# Re: [R] Extract from matrix

From: Gavin Simpson <gavin.simpson_at_ucl.ac.uk>
Date: Fri, 15 Feb 2008 17:10:40 +0000

On Fri, 2008-02-15 at 08:10 -0800, _Fede_ wrote:
> Hi all.
>
> I have certain problem with the extraction of data from matrices. I know how
> extract elements from a matrix, but what I would like to do is to extract a
> concrete number, not an element. For example if I have this matrix X:
>
> [,1] [,2] [,3] [,4]
> [1,] 3 13 23 33
> [2,] 9 19 29 39
> [3,] 10 20 30 40
>
> I can do X[-2] or X[2] in order to eliminate the element of the matrix or to
> extract it, respectively.
>
> But, what I need is to extract or to eliminate a value of the matrix. For
> example the 29.
>
> Can I do this? How?

Is this the sort of thing you want?

> mat <- matrix(c(3,9,10,13,19,20,23,29,30,33,39,40), ncol = 4)
> mat

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

```[1,]    3   13   23   33
[2,]    9   19   29   39
[3,]   10   20   30   40
```

> which(mat == 29)

[1] 8
> mat[which(mat == 29)]

[1] 29
> mat[which(mat == 29)] <- NA
> mat

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

```[1,]    3   13   23   33
[2,]    9   19   NA   39
[3,]   10   20   30   40

```

HTH G

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