Why in R are grep & !grep not logically consistent when grepl and !grepl ARE logically consistent?

Question

R functions grep and !grep (NOT grep) are not logically consistent; unlike grepl and !grepl (NOT grepl) which ARE logically consistent.

grepl returns a logical vector equal in length to the number of items being searched. For example, if the target is found in items 2 and 3 of a 5-item vector the following is returned:

FALSE TRUE TRUE FALSE FALSE 

If grepl is replaced by !grepl , then the "opposite" logical result is returned:

TRUE FALSE FALSE TRUE TRUE 

grep , on other hand returns a vector of the 2 positions of the found items: 2 3

What does !grep return in the same scenario? Logically it should return 1 4 5 , instead it returns FALSE FALSE . How can that be a logically consistent returned value? Can anyone explain?

Answer 1

You are looking for the invert argument to grep() .

From help(grep) , under Arguments :

invert - logical. If TRUE return indices or values for elements that do not match.

Sounds like exactly what you want. Let's see an example.

x <- c("ab", "cd", "bc", "def", "abc")

grep("b", x)
# [1] 1 3 5
grep("b", x, invert=TRUE)
# [1] 2 4

Answer 2

grep(value = FALSE) - returns a vector of the indices of the elements of x that yielded a match.

grepl returns a logical vector (match or not for each element of x).

grep vs grepl in R