question on indexing: How to extract rows from a matrix that crossponds to certain values in another vector?

Ro =

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

Given an arbitart matrix like this such that for every row in Ro crossponds a rank in ranks:

     ranks=[1;2;3;4;3]

I need to extract all the rows in Ro that crossponds to ranks=3 but I need some general rule because the matrices Ro and ranks changes in the code

I thought of the following:

I made a function:

function [ pos ] = Position( x,n )

%x is the input row vector
%n is the number (in x) that we want to find its position(s) vector w.r.t
%x

pos=find(x==n);

end

FF=[];
P=Position(ranks,3);
lenP=length(P);
for i=1:lenP
    FF=[FF;Ro(P(i),1:end)]
end

The problem is that FF=[1 0 0 0; 1 0 0 0];

I need it only be FF=[ 1 0 0 0]; I don't want repition

so can you help :)?

Answer 1

[JESUS DAVID ARIZA ROYETH]

FF=unique(Ro(ranks==3,:),'rows')

Answer 2

[Stephen23]

Just call find with its optional 2nd and 3rd input arguments to specify that you only want one result returned:

>> n = 3;
>> pos = find(ranks==n, 1, 'first');
>> FF = Ro(pos,:)
FF =
   1   0   0   0