[R] Oddity: I seem to have a variable in a dataframe that doesn't show in colnames() - can anyone advise?

[Phil Spector]

Chris -
     If you check the documentation for the "$" operator, 
for example by typing

help("$")

you'll find (among a lot of other information):

     name: A literal character string or a name (possibly backtick
           quoted).  For extraction, this is normally (see under
           ‘Environments’) partially matched to the ‘names’ of the
           object.

So when you use the "$" operator (but not "[" or "[["), partial
matching is performed.  For example:

> x = data.frame(PHQ9=1:10)
> x$PHQ
  [1]  1  2  3  4  5  6  7  8  9 10
> x[,'PHQ']
Error in `[.data.frame`(x, , "PHQ") : undefined columns selected
> x[['PHQ']]
NULL

So if you don't want this "feature", you can use brackets instead
of the dollar sign for extraction.

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu


On Sun, 29 May 2011, Chris Evans wrote:

> I may be being dopey, I surely am, but I'm baffled by this.  I've been
> working, on and off for a few days in R version 2.13.0 (2011-04-13)
> i386-pc-mingw32/i386 (32-bit) working it through ESS.
>
> I've got a dataframe created a couple of days back, during the session:
>> dim(AllDat)
> [1] 27270    94
>
> I came back this morning and misremembered my variables and thought I
> had a variable AllDat$PHQ and started using it and everything seemed
> fine until I realised that I shouldn't have it (!) and that the variable
> I was thinking of is AllDat$PHQ9 and that's there:
>> colnames(AllDat)[grep("PHQ",colnames(AllDat))]
> [1] "PHQ9"    "HasPHQ"  "ZeroPHQ"
>
> and, as you can see, AllDat$PHQ.  But I can I do:
>
>> head(table(AllDat$PHQ))
>  0   1   2   3   4   5
> 731 527 764 845 872 915
>
> Ooops ... so AllDat$PHQ _DOES_ exist.  Its contents exactly match
> AllDat$PHQ9:
>> table(abs(AllDat$PHQ - AllDat$PHQ9))
>    0
> 19032
>
> I have searched back through my ESS transcript back to the start of the
> session and I can't see anywhere I've assigned to AllDat$PHQ (and I've
> never used "attach").
>
> However, I guess that somehow I must have managed to duplicate AllDat in
> more than one open environment so I check out and I have 16 environments
> (I'm sure that's not right terminology, apologies):
>> search()
> [1] ".GlobalEnv"        "package:reshape2"
> [3] "package:Hmisc"     "package:survival"
> [5] "package:splines"   "package:nnet"
> [7] "package:MASS"      "package:gdata"
> [9] "package:stats"     "package:graphics"
> [11] "package:grDevices" "package:utils"
> [13] "package:datasets"  "package:methods"
> [15] "Autoloads"         "package:base"
>
> So I try:
>> for (i in 1:16) { print(paste("i =",i,exists("AllDat",i,inherits =
> FALSE))) }
> [1] "i = 1 TRUE"
> [1] "i = 2 FALSE"
> [1] "i = 3 FALSE"
> [1] "i = 4 FALSE"
> [1] "i = 5 FALSE"
> [1] "i = 6 FALSE"
> [1] "i = 7 FALSE"
> [1] "i = 8 FALSE"
> [1] "i = 9 FALSE"
> [1] "i = 10 FALSE"
> [1] "i = 11 FALSE"
> [1] "i = 12 FALSE"
> [1] "i = 13 FALSE"
> [1] "i = 14 FALSE"
> [1] "i = 15 FALSE"
> [1] "i = 16 FALSE"
>
> So I don't think I do have two different AllDat dataframes.
>
> Can anyone throw light on what's going on?  I have searched archives
> etc. but can't think of sensible keywords and so far turned up nothing.
> Happy to be told RTFM or the equivalent but could someone point me to a
> specific location?  Also happy to try any diagnostics anyone recommends.
>
> Many thanks in advance,
>
> Chris
>
> -- 
> Chris Evans <chris at psyctc.org> Skype: chris-psyctc
> Consultant Psychiatrist in Psychotherapy, Notts. PDD network;
> Professor, Psychotherapy, Nottingham University
> *If I am writing from one of those roles, it will be clear. Otherwise*
> *my views are my own and not representative of those institutions    *
> If you have difficulty Emailing me on this address or getting a reply,
> send again but cc to:       chris dot evans at nottshc dot nhs dot uk
> and to:                     c dot evans at nottingham dot ac dot uk
>
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