haha nice, SO is lucky to have you today ;) Agreed that speedup should be the primary reason for vectorization! Well MATLAB is coming off age with its PCT to speed things up. I really hope they talk more of the GPU when speeding things up here on SO, which they don't sadly.

f10w Over a year ago

Hi ASantosRibeiro and @Divakar. Thank you very much for your answers. I tested both solutions and ASantosRibeiro's one is slightly faster (but the difference is very small). Since Divakar posted his answer first and also suggested a correction to ASantosRibeiro's, I think credits should go to him first. (I wish I could mark both as answers :( )

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