Dear Julia,
Try this:

DF<-read.table(textConnection("
x1 x2 x3
1   2   3
4   5   6
7   8   9"),header=TRUE)
closeAllConnections()

DF$x4<-DF$x1+DF$x2
DF


HTH,

Jorge



On Tue, Jan 20, 2009 at 2:12 PM, Julia Zhou <ms.juliazhou@gmail.com> wrote:

> Hi,
>
> I would like to operate on certain columns in a dataframe, but not
> others. My data looks like this:
>
> x1 x2 x3
> 1   2   3
> 4   5   6
> 7   8   9
>
> I want to create a new column named x4 that is the sum of x1 and x2,
> but NOT x3. I looked at colSums and apply, but those functions seem to
> use all the columns in a dataframe. How do I only use select columns?
>
> If it helps, in Stata this would be gen x4 = x1 + x2.
>
> Thanks!
>
> ______________________________________________
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
>

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