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finding index of several values within a vector efficiently

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Jimmian Lin
Jimmian Lin 2012-12-2
I am trying to find the index of 'several' values within an array/vector. For instance, I have a vector [1 2 5] and matrix X=[1 5 4 3 2]. I want it to return [1 5 2]. I could write a for loop that finds each values in that vector such as find(X==1), then find(X==2) ..., but I was hoping there is a vectorized way to do this. Thanks.

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采纳的回答

the cyclist
the cyclist 2012-12-2
v = [1 2 5];
X = [1 5 4 3 2];
[tf loc] = ismember(v,X);
"loc" is the vector you want.

更多回答（1 个）

Matt Fig
Matt Fig 2012-12-2
编辑：Matt Fig 2012-12-2
idx = [1 2 5];
X = [1 5 4 3 2];
vals = X(idx) % linear indexing.
  2 个评论
the cyclist
the cyclist 2012-12-2
If I am interpreting Jimmian's question correctly, this method only gives the correct answer to the example because of a numerical coincidence. For example, if
X = [0 0 0 0 0 1 5 4 3 2]
then I think he wants the output to be
loc = [6 10 7]
and not
[0 0 0].
Matt Fig
Matt Fig 2012-12-2
编辑：Matt Fig 2012-12-2
Exactly how I interpreted your answer when I read it (works because of a coincidence).
:-)
It would be nice to have an example from OP that is unambiguous......

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