Error in parse(text = expression) : <text>:2:1: unexpected ','

Question

Solutions to prior similar questions 1 and 2 do not work.

f_letter <- 'a'
l_letter <- 'c'
sep <- c(':', ',')
to <- 1
and <- 2

expression <- noquote(c(which(letters == f_letter), 
   sep[and], which(letters == l_letter)))
expression

eval(parse(text=expression))

Error in parse(text = expression) : :2:1: unexpected ',' 1: 1 2: , ^

Expected output:

#for sep[and]
1 3

#for sep[to]
1 2 3

Answer 1

Based on question 1, you could try a similar approach.

First you should use c instead of , since the first is a function, the latter is not. So you could do something like this:

f_letter <- 'a'
l_letter <- 'c'
sep <- c(":", "c")
to <- 1
and <- 2

do.call(sep[to], list( which(letters == f_letter), which(letters == l_letter) ))
[1] 1 2 3


do.call(sep[and], list( which(letters == f_letter), which(letters == l_letter) ))
[1] 1 3