Using system.time() you can check out the performance yourself. E.g.,
the following compares head(y,-1), y[-length(y)], and y[seq_len(length(y)-1))].
> y <- 1:10
> all.equal(head(y,-1), y[-length(y)])
[1] TRUE
> all.equal(head(y,-1), y[seq_len(length(y)-1)])
[1] TRUE
> system.time(for(i in 1:1e5)head(y,-1))
user system elapsed
1.53 0.00 1.53
> system.time(for(i in 1:1e5)y[-length(y)])
user system elapsed
0.09 0.00 0.10
> system.time(for(i in 1:1e5)y[seq_len(length(y)-1)])
user system elapsed
0.11 0.00 0.11
Performance may depend on the size or type of the input, so check it on
data similar to what you intend to work with. E.g., for long vectors the
seq_len approach looks better.
> y <- runif(1e6)
> system.time(for(i in 1:100)head(y,-1))
user system elapsed
1.26 0.25 1.51
> system.time(for(i in 1:100)y[-length(y)])
user system elapsed
1.10 0.36 1.45
> system.time(for(i in 1:100)y[seq_len(length(y)-1)])
user system elapsed
0.67 0.28 0.95
Also note that the relative performance of these idioms may change as R evolves.
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
> -----Original Message-----
> From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
> Behalf
> Of Sepp Tannhuber
> Sent: Saturday, August 25, 2012 2:24 AM
> To: r-help@r-project.org
> Subject: Re: [R] extract vector elements of unknown range
>
> Hi,
>
> thanks for your quick answers! These solve my problem.
>
> Now I have another question. I think I can use
> head(y, -1)
> instead of
> y[-length(y)]
>
> Are there differences in terms of performance?
>
> Best regards
> Joseph
>
> ______________________________________________
> R-help@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.
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
