[R] indexing in data frames

[peter dalgaard]

On Aug 10, 2012, at 01:36 , William Dunlap wrote:

> And if you are extremely concerned with speed, do
> not compute a$b and a$c in every iteration of the loop.
> E.g., change
>   lapply(seq_along(a$c),function(x) a$b[x]-a$c[[x]])
> to something like
>   with(a, lapply(seq_along(c), function(x)b[x] - c[[x]]))
> 

This seems to be working fine as well:

> a <- data.frame(b = 1:2, c = I(list(2, 1:2)))
> a <- within(a, dif <- mapply("-", b, c))
> a
  b    c  dif
1 1    2   -1
2 2 1, 2 1, 0


> 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 R. Michael Weylandt
>> Sent: Thursday, August 09, 2012 4:22 PM
>> To: arun
>> Cc: R help; jimi adams
>> Subject: Re: [R] indexing in data frames
>> 
>> On Thu, Aug 9, 2012 at 5:30 PM, arun <smartpink111 at yahoo.com> wrote:
>>> 
>>> lapply(1:length(a$c),function(x) a$b[x]-a$c[[x]])
>> 
>> Arun,
>> 
>> I've seen you use this idiom a few times lately and I'd just like to note that
>> 
>> seq_along()
>> 
>> is an (underutilized) primitive and a safer and faster alternative
>> (avoiding the pathological length(x) = 0 case).
>> 
>> Cheers,
>> Michael
>> 
>> ______________________________________________
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>> 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.

-- 
Peter Dalgaard, Professor
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com