adding matrices inside a big matrix
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Hello everybody,
I am wandering if there is a way for not using loops to add many small matrices inside a big matrix. For example I have a big 40000x240000 matrix and I want to add all the numbers of 4x4 matrices inside that matrix, resulting in a 10000x60000 matrix.
thank you all for your answers
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Sean de Wolski
le 15 Nov 2011
And this fun answer:
sum4x4 = reshape(sum(reshape(reshape(sum(reshape(B,4,[])),size(B,1)/4,[])',4,[])),size(B)./4);
sum4x4 = reshape(sum4x4,size(sum4x4,2),size(sum4x4,1))'
where B is your matrix.
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Sean de Wolski
le 16 Nov 2011
Good catch!
Just add this line will do the same thing.
sum4x4 = reshape(sum4x4,size(sum4x4,2),size(sum4x4,1))'
Plus de réponses (3)
Fangjun Jiang
le 15 Nov 2011
Because the size of your matrix, you might want to re-use the matrix so replace the variable "e" and "f" with "d".
d=rand(40,24);
[M,N]=size(d);
e=mat2cell(d,4*ones(M/4,1),4*ones(N/4,1));
f=cellfun(@(x) sum(x(:)),e);
Sean de Wolski
le 15 Nov 2011
How about blkproc if you have the Image Processing Toolbox (or blockproc in newer versions).
blkproc(magic(16),[4 4], @(x)(sum(x(:))))
I'm surprised conversion to cell is faster than a well done for-loop. Perhaps you could post what you had for the nested for-loops?
4 commentaires
Sean de Wolski
le 15 Nov 2011
in blkproc, did you change magic(16) to 'a'? the magic(16) was for demo purposes only.
Sean de Wolski
le 15 Nov 2011
Your for-loop could be sped up a little by building the j-index vectors outside of the k for-loop. Overall it's good though!
Andrei Bobrov
le 17 Nov 2011
e.g.
A = randi(25,12,16);
m = 4;
n = 4;
[M,N] = size(A);
out = reshape(sum(sum(reshape(A.',n,N/n,m,[]),3)),n,[])';
or
out2 = reshape(sum(sum(reshape(A,m,M/m,n,[]),3)),[],n);
or2
out3 = blockproc(A,[m n],@(block_struct)sum(block_struct.data(:)));
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