[R] created data doesn't remain when split...

[Prof Brian Ripley]

On Thu, 6 Nov 2003, Jeff D. Hamann wrote:

> I've been trying to figure out why the following is happening....
> 
> I've got some data I'll load in from a file...
> 
> rm(list=ls(all=TRUE))
> trees <- read.table( "c:/cruisepak/data.txt", header=T)
> trees$ct <- 1

That is incorrect usage: you set the last element to 1, and you used list
indexing on a data frame.  Either use

tree["ct"] <- 1

or, better,  use a value of the correct length.  (As from 1.8.0 using
$ with data frames is supported.)

> And when I create some temp variable, then split the data to perform further
> processing, the additional column doesn't maintain the data correctly....

Actually, it was maintained correctly.

> mtrees <- trees[trees$m == 1,]
> ctrees <- trees[trees$m == 0,]
> 
> The results are as follows...
> 
> > trees
> plot tree m sp dbh tht ct
> 1      1    1 1 DF  44 185 1
> 3      1    3 0 DF  40 192  1
> .....blah, blah, blah......
> 6      1    6 0 DF  26 156   1
> 8      1    8 0 DF  26 155   1

That's a bug that got fixed in 1.8.0.
 
> but,
> >mtrees
>     plot tree m sp dbh tht ct
> 1      1    1 1 DF  44 185     1
> 2      1    2 1 DF  38 188    NA
> 17     2    6 1 DF  26 174    NA
> 26     3    1 1 DF  42 185    NA
> 
> and
> 
> >ctrees
> > ctrees
>     plot tree m sp dbh tht  ct
> 3      1    3 0 DF  40 192    NA
> 4      1    4 0 DF  33 148    NA
> 5      1    5 0 DF  43 182    NA
> 
> when the value of ct for all the records in all the data.frames should be 1,
> not NA.
> 
> Why is that? Am I missing a step here? I'm running R 1.7.1 on Win2k.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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