Converting 4D matrix to 2D with multiple for-loop
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Hello, I'm trying to convert 4D matrix to 2D matrix instead of for-loop.
Here is my code.
X = randi(10,100,10,100);
A = ones*(10,100);
SUM = zeros(10,100);
for k=1:10
for l=1:100
for m=1:10
for n=1:100
SUM(m,n) = SUM(m,n) + A(k,l) * X(k,l,m,n);
end
end
end
end
I want to know how to change 4D to 2D for simpler calculation without for-loops. Thank you.
0 Comments
Accepted Answer
Andrei Bobrov
on 17 Feb 2016
Edited: Andrei Bobrov
on 17 Feb 2016
X = randi(100,10,100,10,100);
A = randi(20,10,100);
[k,l,m,n] = size(X);
z = bsxfun(@times,X,A);
out = reshape(sum(reshape(z,k*l,1,m,n),1),m,n);
or
out = squeeze(sum(reshape(z,k*l,1,m,n)));
3 Comments
More Answers (1)
John BG
on 17 Feb 2016
Edited: John BG
on 19 Feb 2016
1.- your first command
randi(10,100,10,100)
does not generate a 4D matrix, but 3D the first input field of randi is the range [1:10] within the output values fall within. 4D would be
X=randi(10,10,100,10,100)
2.- reshaping matrices is fairly simple with command reshape
[s1,s2,s3,s4]=size(X)
Total=s1*s2*s3*s4
to a square matrix
X2=reshape(X,[Total^.5,Total^.5])
or to
X3=reshape(X,[10,Total/10])
3.- However, your 4 for loops do more than 'reshaping'. You take a slice of X, multiply it by A, and then cumulatively sum it, don't you?
check if the following 2 for loops are ok:
X=randi(10,10,100,10,100)
A = ones(10,100)
[s1,s2,s3,s4]=size(X)
SUM = zeros(s3,s4)
for m=1:s3
for n=1:s4
X2=X(:,:,m,n)
X3=A.*X2
SUM = SUM+ X3
end
end
If this answer helps in any way to solve your question please click on the thumbs-up vote link above, thanks in advance
John
4 Comments
Andrei Bobrov
on 19 Feb 2016
Hi John! Simple example:
X = randi(20,4,5,3,2);
A = randi(20,4,5);
SUM = zeros(3,2);
for k=1:4
for l=1:5
for m=1:3
for n=1:2
SUM(m,n) = SUM(m,n) + A(k,l) * X(k,l,m,n);
end
end
end
end
[k,l,m,n] = size(X);
z = bsxfun(@times,X,A);
out = reshape(sum(reshape(z,k*l,1,m,n),1),m,n);
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