[R] splitting a data.frame

[Thomas Lumley]

On Tue, 22 Jan 2002, Gary Collins wrote:

>  I have the following (simple!?) problem which I am unable to find a
>  relatively trivial solution to.
>  If I have a dataframe,
>
>  A    1
>  A    7
>  B    4
>  B    5
>  C    3
>  D    3
>  D    2
>  E    5
>  F    5
>  F    6
>
>  I would like to create a new data.frame in the form
>
>  ID    pt1    pt2
>  A    1    7
>  B    4    5
>  C    3    NA
>  D    3    2
>  E    5    NA
>  F    5    6
>

In addition to the specific suggestions already given there is a general
solution to this sort of problem with reshape()

You would need to create a time variable to indicate which is the first
or second observation, which in your case could be
  df$time<-c(0,df$ID[-1]==df$ID[-10])
then
  newdf<-reshape(df,timevar="time",idvar="ID",direction="wide")


In your case this isn't a big saving over the other approaches. It becomes
really useful when you have many variables, especially if some are
constant over time and others aren't.


	-thomas

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