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## Re: Easy about Matrices

 From: Mike Miller Subject: Re: Easy about Matrices Date: Mon, 18 Apr 2005 18:55:38 -0500 (CDT)

```On Mon, 18 Apr 2005, Quentin Spencer wrote:

```
I'm not completely sure if this is what you're looking for, but maybe this example helps:
```
x = randn(1,1000);
x_positive = x(find(x>=0));
x_negative = x(find(x<0));
```
```

```
That seems like it must have been an excellent answer for Alvaro. I have a related question. Suppose I have a vector like this:
```
X=[1, 3, 9, 2, 4, 2, 7, 5, 3, 2, 9, 7]';

```
And suppose I want the indices of all elements of X that are in this vector Y:
```
Y=[2, 3, 9]';

I can see how to do it with a loop...

index=find(X==Y(1)); for i=2:length(Y), index=[index ; find(X==Y(i))]; end

```
...but can it be done without a loop? (Also, my method is a little awkward and I wouldn't mind hearing about how I could do it better.)
```
Mike

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