# [R] learning R

David Winsemius dwinsemius at comcast.net
Wed Feb 25 06:12:28 CET 2009

```On Feb 24, 2009, at 11:36 PM, Fuchs Ira wrote:

> I was wondering why the following doesn't work:
>
> > a=c(1,2)
> > names(a)=c("one","two")
> > a
> one two
>  1   2
> >
> > names(a[2])
> [1] "two"
> >
> > names(a[2])="too"
> > names(a)
> [1] "one" "two"
> > a
> one two
>  1   2
>
> I must not be understanding some basic concept here.
> Why doesn't the 2nd name change to "too"?

I cannot tell you why, perhaps you are not actually working with the
names of a, but I can show you that:

> names(a)[2] <- "too"
> a[2]
too
2
> a
one too
1   2

And this is seen as well in the help page examples. The help page also
says the following, which I cannot understand:

It is possible to update just part of the names attribute via the
general rules: see the examples. This works because the expression
there is evaluated as z <- "names<-"(z, "[<-"(names(z), 3, "c2")).

>
> also unrelated:  if I have two vectors and I want to combine them to
> form a matrix ,is cbind (or rbind) the most direct way to do this?
>
> e.g.
>
> x=c(1,2,3)
> y=c(3,4,5)
> z=rbind(x,y)

That is ok. Also effective would be:

z <- matrix( c(x,y), ncol=length(x) )
>
>
> alternatively: is there a way to make a matrix with dim=2,3 and then
> to replace the 2nd row with y
>
> something like this (which doesn't work but perhaps there is another
> way to do the equivalent?)
>
> attr(x,"dim")=c(2,3)
> x[2,]=y

Not sure why you are trying to do that to x since it is a vector but
it can be done if you make it a matrix first. Take a look at these and
see if you can figure out what is happening:

> x <- matrix(x,2,3)  # second and third arguments of matrix function
are nrow and ncol.
> x
[,1] [,2] [,3]
[1,]    1    3    2
[2,]    2    1    3

And then try:
> x = c(1,2,3)
> x <- matrix(x,2,3,byrow=TRUE)
> x
[,1] [,2] [,3]
[1,]    1    2    3
[2,]    1    2    3

And now that x is a matrix, this will work:

> x[2,] <- y
> x
[,1] [,2] [,3]
[1,]    1    2    3
[2,]    3    4    5

-- David Winsemius
>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help