[R] avoiding loop

Dimitris Rizopoulos d.rizopoulos at erasmusmc.nl
Sat Oct 31 22:14:28 CET 2009


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
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