Question

If I want to number all elements in two vectors, vector 1 gets all odd bumbers and vector 2 gets all even numbers, I can do this assuming the vectors are of length 10.

seq(1, 10, by=2)
[1] 1 3 5 7 9

seq(2, 11, by=2)
[1]  2  4  6  8 10

but if my vector has only one element I will run into problems:

 seq(2)
[1] 1 2

so I use:

seq_along(2)
[1] 1

BUT I cant use by= in seq_long(). How do i get the reliability of seq_along with the functionality of seq()?

---

This example might clear things.

Imagine I ahve two lists:

list1 <- list(4)
list2 <- list(4)

list1 must get even names along the element of the list.
list2 must get odd names along the element of the list.

I dont know how long the list elements will be.

seq_along(list1[[1]]) # this will know to only give one name but I cant make it even

seq(list2[[1]]) # this know to give 1 name
#and
seq(2, list1[[1]], by=2) # this gives me even but too nay names

Answer 1

Here’s a function that adds a ‘by’ argument to seq_along:

seq_along_by = function(x, by=1L, from = 1L) (seq_along(x) - 1L) * by + from

and some test cases

> seq_along_by(integer(), 2L)
integer(0)
> seq_along_by(1, 2L)
[1] 1
> seq_along_by(1:4, 2L)
[1] 1 3 5 7
> seq_along_by(1:4, 2.2)
[1] 1.0 3.2 5.4 7.6
> seq_along_by(1:4, -2.2)
[1]  1.0 -1.2 -3.4 -5.6