R help - Aug 2008 - parse and eval character vector

Dear R-help,

I have a character vector, some elements will be numeric, some not,
and some even empty. E.g.:

temp1 <- c("abcd","  2 ","")

I'm only interested in the numeric elements, the rest I can just throw
away. It is easy enough to loop through the vector:

temp <- try(eval(parse(text=temp1[1])), silent=TRUE); class(temp) # try-error
temp <- try(eval(parse(text=temp1[2])), silent=TRUE); class(temp) # numeric
temp <- try(eval(parse(text=temp1[3])), silent=TRUE); class(temp) # NULL

and then throw away the non-numeric/NULL stuff. But, as this vector
will be long, I would really like to speed things up by not using a
loop, and I thought that "lapply" might do the trick. However:

temp.fn <- function(x)
  try(eval(parse(text=x)), silent=TRUE)

temp2 <- lapply(temp1, FUN=temp.fn)
class(temp2[2]) # list, for elements 1, 2, and 3

and I don't know how to extract the numeric elements from here. So,
can I either use lapply as above and somehow get the information I
need out of "temp2" (I've tried using "unlist" but had
no success), or
is there some other function that I can apply to my character vector
to avoid looping?

Rob.

Just use as.numeric. Non numeric will be NA. So the solution of your
problem is na.omit(as.numeric(temp1))

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx op inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces op r-project.org [mailto:r-help-bounces op r-project.org]
Namens Rob Foxall
Verzonden: dinsdag 26 augustus 2008 10:36
Aan: r-help op r-project.org
Onderwerp: [R] parse and eval character vector

Dear R-help,

I have a character vector, some elements will be numeric, some not,
and some even empty. E.g.:

temp1 <- c("abcd","  2 ","")

I'm only interested in the numeric elements, the rest I can just throw
away. It is easy enough to loop through the vector:

temp <- try(eval(parse(text=temp1[1])), silent=TRUE); class(temp) #
try-error
temp <- try(eval(parse(text=temp1[2])), silent=TRUE); class(temp) #
numeric
temp <- try(eval(parse(text=temp1[3])), silent=TRUE); class(temp) # NULL

and then throw away the non-numeric/NULL stuff. But, as this vector
will be long, I would really like to speed things up by not using a
loop, and I thought that "lapply" might do the trick. However:

temp.fn <- function(x)
  try(eval(parse(text=x)), silent=TRUE)

temp2 <- lapply(temp1, FUN=temp.fn)
class(temp2[2]) # list, for elements 1, 2, and 3

and I don't know how to extract the numeric elements from here. So,
can I either use lapply as above and somehow get the information I
need out of "temp2" (I've tried using "unlist" but had
no success), or
is there some other function that I can apply to my character vector
to avoid looping?

Rob.

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