[R] 1st el of a list of vectors

[arun]

Or
rapply(l,function(x) x[1])
#[1] 1 3 7


set.seed(42)
 l1 <- replicate(1e6, list(sample(1:5,sample(8),replace=T)))
system.time(r1 <- sapply(l1, `[`, 1))
 #  user  system elapsed 
 # 1.324   0.000   1.326 

system.time(r2 <- rapply(l1, function(x) x[1]))
#   user  system elapsed 
#  0.736   0.004   0.741 

identical(r1,r2)
#[1] TRUE

system.time({
eltlens <- elementLengths(l1)
 r3 <- unlist(l1, use.names=FALSE)[cumsum(eltlens) - eltlens + 1L]
})
# user  system elapsed 
#  0.153   0.000   0.154 


A.K.


On Tuesday, July 22, 2014 12:11 AM, Richard M. Heiberger <rmh at temple.edu> wrote:
l = list(c(1,2), c(3,5,6), c(7))

sapply(l, `[`, 1)

On Mon, Jul 21, 2014 at 3:55 PM, carol white <wht_crl at yahoo.com> wrote:
> Hi,
> If we have a list of vectors of different lengths, how is it possible to retrieve the first element of the vectors of the list?
>
>
> l = list(c(1,2), c(3,5,6), c(7))
>
> 1,3,7 should be retrieved
>
> Thanks
>
> Carol
>         [[alternative HTML version deleted]]
>
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