R: selecting element from list

Question

I have two elements:

id1 <- "dog"
id2 <- "cat"

I want to extract any combination of these elements (dogcat or catddog) from a vector

L <- c("gdoaaa","gdobbb","gfoaaa","ghobbb","catdog")
L

I tried:

L[grep(paste(id1,id2,sep="")),L]
L[grep(paste(id2,id1,sep="")),L]

but this gives an error.

I would be grateful for your help in correcting the above.

L[grep(paste(id1,id2,sep=""),L)] L[grep(paste(id2,id1,sep=""),L)] — HubertL
– HubertL, Commented Mar 8, 2016 at 23:42
A simple non-regex solution could be grepl(id1, L) & grepl(id2, L). You can add fixed = TRUE to both if efficiency is important. — David Arenburg
– David Arenburg, Commented Mar 8, 2016 at 23:44
I don't understand it, but apparently grepl("(dog(cat)?)", L) works courtesy of stackoverflow.com/questions/1177081/… — thelatemail
– thelatemail, Commented Mar 9, 2016 at 0:32
That pattern matches dogcat but also dog, as in: grepl("(dog(cat)?)", "dog"). — effel
– effel, Commented Mar 9, 2016 at 1:09

effel · Accepted Answer · 2016-03-09 01:28:26Z

2

The error is from misplaced parentheses, so these minor variations on your code will work.

L[grep(paste(id1,id2,sep=""), L)]
# character(0)
L[grep(paste(id2,id1,sep=""), L)]
# [1] "catdog"

Alternatively this is a regex one-liner:

L[grep(paste0(id2, id1, "|", id1, id2), L)]
# [1] "catdog"

That and some patterns in the comments will also match dogcatt. To avoid this you could use ^ and $ like so:

x <- c("dogcat", "foo", "catdog", "ddogcatt")
x[grep(paste0("^", id2, id1, "|", id1, id2, "$"), x)]
# [1] "dogcat" "catdog"

answered Mar 9, 2016 at 1:15

effel

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