[R] avoiding loop

[David Winsemius]

This is pretty much equivalent:

tapply(DF$value[DF$choice==1], DF$time[DF$choice==1], sum) /
         tapply(DF$value, DF$time, sum)

And both will probably fail if the number of groups with choice==1 is  
different than the number overall.

-- 
David.

On Oct 31, 2009, at 5:14 PM, Dimitris Rizopoulos wrote:

> one approach is the following:
>
> # say 'DF' is your data frame, then
> with(DF, {
>    ind <- choice == 1
>    n <- tapply(value[ind], time[ind], sum)
>    d <- tapply(value, time, sum)
>    n / d
> })
>
>
> I hope it helps.
>
> Best,
> Dimitris
>
>
> parkbomee wrote:
>> Hi all,
>> I am trying to figure out a way to improve my code's efficiency by  
>> avoiding the use of loop.
>> I want to calculate a conditional mean(?) given time.
>> For example, from the data below, I want to calculate sum((value| 
>> choice==1)/sum(value)) across time.
>> Is there a way to do it without using a loop?
>> time  cum_time  choice    value
>> 1         4             1           3
>> 1         4              0           2
>> 1          4             0           3
>> 1          4             0           3
>> 2         6             1           4
>> 2         6             0           4
>> 2         6             0           2
>> 2         6             0           4
>> 2         6             0           2
>> 2         6             0           2 3         4              
>> 1           2 3         4             0           3 3          
>> 4             0           5 3         4             0           2  
>> My code looks like
>> objective[1] = value[1] / sum(value[1:cum_time[1])
>> for (i in 2:max(time)){
>>     objective[i] = value[cum_time[i-1]+1] /  
>> sum(value[(cum_time[i-1]+1) : cum_time[i])])
>> }
>> sum(objective)
>> Anyone have an idea that I can do this without using a loop??
>> Thanks.
>> 		 	   		   
>> _________________________________________________________________
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>
> -- 
> Dimitris Rizopoulos
> Assistant Professor
> Department of Biostatistics
> Erasmus University Medical Center
>
> Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
> Tel: +31/(0)10/7043478
> Fax: +31/(0)10/7043014
>
> ______________________________________________
> 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.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT