Why this behavior when coercing a list to character via as.character()?

For non-trivial lists, as.character uses deparse to generate the strings.

1. Only if the vector is integer and 1,2,3,...,n - then it deparses as 1:n.

   c(1,2,3) is double whereas 1:3 is integer...

2. No idea :-)

...but look at deparse if you want to understand as.character here:

deparse(c(1L, 2L, 3L)) # 1:3
deparse(c(3L, 2L, 1L)) # c(3L, 2L, 1L)
deparse(c(1, 2, 3))    # c(1, 2, 3)

The help file does say

For lists it deparses the elements individually, except that it extracts the first element of length-one character vectors.

I'd seen this before in trying to answer a question [not online] about grep. Consider:

> x <- list(letters[1:10],letters[10:19])
> grep("c",x)
[1] 1 2

grep uses as.character on x, with the result that, since both have c( in them, both components match. That took a while to figure out.

On "Why does it do this?", I'd guess that one of the members of R core wanted it to do this.