On Jul 15, 2010, at 9:48 AM, AndrewPage wrote:
>
> Hi all,
>
> I'm trying to write a function that will search and extract from a long
> character string, but with a twist: I want to use the characters before and
> the characters after what I want to extract as reference points. For
> example, say I'm working with data entries that looks like this:
>
> Drink=Coffee:Location=Office:Time=Morning:Market=Flat
>
> Drink=Water:Location=Office:Time=Afternoon:Market=Up
>
> Drink=Water:Location=Gym:Time=Evening:Market=Closed
>
> Drink=Wine:Location=Restaurant:Time=LateEvening:Market=Closed
>
>
> ...
>
> For my function, I'd like to find what's located between "Location=", and
> ":Time=" in every instance, and extract it, to return something like
> "Office, Office, Gym, Restaurant".
>
> In a previous discussion I found
> (http://tolstoy.newcastle.edu.au/R/help/05/03/0344.html), someone wrote a
> function where you could find and substitute characters in a string, based
> on "pre" and "post" variables:
>
> interp <- function(x, e = parent.frame(), pre = "\\$", post = "" ) {
> for(el in ls(e)) {
> tag <- paste(pre, el, post, sep = "")
> if (length(grep(tag, x))) x <- gsub(tag, eval(parse(text = el),
> e), x)
> }
> x
> }
>
> I'm not sure how to modify it, however, to do what I want it to do. Any
> suggestions?
>
> Thanks in advance,
>
> Andrew
> Vec
[1] "Drink=Coffee:Location=Office:Time=Morning:Market=Flat"
[2] "Drink=Water:Location=Office:Time=Afternoon:Market=Up"
[3] "Drink=Water:Location=Gym:Time=Evening:Market=Closed"
[4] "Drink=Wine:Location=Restaurant:Time=LateEvening:Market=Closed"
> gsub(".*Location=(.+):Time=.*", "\\1", Vec)
[1] "Office" "Office" "Gym" "Restaurant"
This returns the back reference within the parens, found between the two
bounding sets of characters.
HTH,
Marc Schwartz
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