1st el of a list of vectors

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l = list(c(1,2), c(3,5,6), c(7))

sapply(l, `[`, 1)
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
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Hi Carol,
l = list(c(1,2), c(3,5,6), c(7))

sapply(l, `[`, 1)
Using sapply() works but won't be very efficient if you have a very long
list. If you worry about efficiency, you can do the following (using the
IRanges package from Bioconductor):

   > library(IRanges)
   > eltlens <- elementLengths(l)
   > unlist(l, use.names=FALSE)[cumsum(eltlens) - eltlens + 1L]
   [1] 1 3 7

Only worth if the length of your list is > 100000 though...

Cheers,
H.

PS: See http://bioconductor.org/packages/release/bioc/html/IRanges.html
for how to install the IRanges package.
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]]

______________________________________________
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.

Herv? Pag?s

Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N, M1-B514
P.O. Box 19024
Seattle, WA 98109-1024

E-mail: hpages at fhcrc.org
Phone:  (206) 667-5791
Fax:    (206) 667-1319
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.
l = list(c(1,2), c(3,5,6), c(7))

sapply(l, `[`, 1)
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]]

______________________________________________
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.
______________________________________________
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.
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
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]]

______________________________________________
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.