[R] applying a function in list of indexed elements of a vector:

[aldi]

Hi,

I have a vector
Y1 <-c(8, 11,  7,  5,  6,  3,  6, 3,  3)
and an index
iy <-c(c(1, 2),c(1 2), c(1, 2, 3, 4), c(2, 3, 5), c(4), c(5, 6, 7), c(7, 
8, 9))

how can I produce the mean, or the sum of the elements specified in the 
index iy from the vector Y1?

expecting something like this for the sum:
Y2
19 19 31 24 5 15 12

I thought lapply function may perform this, but does not work:
Y2<-lapply(Y1[iy],sum)

Any suggestion?
TIA,

Aldi

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