CODEWITHSUNDEEP
matlabmatrix
Farid
Farid

Reputation: 23

How to index a matrix with the column maxima of other matrix

I have 2 matrices A and B.

I find the max values in the columns of A, and keep their indices in I. So far so good. Now, I need to choose those arrays of B with the same index as stored in I. I don't know how to do this. See below:

A = [1,2,3; 0,8,9]
B = [0,1,2; 4,2,3]
[~,I] = max(A)
h = B(I)

I need to get these values of B:

h = [0 2 3]

But the code results in a different one. How can I fix it?

A =

 1     2     3
 0     8     9


B =

 0     1     2
 4     2     3


I =

 1     2     2


h =

 0     4     4

Thanks in advance

Upvotes: 2

Views: 349

Answers (1)

Robert Seifert
Robert Seifert

Reputation: 25232

The max function how you used it works like

If A is a matrix, then max(A) is a row vector containing the maximum value of each column.

so M = max(A) is equivalent to M = max(A,[],1). But rather use the third input if you're not sure.

If you use max to find the maxima in the columns of the matrix, it returns the row indices. The column indices are for your case simply 1:size(A,2) = [1 2 3].

Now you need to convert your row and column indices to linear indices with sub2ind:

%// data
A = [1,2,3; 0,8,9]
B = [0,1,2; 4,2,3]

%// find maxima of each column in A
[~, I] = max( A, [], 1 )  %// returns row indices

%// get linear indices for both, row indices and column indices
I = sub2ind( size(A), I, 1:size(A,2) ) 

%// index B
h = B(I)

returns:

h =

     0     2     3

Upvotes: 2

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