R: Get indices of all elements in a vector?

Question

Simple questions, but I can\'t find an answer.

I have a vector

a<-c(5,6,7)

how can I get back the indices for all elements in my vec

Answer 1

If you like something elementary, you can try:

a<-c(5,6,7)
b <- 1:length(a)

Answer 2

This is the purpose served by the seq_along function:

seq_along(a)
[1] 1 2 3
> cbind( seq_along(a), a)
         a  
[1,] "1" "a"
[2,] "2" "b"
[3,] "3" "c"
>  data.frame( sq = seq_along(a), a)
  sq a
1  1 a
2  2 b
3  3 c