Answering my own question: if I explicitly garbage collecte before the 
benchmark then 'index' always wins, which probably also answers the 
original question.

v<-rep(1:1000,1:1000); x<-5; gc(); benchmark(replications=200, 
columns=c("test","elapsed"), order="elapsed", which=length(which(x==v)), 
index=length(v[v==x]), sum=sum(v==x))

Allan


On 19/06/09 16:51, Allan Engelhardt wrote:
> When trying out a couple of different approaches to this problem I get 
> rather different answers between runs.  Anybody know why?
>
> > library("rbenchmark")
> > v<-rep(1:1000,1:1000); x<-5; benchmark(replications=200, 
> columns=c("test","elapsed"), order="elapsed", 
> which=length(which(x==v)), index=length(v[v==x]), sum=sum(v==x))
>    test elapsed
> 3   sum   2.513
> 2 index   5.512
> 1 which   6.712
> > v<-rep(1:1000,1:1000); x<-5; benchmark(replications=200, 
> columns=c("test","elapsed"), order="elapsed", 
> which=length(which(x==v)), index=length(v[v==x]), sum=sum(v==x))
>    test elapsed
> 3   sum   2.502
> 2 index   3.779
> 1 which   6.650
> > v<-rep(1:1000,1:1000); x<-5; benchmark(replications=200, 
> columns=c("test","elapsed"), order="elapsed", 
> which=length(which(x==v)), index=length(v[v==x]), sum=sum(v==x))
>    test elapsed
> 2 index   3.796
> 3   sum   5.808
> 1 which   6.633
>
> This pattern appears to repeat (so on the next two runs "sum" will win 
> followed by "index" followed by "sum" twice followed by "index" ...)
>
> Allan.
>
>
>
>
> On 19/06/09 14:55, Praveen Surendran wrote:
>> Hi,
>>
>>
>>
>> I have a vector "v" and would like to find the number of occurrence of
>> element "x" in the same.
>>
>> Is there a way other than,
>>
>>
>>
>> sum(as.integer(v==x)) or length(which(x==v))
>>
>>
>>
>> to do the this.
>>
>>
>>
>> I have a huge file to process and do this.  Both the above described 
>> methods
>> are pretty slow while dealing with a large vector.
>>
>> Please have your comments.
>>
>>
>>
>> Praveen Surendran.
>>
>>
>>
>>
>>     [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> 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.
>>
>
> ______________________________________________
> 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.

	[[alternative HTML version deleted]]


