combine specific objects in a list

Question

"a" is a list.

> a<-list(1,2,3,c(4,5),6,7)
> a
[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
[1] 3

[[4]]
[1] 4 5

[[5]]
[1] 6

[[6]]
[1] 7

"b" is a transform indicator.

b<-c(3,2,1)

I want group(or combine) objects in "a" according to the number in "b", that means the first 3 objects group together, then the next two, finally the last, the expected result is as follows:

[[1]]
[1] 1 2 3

[[2]]
[1] 4 5 6

[[3]]
[1] 7

I can only use "cumsum" to sum the three groups, but don't know how to display each object as listed above. Thanks.

Answer 1

tapply(a, rep(seq_along(b), b), Reduce, f = `c`)
$`1`
[1] 1 2 3

$`2`
[1] 4 5 6

$`3`
[1] 7

Answer 2

Another option is

lapply(split(a, cumsum(sequence(b)==1)), unlist)
#$`1`
#[1] 1 2 3

#$`2`
#[1] 4 5 6

#$`3`
#[1] 7