Create a numeric vector from 1 to n each time, given a vector of ns

Question

It sounds confusing from the title, but it should be very clear with the following examples.

Say I have the following vector a:

a=c(3,10,6,7)

I want a vector b, so that it covers from 1 to each of the numbers in a each time, resulting in:

b=c(1,2,3,1,2,3,4,5,6,7,8,9,10,1,2,3,4,5,6,1,2,3,4,5,6,7)

To complete the task, if a number in a is ==1, I don't want that 1 alone, but rather an NA. In this case, if a is:

a=c(3,10,6,1,7)

b should be:

b=c(1,2,3,1,2,3,4,5,6,7,8,9,10,1,2,3,4,5,6,NA,1,2,3,4,5,6,7)

How should I do that easily with base tools? Thanks!

Answer 1

Yes, sequence(a) should be preferred here over, e.g., unlist(lapply(a, seq_len)). And once you work out where the length-1 subsequences begin, you can subassign NA:

a <- c(3, 10, 6, 1, 7)
replace(sequence(a), cumsum(a)[a == 1], NA)
##  [1]  1  2  3  1  2  3  4  5  6  7
## [11]  8  9 10  1  2  3  4  5  6 NA
## [21]  1  2  3  4  5  6  7