find repeated columns of a matrix

Hello,

Consider a 8x100 matrix.

It consists of 5 8x1 columns that are repeated randomly, namely the col1,...,col5.

How can I find the indexes of the columns that contain the col1?

Thank you.

Pavlos

Answer 1

[Azzi Abdelmalek]

A=repmat(rand(8,5),1,100/5)
A=A(:,randperm(100)) % your matrix
col1=A(:,1)
idx=find(~any(bsxfun(@minus, A,col1)))

Answer 2

[Matt J]

indices=find(ismember(yourmatrix.',col1.','rows'))

[Sean de Wolski]

Better answer if you only care about one column.

[Sean de Wolski]

Actually using the two output option of ismember, better all around:

[ix iy] = ismember([3 4;1 2;1 2],[3 4; 1 2],'rows')

[Matt J]

Hah! I hit 500 points because of your upvote. Thanks!

[pavlos]

thanks for your help