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Short question: Using the find command

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MiauMiau
MiauMiau 2012-12-28
关闭： MATLAB Answer Bot 2021-8-20
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

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回答（2 个）

Andrei Bobrov
Andrei Bobrov 2012-12-28
[~,g1] = unique(X(:,3),'first');
G = sort(g1);
Jan
Jan 2012-12-28
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.

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