# [R] to modify a matrix

Sun Feb 9 10:24:03 CET 2003

```May I suggest you learn the handy function called which().

yes.rows <- which(x[,1]==3)        # Row that meets this condition
x[ yes, 2] <- x[ yes.rows, 2] * 5  # Perform the desired change for the
row that meets this condition.

At 12:04 AM 2/9/2003 +0900, Mitsuo Igarashi wrote:
>Hi All.
>
>I am quite a newbie to R.
>This is a next basic question.
>
>I have a matrix;
> > x <- matrix(1:10.,5)
> > x
>      [,1] [,2]
>[1,]    1    6
>[2,]    2    7
>[3,]    3    8
>[4,]    4    9
>[5,]    5   10
>
>I like to get a modified matrix as follows;
>      [,1] [,2]
>[1,]    1    6
>[2,]    2    7
>[3,]    3    8 * 5 -> 40
>[4,]    4    9
>[5,]    5   10
>
>The following expression does not work.
>  if (x[x[,1]==3])  x[x[,1]==3],2]*5
>
>How can I obtain a right expresion to get a modified matrix?

-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

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