Update matrix using vector of column index
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Georgio Mazarin
on 25 Mar 2017
Commented: Georgio Mazarin
on 25 Mar 2017
I have a 2-dim matrix like this:
0.3436 0.3283 0.3890 0.0274
0.1137 0.4443 0.3484 0.0889
0.2748 0.3124 0.4074 0.0439
and a vector with column indexes for each row
3
2
2
Task is to invert values to get result like this:
0.3436 0.3283 -0.3890 0.0274
0.1137 -0.4443 0.3484 0.0889
0.2748 -0.3124 0.4074 0.0439
I believe there is a simple way to do that efficiently without for-loop cycles. Any idea?
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Accepted Answer
the cyclist
on 25 Mar 2017
Edited: the cyclist
on 25 Mar 2017
M = [0.3436 0.3283 0.3890 0.0274
0.1137 0.4443 0.3484 0.0889
0.2748 0.3124 0.4074 0.0439];
v = [3 2 2];
[m,n] = size(M);
linearIndex = sub2ind([m n],1:m,v);
M(linearIndex) = -M(linearIndex)
Note that the 2nd and 3rd arguments to sub2ind have to be the same shape, so make sure they are both column vectors, or both row vectors, but not mixed.
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