enum() creates a vector of integers from 1 to length of the object (it
enumerates items in the object), except if the object is empty. It is
particularly useful in the for(i in enum(object)) construct.
Note
The pattern for(i in 1:length(object)) is often found, but it fails
in case length(object) == 0! enum() is indeed a synonym of seq_along(),
but the later one is less expressive in the context.
Examples
enum(letters)
#> [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
#> [26] 26
enum(numeric(0))
#> integer(0)
# Compare with:
1:length(numeric(0))
#> [1] 1 0
enum(NULL)
#> integer(0)
letters5 <- letters[1:5]
for (i in enum(letters5)) cat("letter", i, "=", letters5[i], "\n")
#> letter 1 = a
#> letter 2 = b
#> letter 3 = c
#> letter 4 = d
#> letter 5 = e