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Find the indices of elements of matrix that were multiplied

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Astrik
Astrik on 15 Sep 2016
Answered: Andrei Bobrov on 15 Sep 2016
I have a task to find the maximum product of elements in a matrix. The input arguments are the matrix and the scalar n that tells me how many elements to multiply. If n=2, then I should multiply two by two. The maximum product can lie either in rows, or in columns on in diagonal. For example in this case A's elements were multiplied 2 by 2 along rows (B) and columns (C).
A =
8 1 6
3 5 7
4 9 2
B =
8.0000 6.0000
15.0000 35.0000
36.0000 18.0000
C =
24.0000 5.0000 42.0000
12.0000 45.0000 14.0000
I do it using the loop
c=[]
for ii = 1:(length(A)-n+1)
p = prod(A(:,ii:ii+n-1));
c=[c p];
end
or
for i=1:size(A,2)
B(i,:)=real(exp(conv(log(A(i,:)),ones(1,n),'valid')));
C(:,i)=real(exp(conv(log(A(:,i)),ones(1,n),'valid')));
end
In both cases I receive the product but when it comes to getting the maximum among that products, (in my case that is the product of A(3,1)*A(3,2)=45) I cannot find the indices of original matrix elements that formed that product. Any help will be appreciated.

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Answers (1)

Andrei Bobrov
Andrei Bobrov on 15 Sep 2016
A =[ 8 1 6
3 5 7
4 9 2];
n = 2;
s = size(A);
B = real(exp(conv2(log(A),ones(1,n),'valid')));
C = real(exp(conv2(log(A),ones(n,1),'valid')));
[b,bm] = max(B(:));
[c,cm] = max(C(:));
if b > c
[ii,jb] = ind2sub([s(1),s(2) - n + 1],bm);
out = {ii,jb + (0:n-1)};
else
[ic,jj] = ind2sub([s(1) - n + 1,s(2)],cm);
out = {ic + (0:n-1),jj};
end

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