collect rows of a matrix having a desired number in the first row

Question

Christopher 2014-11-21

评论： Azzi Abdelmalek 2014-11-21

If I have a matrix, what is the fastest way to collect the columns which satisfy a logical operation in the first row?

So if I have the matrix

5 3 6 4 5 8 4 7 5 1
6 7 5 6 7 8 9 4 5 2
4 3 5 8 3 5 4 4 5 1
6 7 1 6 3 7 4 5 3 8
2 9 5 0 4 2 5 6 3 4

if I wanted to collect all rows which have a 5 in the first row I would have

It is important that it is as fast as possible.

Answer 1

Azzi Abdelmalek 2014-11-21

编辑：Azzi Abdelmalek 2014-11-21

A=[0 5 3 6 4 5 8 4 7 5 1
5 6 7 5 6 7 8 9 4 5 2
3 4 3 5 8 3 5 4 4 5 1
9 6 7 1 6 3 7 4 5 3 8
9 2 9 5 0 4 2 5 6 3 4]
B=A(:,A(1,:)==5)

Christopher 2014-11-21

Sorry I made a mistake. How, instead, do I get the following index from the original matrix:

1 0 0 0 1 0 0 0 1 0
1 0 0 0 1 0 0 0 1 0
1 0 0 0 1 0 0 0 1 0
1 0 0 0 1 0 0 0 1 0
1 0 0 0 1 0 0 0 1 0

Azzi Abdelmalek 2014-11-21

repmat(A(1,:)==5,size(A,1),1)