CODEWITHSUNDEEP
rlist
lightsnail
lightsnail

Reputation: 788

combine specific objects in a list

"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.

Upvotes: 4

Views: 74

Answers (3)

thelatemail
thelatemail

Reputation: 93813

Another attempt:

grps <- rep(rep(seq_along(b), b), lengths(a))
relist(unlist(a), split(grps,grps) )

#$`1`
#[1] 1 2 3
#
#$`2`
#[1] 4 5 6
#
#$`3`
#[1] 7

Upvotes: 1

akrun
akrun

Reputation: 886948

Another option is

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

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

#$`3`
#[1] 7

Upvotes: 2

Julius Vainora
Julius Vainora

Reputation: 48191

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

$`2`
[1] 4 5 6

$`3`
[1] 7

Upvotes: 4

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