Create integer sequences defined by 'from' and 'to' vectors

Question

I have two vectors which define start (from) indices and finish (to) indices:

Start = c(1, 10, 20)
Finish = c(9, 19, 30)

I want to create a list of all Start:Finish sequences along the two vectors, i.e. generate the sequences Start[1]:Finish[1] (1:9); Start[2]:Finish[2], and so on.

## [[1]]
## [1] 1 2 3 4 5 6 7 8 9
## 
## [[2]]
##  [1] 10 11 12 13 14 15 16 17 18 19
## 
## [[3]]
##  [1] 20 21 22 23 24 25 26 27 28 29 30

Preferably in some vectorized way. The values in 'Start' vector will always be larger than the corresponding elements in 'Finish' vector.

Answer 1

Just use mapply:

Start = c(1,10,20)
Finish = c(9,19,30)
mapply(":", Start, Finish)
## [[1]]
## [1] 1 2 3 4 5 6 7 8 9
## 
## [[2]]
##  [1] 10 11 12 13 14 15 16 17 18 19
## 
## [[3]]
##  [1] 20 21 22 23 24 25 26 27 28 29 30
## 

You could, of course, also use Vectorize, but that's just a wrapper for mapply. However, Vectorize cannot be used with primitive functions, so you'll have to specify seq.default rather than seq, or seq.int.

Example:

Vectorize(seq.default)(Start, Finish)
## [[1]]
## [1] 1 2 3 4 5 6 7 8 9
## 
## [[2]]
##  [1] 10 11 12 13 14 15 16 17 18 19
## 
## [[3]]
##  [1] 20 21 22 23 24 25 26 27 28 29 30
## 

Answer 2

Agree with @ColonelBeauvel and @nicola, though you could use seq instead of :, hence

Start = c(1, 10, 20)
Finish = c(9, 19, 30)
Map(seq, Start, Finish)