[R] Extract names from vector according to their values

[David Winsemius]

On Aug 2, 2011, at 2:21 PM, Sverre Stausland wrote:

> Dear helpers,
>
> I can create a vector with the priority of the packages that came with
> R, like this:
>
>> installed.packages()[,"Priority"]->my.vector
>> my.vector
>         base          boot         class       cluster     codetools
>       "base" "recommended" "recommended" "recommended" "recommended"
>     compiler      datasets       foreign      graphics     grDevices
>       "base"        "base" "recommended"        "base"        "base"
>         grid    KernSmooth       lattice          MASS        Matrix
>       "base" "recommended" "recommended" "recommended" "recommended"
>      methods          mgcv          nlme          nnet         rpart
>       "base" "recommended" "recommended" "recommended" "recommended"
>      spatial       splines         stats        stats4      survival
> "recommended"        "base"        "base"        "base" "recommended"
>        tcltk         tools         utils
>       "base"        "base"        "base"
>
> How can I extract the names from this vector according to their
> priority? I.e. I want to create a vector from this with the names of
> the "base" packages, and another vector with the names of the
> "recommended" packages.

 > names( my.vector[which(my.vector=="recommended")])
  [1] "boot"       "class"      "cluster"
  [4] "codetools"  "foreign"    "KernSmooth"
  [7] "lattice"    "MASS"       "Matrix"
[10] "mgcv"       "nlme"       "nnet"
[13] "rpart"      "spatial"    "survival"

Note that some people may tell you that this form below should be  
preferred because the 'which' is superfluous. It is not. The "["  
function returns all the NA's fr reasons that are unclear to me. It is  
wiser to use `which` so that you get numerical indexing.
 > names(my.vector[my.vector=="recommended"])

On my system it produces 493 items most of them NA's.

-- 

David Winsemius, MD
West Hartford, CT