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How to multiply -1 with specific vector of a matrix ?

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D Bhuiyan
D Bhuiyan 2015-1-29
评论： D Bhuiyan 2015-1-30
Dear all,
I would like to multiply -1 in all elements of column vector of a matrix in the following conditions: 1) if all elements of the column vectors of the matrix are negative. 2) if some elements of the column vectors are negative and rest of them are positive 3) if all elements of the column vectors are positive, don't need to multiply with -1
would you please give me some idea how can i proceed??? for example the matrix is
A=[-0.5 2 3 -5; -0.8 -7 4 6; -0.9 2 3 -9; -0.3 -6 3 1; -0.2 5 3 -7]
result should be like that Result_A=[0.5 -2 3 5; 0.8 7 4 -6; 0.9 -2 3 9; 0.3 6 3 -1; 0.2 -5 3 7]
Thanks in advance.

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Sean de Wolski
Sean de Wolski 2015-1-29
编辑：Sean de Wolski 2015-1-29
The easiest and likely fastest way to do this would be with a simple for-loop over columns.
C = A;
for ii = 1:size(C,2)
if any(sign(C(:,ii)) == -1)
C(:,ii) = -C(:,ii);
end
end
But just for fun, here's a vectorized way:
A=[-0.5 2 3 -5; -0.5 -2 4 6; -0.5 2 3 -9; -0.5 -2 3 1; -0.5 2 3 -7]
B = bsxfun(@times,A,-((double(any(sign(A)==-1)))-0.5)*2)
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Sean de Wolski
Sean de Wolski 2015-1-29
Closing parenthesis was in the wrong place. Try it now.
And, Sean, this is why you should always assert!
assert(isequal(B,C))

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