R help - Aug 2012 - extract vector elements of unknown range

Dear all,

I have two vectors
? x <- c(1:20)
? y <- c(1,5,10,14)

Now I would like to extract
? x[ (y[n] + 2):(y[n+1] - 1) ]
for all elements except last one in y. This means I want to have
??x[ c( (y[1]+2):(y[2]-1), (y[2]+2):(y[3]-1), (y[3]+2):(y[4]-1) ) ]
How is this possible if y is a vector of unknown length?

Best regards
Joseph

Hi,

try below:

x <- c(1:20)
y <- c(1, 5, 10, 14)

x[ c( (y[1]+2):(y[2]-1), (y[2]+2):(y[3]-1), (y[3]+2):(y[4]-1) ) ]
x[unlist(lapply(1:(length(y) - 1), function (i) (y[i] + 2) : (y[i + 1] - 1)))]
x[unlist(mapply(seq, y[-length(y)] + 2, y[-1] - 1, SIMPLIFY = FALSE))]

-- 
Noia Raindrops
noia.raindrops at gmail.com

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

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-bounces at r-project.org [mailto:r-help-bounces at
r-project.org] On Behalf
> Of Sepp Tannhuber
> Sent: Saturday, August 25, 2012 2:24 AM
> To: r-help at 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 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.

On Aug 25, 2012, at 4:23 AM, Sepp Tannhuber <sepp.tannhuber at yahoo.de>
wrote:
> 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?
The latter is marginally more efficient. Take a look at head.matrix()

Cheers,
Michael

> 
> Best regards
> Joseph
> 
> ______________________________________________
> 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.