[R] Extract values from vector and repeat by group

[PIKAL Petr]

Hi

probably not most elegant and also not general but

rep(df$var[df$time==4],rle(df$group)$lengths)

or

rep(df$var[df$time==4],  sapply(split(df$var,df$group), length))

shall give you desired vector.

Regards
Petr


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Benjamin Gillespie
> Sent: Sunday, November 17, 2013 3:47 PM
> To: r-help at r-project.org
> Subject: [R] Extract values from vector and repeat by group
> 
> Hi all,
> 
> I hope you can help.
> 
> I have a data frame 'df':
> 
> group=c(rep(1,8),rep(2,10),rep(3,11))
> var=rnorm(29)
> time=c(seq(1,8),seq(1,10),seq(1,11))
> df=data.frame(group,var,time)
> 
> I would like to extract the value from 'var' for each 'group' at
> 'time'=4 and repeat these extracted values in a new vector ('new') n
> times where n is the number of rows for each group. I did this by hand
> as below, but there must be a quicker way:
> 
> subset=subset(df,df$time==4)
> subset
> group        var time
> 4      1  0.2531270    4
> 12     2 -0.3600128    4
> 22     3  0.4194730    4
> 
> df$new=c(rep(0.2531270,8),rep(-0.3600128,10),rep(0.4194730,11))
> 
> Any questions please ask,
> 
> Many thanks in advance,
> 
> Ben Gillespie, Research Postgraduate
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