[R] subset using noncontiguous variables by name (not index)

[François Pinard]

[Muenchen, Robert A (Bob)]

>I'm using the subset function to select a list of variables, some of
>which are contiguous in the data frame, and others of which are not. It
>works fine when I use the form:

>subset(mydata,select=c(x1,x3:x5,x7))

>In reality, my list is far more complex. So I would like to store it in
>a variable to substitute in for c(x1,x3:x5,x7) but cannot get it to
>work. That use of the c function seems to violate R rules, so I'm not
>sure how it works at all. A small simulation of the problem is below.  

>mydata <- data.frame(
>  x1=c(1,2,3,4,5),
>  x2=c(1,2,3,4,5),
>  x3=c(1,2,3,4,5),
>  x4=c(1,2,3,4,5),
>  x5=c(1,2,3,4,5),
>  x6=c(1,2,3,4,5),
>  x7=c(1,2,3,4,5)
>)
>mydata

># This does what I want.
>summary(subset(mydata, select=c(x1, x3:x5, x7)))

Maybe:

  variables <- expression(c(x1, x3:x5, x7))

and later:

  summary(subset(mydata, select=eval(variables)))

However, I do not know how one computes the expression piecemeal, that 
is, better than by building a string and parsing the result.

-- 
François Pinard   http://pinard.progiciels-bpi.ca