How to use indices returned from [Y,I] = min(X) when X is a 2D matrix?

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Gabe 2015-4-13

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评论： Titus Edelhofer 2015-4-13

I'm sure there is an answer here but I'm just not finding the right keywords...

I have a large matrix X, and I want the minimum values from each column. min(X) does exactly what I want.

But how would I use the indices returned in "I"? X(I) does not return the minimum values from min(X). Is there notation to address X columnwise, so that I would get back a row vector that matches min(X)?

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Answer 1

Jan 2015-4-13

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编辑：Jan 2015-4-13

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The replied indices are realted to each column. So you need sub2ind to get the linear index to obtain the minimal values:

x = rand(3, 3);
[v, ind] = min(x);
w = x(sub2ind(size(x), ind, 1:size(x, 2))));

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Gabe 2015-4-13

Thanks! sub2ind was just what I needed.

Titus Edelhofer 2015-4-13

Hi Gabe,

fine, please go ahead then and mark Jan's answer as "the" answer and the question answered ...

Thanks,

Titus

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Answer 2

Stephen23 2015-4-13

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编辑：Stephen23 2015-4-13

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Although the documentation is not very clear on this, the function min returns indices along the dimension that it is working along. So when you sum along each column, the indices are for the position in that column. You can use sub2ind to convert these indices into linear indices:

>> X = randi(9,3,3)
X =
     8     9     7
     6     3     4
     5     7     6 
>> [Y,I] = min(X,[],1)
Y =
     5     3     4
I =
     3     2     2
>> Z = sub2ind(size(X),I,1:size(X,2))
Z =
     3     5     8
>> X(Z)
ans =
     5     3     4