How to grep a word exactly

Question

I'd like to grep for "nitrogen" in the following character vector and want to get back only the entry which is containing "nitrogen" and nothing of the rest (e.g. nitrogen fixation):

varnames=c("nitrogen", "dissolved organic nitrogen", "nitrogen fixation", "total dissolved nitrogen", "total nitrogen")

I tried something like this:

grepl(pattern= "![[:space:]]nitrogen![[:space:]]", varnames)

But this doesn't work.

Answer 1

Although Dason's answer is easier, you could do an exact match using grep via:

varnames=c("nitrogen", "dissolved organic nitrogen", "nitrogen fixation", "total dissolved nitrogen", "total nitrogen")

grep("^nitrogen$",varnames,value=TRUE)
[1] "nitrogen"

grep("^nitrogen$",varnames)
[1] 1

Answer 2

To get the indices that are exactly equal to "nitrogen" you could use

which(varnames == "nitrogen")

Depending on what you want to do you might not even need the 'which' as varnames == "nitrogen" gives a logical vector of TRUE/FALSE. If you just want to do something like replace all of the occurances of "nitrogen" with "oxygen" this should suffice

varnames[varnames == "nitrogen"] <- "oxygen"