# [R] apply mean function to a subset of data

Pedro Mardones mardones.p at gmail.com
Mon Apr 4 14:07:45 CEST 2016

```Thanks David
It works perfectly
Pedro

Sent from my iPhone

> On Apr 3, 2016, at 17:44, David L Carlson <dcarlson at tamu.edu> wrote:
>
> Here are several ways to get there, but your original loop is fine once it is corrected:
>
>> for (i in 1:2)  smean[i] <- mean(toy\$diam[toy\$group==i][1:nsel[i]])
>> smean
> [1] 0.271489 1.117015
>
> Using sapply() to hide the loop:
>> smean <- sapply(1:2, function(x) mean((toy\$diam[toy\$group==x])[1:nsel[x]]))
>> smean
> [1] 0.271489 1.117015
>
>> smean <- sapply(1:2, function(x) mean(head(toy\$diam[toy\$group==x], nsel[x])))
>> smean
> [1] 0.271489 1.117015
>
> Or mapply() instead of sapply
>> smean <- mapply(function(x, y) mean(head(x, y)) , x=split(toy\$diam, toy\$group), y=nsel)
>> smean
>       1        2
> 0.271489 1.117015
>
> ------------------------------
> David L. Carlson
> Department of Anthropology
> Texas A&M University
>
> -----Original Message-----
> From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Jim Lemon
> Sent: Saturday, April 2, 2016 6:14 PM
> To: Pedro Mardones <mardones.p at gmail.com>
> Cc: r-help mailing list <r-help at r-project.org>
> Subject: Re: [R] apply mean function to a subset of data
>
> Hi Pedro,
> This may not be much of an improvement, but it was a challenge.
>
> selvec<-as.vector(matrix(c(nsel,unlist(by(toy\$diam,toy\$group,length))-nsel),
> ncol=2,byrow=TRUE))
> TFvec<-rep(c(TRUE,FALSE),length.out=length(selvec))
> toynsel<-rep(TFvec,selvec)
> by(toy[toynsel,]\$diam,toy[toynsel,]\$group,mean)
>
> Jim
>
>> On 4/3/16, Pedro Mardones <mardones.p at gmail.com> wrote:
>> Dear all;
>>
>> This must have a rather simple answer but haven't been able to figure it
>> out: I have a data frame with say 2 groups (group 1 & 2). I want to select
>> from group 1 say "n" rows and calculate the mean; then select "m" rows from
>> group 2 and calculate the mean as well. So far I've been using a for loop
>> for doing it but when it comes to a large data set is rather inefficient.
>> Any hint to vectorize this would be appreciated.
>>
>> toy = data.frame(group = c(rep(1,10),rep(2,8)), diam =
>> c(rnorm(10),rnorm(8)))
>> nsel = c(6,4)
>> smean <- c(0,0)
>> for (i in 1:2)  smean[i] <- mean(toy\$diam[1:nsel[i]])
>>
>> Thanks
>>
>> Pedro
>>
>>    [[alternative HTML version deleted]]
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help