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apply mean function to a subset of data

5 messages · Pedro Mardones, Boris Steipe, Jim Lemon +1 more

Pedro Mardones
# 
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
Boris Steipe
# 
Your toy code does not reproduce what you describe: mean(toy$diam[1:nsel[i]]) both times selects from elements of group 1. YOu probably want to subset like toy$diam[toy$group == i]. Also, if there is any real inefficiency here, it is _not_ because you are executing a for-loop for two iterations. What makes you think you have an efficiency problem?


B.
On Apr 2, 2016, at 2:46 PM, Pedro Mardones <mardones.p at gmail.com> wrote:

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Jim Lemon
    

    

      
      #
    

  


  

      
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:

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      David L Carlson
    

    

      
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Here are several ways to get there, but your original loop is fine once it is corrected:

            

      
      
      
    

        
[1] 0.271489 1.117015

Using sapply() to hide the loop:

            

      
      
      
    

        
[1] 0.271489 1.117015

Or use head()

            

      
      
      
    

        
[1] 0.271489 1.117015

Or mapply() instead of sapply

            

      
      
      
    

        
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:

            

      
      
      
    

        

      
      
    

        
______________________________________________
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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.

  
  


  


      

    

    

      
      

        


  

        

      Pedro Mardones
    

    

      
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Thanks David
It works perfectly
Pedro

Sent from my iPhone