[R] to modify a matrix

[John Fox]

Dear Mitsuo,

It isn't completely clear to me what the criterion is for selecting 
elements, but if you want to change 8 to 40, but aren't sure where 8 is (or 
8s are), then the following will work:

         x[x==8] <- 40

The trick here is to index the matrix as a vector (since matrices in R are 
vectors with a dim attribute).

More generally, there's a great deal of free information available about 
how to use R that can help you to answer questions like this, including 
manuals supplied with the R distribution and other material available from 
the R web site.

John

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
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
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