R grep(): How to search for letter "l"? - r

Question

I have a problem with R's grep() function apparently finding an "l" everywhere:
> l <- list(list(), list("a"), list("a","l"))
> grep("a",l)
[1] 2 3
> grep("l",l)
[1] 1 2 3
> grep("l",l,fixed=TRUE)
[1] 1 2 3
This problem seems to occur only with the letter "l". Does anyone have a hint on that?
Many thanks,
Cord

Answer 1

If you look at the documentation for the argument x in grep you'll see that it should be
a character vector where matches are sought, or an object which can be coerced by as.character to a character vector.
If you try that operation you'll see what goes wrong:
> as.character(l)
[1] "list()" "list(\"a\")" "list(\"a\", \"l\")"
so the same "problem" happens if you grep for i, s etc.
You could try the following instead
sapply(l, function(i) grep("l", i))
which produces
[[1]]
integer(0)
[[2]]
integer(0)
[[3]]
[1] 2

Answer 2

Interesting post, I never knew grep convert the x vector like this:
l <- list(list(), list("a"), list("a","l"))
l
#> [[1]]
#> list()
#>
#> [[2]]
#> [[2]][[1]]
#> [1] "a"
#>
#>
#> [[3]]
#> [[3]][[1]]
#> [1] "a"
#>
#> [[3]][[2]]
#> [1] "l"
Internally grep is converting l to a character vector
grep
#> function (pattern, x, ignore.case = FALSE, perl = FALSE, value = FALSE,
#> fixed = FALSE, useBytes = FALSE, invert = FALSE)
#> {
#> if (!is.character(x))
#> x <- structure(as.character(x), names = names(x))
#> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#> .Internal(grep(as.character(pattern), x, ignore.case, value,
#> perl, fixed, useBytes, invert))
#> }
#> <bytecode: 0x0000000012e18610>
#> <environment: namespace:base>
So now l is actually:
structure(as.character(l), names = names(l))
#> [1] "list()" "list(\"a\")" "list(\"a\", \"l\")"
Which has "l" in each.
You could unlist l first to get expected results:
ul <- unlist(l)
ul
#> [1] "a" "a" "l"
grep("a",l)
#> [1] 2 3
grep("a",ul)
#> [1] 1 2
grep("l",l)
#> [1] 1 2 3
grep("l",ul)
#> [1] 3
grep("l",l,fixed=TRUE)
#> [1] 1 2 3
grep("l",ul,fixed=TRUE)
#> [1] 3