Negating every second entree in a matrix column
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My goal is pretty simple: I have an arbitrary sized matrix holding only positive real numbers and there will not be anymore than two nonzero elements per column at any time, for example:
b = [1 0 1; 0 1.2 2; 1 3 0];
or
b = [1 2 1 0; 0 1.2 0 0; 0 0 0 3; 1.6 0 4 2.2];
Now what I want is to negate every second nonzero entree of a column. The solution I came up with works, but is probably not the most elegant and/or efficient:
for n=1:numel(b(1,:))
isPositive = 0;
for m=1:numel(b(:,n))
if (isPositive == 1 && b(m,n) ~= 0)
b(m,n) = b(m,n)*-1;
break;
end
if (abs(b(m,n)) == b(m,n) && b(m,n) ~= 0)
isPositive = 1;
end
end
end
I am quite new to MATLAB, so if anybody knows a more elegant solution, perhaps not involving any for loops, please share.
Thanks in advance
2 Comments
Jan
on 13 Dec 2011
The BREAK wil stop the "for m" loop - is this intented?
Instead of "(abs(b(m,n)) == b(m,n) && b(m,n) ~= 0)" you could write "b(m,n) > 0", but if the matrix b has positive values only at first, the test "abs(b(m,n)) == b(m,n)" is useless.
Why do you check if "b(m,n)~=0" before multiplying with -1? In Matlab "-0" is the same as "0".
Accepted Answer
Andrei Bobrov
on 13 Dec 2011
idx = find(b)
b(idx(2:2:end)) = - b(idx(2:2:end))
more variant
[i1 j1] = find(b)
[m m] = unique(j1,'first')
k = sub2ind(size(b),i1(m+1) ,j1(m+1))
b(k) = -b(k)
7 Comments
Jan
on 13 Dec 2011
@Michael: Yes, it does not create the correct results.
It is always a good idea to test the code before accepting an answer, even if the code looks nice and even if it comes from a high-skilled programmer like Andrei.
More Answers (2)
Jan
on 13 Dec 2011
This negates every second element per column:
b = [1 2 1 0; 0 1.2 0 0; 0 0 0 3; 1.6 0 4 2.2];
gt0 = (b > 0);
even = mod(cumsum(gt0, 1) - 1, 2);
idx = and(gt0, even);
b(idx) = -b(idx);
[EDITED]: After reading your comment, I understand, that you want to negate one element per column only:
b = [1 2 1 0; 0 1.2 0 0; 0 0 0 3; 1.6 0 4 2.2];
gt0 = (b > 0);
idx = and(gt0, cumsum(gt0, 1) == 2);
b(idx) = -b(idx);
Or with a cleaner loop:
[nx, ny] = size(b);
for iy = 1:ny
isPositive = false;
for ix = 1:nx
if b(ix, iy) > 0
if isPositive
b(ix, iy) = -b(ix, iy);
break;
else
isPositive = true;
end
end
end
end
Daniel Shub
on 13 Dec 2011
While there are probably more efficient, and some would argue elegant, solutions, the real goal should be to do what you think makes the most sense. It is generally a bad idea to spend time trying to speed up sections of code until you know it is a bottle neck.
I like Andrei's solution, but it might be more confusing 6 months later to figure out what it is doing. I think loops often allow for easier documentation. I think a loop like yours with lots of useful comments is a very elegant solution.
My answer:
for column = 1:size(b, 2)
row = find(b(:, column), 1, 'last');
b(row, column) = -b(row, column);
end
4 Comments
Jan
on 13 Dec 2011
@Daniel: Aaaaarrrgh. "every second" and "not be anymore than 2 nonzero". I still do not get the question completely.
I'm taking a break now and have a cup of coffee. Please fix all problems here. Thanks!
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