[R] 1st el of a list of vectors

[Bert Gunter]

Yes, but note that such comparisons don't necessarily tell you much.
Using your l1 on my computer:

> system.time(r1 <- sapply(l1, `[`, 1))
   user  system elapsed
    1.2     0.0     1.2
> system.time(r2<- rapply(l1,function(x)x[1]))
   user  system elapsed
   0.81    0.00    0.81

## But

> system.time(r3<- unlist(lapply(l1,`[`,1)))
   user  system elapsed
   0.64    0.00    0.66
 ## and

> system.time(r4<- vapply(l1,`[`,1,1))
   user  system elapsed
   0.60    0.00    0.61

sapply() takes a bit of extra time to get the return into the right
data structure. You tell vapply the return type and lapply always
returns a list.

-- Bert



Bert Gunter
Genentech Nonclinical Biostatistics
(650) 467-7374

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
Clifford Stoll




On Tue, Jul 22, 2014 at 1:14 AM, arun <smartpink111 at yahoo.com> wrote:
> 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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>
> ______________________________________________
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.