Column multiplication in the same matrix

2018 3 29
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更新时间：2021 12 13
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I have a 3 by 3 matrix and would like to loop and multiply column 1 with all other columns but not itself and also column 2 with other columns but not itself and column 3 with other columns but not itself.
x11 = [1 2 3; 4 5 3; 2 8 3];
[m12, n12] = size(x11);
for i = 1:m12
for j = 1:n12
x1x1(i, j)=x11(i, j).*x11(i, j)
end
end

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 采纳的回答

Star Strider
Star Strider 2018-3-29

0 个投票

Try this:
x11 = [1 2 3; 4 5 3; 2 8 3];
[m12, n12] = size(x11);
for k1 = 1:n12
x11t = x11; % Copy ‘x11’
idx = setdiff(1:n12, k1) % Columns To Be Multiplied
x11t(:,idx) = bsxfun(@times, x11(:,idx), x11(:,k1)) % Assign Multiplied ‘Columns To Be Multiplied’ To Appropriate Columns Of ‘x11t’
x1x1(:,:,k1) = x11t; % Store In Output Array
end
It appears to do what you want.

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Mahmoud Solomon
Mahmoud Solomon 2018-3-29
Thanks for the respond but I am not getting the desire result. The resulting matrix should look like this [2 6 3; 20 15 12; 16 24 6]. Again, column 1 multiplies columns 2 and 3 and column 3 multiplies column 1.
Star Strider
Star Strider 2018-3-29
Change the ‘x1x1’ assignment to:
x1x1(:,k1) = x11t(:,rem(2*n12+k1,n12)+1) % Store In Output Array
That puts the correct columns of ‘x11t’ into ‘x1x1’ in each iteration.
Mahmoud Solomon
Mahmoud Solomon 2018-3-29
Nice Sir. It works. Thanks again.
Mahmoud Solomon
Mahmoud Solomon 2018-4-3
Sir... I need to generalize the solution to that problem. I have been trying but it's not working. What I want to achieve is when a column is used once, it should be removed from the original matrix thereby reducing the number of column by one. So, column 1 is used to multiple columns 2 to n. The next multiplication should be done between column 2 and 3 up to n, and in that order. Remember, nCr = n!/(n-r)!r!.
Star Strider
Star Strider 2018-4-3
It seems that my original solution (link) will do what you want.
If you begin with an (NxM) matrix, you would end up with a (NxMxM) 3D matrix, or perhaps (Nx(M-1)xM), depending upon what you want. I know of no other way to do that.

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更多回答（2 个）

Elias Gule
Elias Gule 2018-3-29

1 个投票

Here you go:
[nr,nc] = size(x11);
for col = 1 : nc
col_data = x11(:,col);
col_prod = bsxfun(@times,col_data,x11(:,~all(ismember(x11,col_data))));
col_prod % you can store this in another array
end

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Mahmoud Solomon
Mahmoud Solomon 2018-3-29
Thanks for the respond but I am not getting the desire result. The resulting matrix should look like this [2 6 3; 20 15 12; 16 24 6]. Again, column 1 multiplies columns 2 and 3 and column 3 multiplies column 1.
Elias Gule
Elias Gule 2018-3-29
ok. I see what you mean. You are basically multiplying the columns of a matrix while ignoring the currently selected column.
x11 = [1 2 3; 4 5 3; 2 8 3];
[nr,nc] = size(x11);
for col = 1 : nc
col_data = x11(:,col);
tempX11 = x11(:,~all(ismember(x11,col_data)));
col_prod = tempX11(:,1).*tempX11(:,2);
col_prod % you can store this in another array
end

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Elias Gule
Elias Gule 2018-3-29

0 个投票

I believe this works as expected:
x11 = [1 2 3; 4 5 3; 2 8 3];
[nr,nc] = size(x11);
for col = 1 : nc
col_data = x11(:,col);
tempX11 = x11(:,~all(ismember(x11,col_data)));
col_prod = tempX11(:,1).*tempX11(:,2);
col_prod % you can store this in another array
end

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Mahmoud Solomon
Mahmoud Solomon 2018-3-29
It also worked but the result is not in a matrix format. I will work that out. Anyway, thanks.
Yuanli Guo
Yuanli Guo 2021-12-13
@Mahmoud Solomon Hi Mahmoud, do you know how to store these arrays in one matrix? I'm having the same problem now. Thank you^^

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