[R] How to choose columns in data.frame by parts of columns' names?

[Gabor Grothendieck]

On 5/30/06, Guo Wei-Wei <wwguocn at gmail.com> wrote:
> Dear all,
>
> I have a data.frame which has names as following.
> [1] "XG1"  "YG1"  "XEST"     "YEST"
> [2] "XNOEMP1"  "XNOEMP2" "YNOEMP1"  "YNOEMP2"
> [3] "XBUS10"   "XBUS10A" "XBUS10B"  "XBUS10C"
> [4] "YBUS10"   "YBUS10A"  "YBUS10B"  "YBUS10C"
> [5] "XOWNBUS"  "XSELFEST"  "YOWNBUS"  "YSELFEST"
>
> Those columns have names beginning with "X" or "Y". Each "X" is paired
> by a "Y", e.g. "XG1" and "YG1", but they are not in the order of "X Y
> X Y ...". I want to combine "X*" and "Y*" like this:
>
> data.new[,"G1"]  <- (data.old[,"XG1"] + endata.use[,"YG1"])/2
>
> How to choose columns by parts of names? For example, I can pick out
> XG1 and YG1 because they have the common part "G1".
>


This gives all columns whose column name contains G1:

data.old[, regexpr("G1", colnames(data.old)) > 0]