Short question: Using the find command

Hi

I have a matrix X with three columns, say column 1 stores attribute a, column 2 sotres attribute b, and column 3 stores attribute c. Attribute c is not unique (same value might appear in different lines). What I want to do is to create a matrix, such that for each specific value of c, it would show me in which lines of X it is to be find. Why can I not use the following line:

    G = find(X(:,3) == unique(X(:,3)))

to do so? Thanks

Answer 1

[Andrei Bobrov]

[~,g1] = unique(X(:,3),'first');
G = sort(g1);

Answer 2

[Jan]

You cannot use

find(X(:,3) == unique(X(:,3))),   % !!! wrong

because X(:,3) and unique(X(:, 3)) have a different number of elements in general. The result of

[1,2,3] == [1,2,3,4]

cannot be defined easily.

As Andrei has suggested already, the 2nd output of unique() will help you to solve the problem.