[R] Re: none

[Douglas Bates]

>>>>> "Troels" == Troels Ring <tring at mail1.stofanet.dk> writes:

  Troels> This is again a very simple question, and I
  Troels> hope you will accept it. I have a dataset with variables
  Troels> in columns all with their names. Now I want to add a
  Troels> column indicating simply identity of patients, numbered
  Troels> according to row. There are 34 rows.  So I did:
  >> ID <- c(1:34)
  Troels> but was not allowed to
  >> Ptt <- cbind(patients,ID)=20
  Troels> and was informed that
  >> Error: names attribute must be the same length as the vector

There may be a problem with the way R is handling your request but I
think we would need to know a bit more about the form of the data
before we could decide.

Your initial description sounds as if the data are in a "data frame",
the class of object created by the "read.table" function.  One way to
discover the form of the data is to use the "class" or, more
generally, the "data.class" functions.  For example, data I am using
in a lecture today has this form
 R> T10.5  # Average SAT math scores by sex - Hocking, table 10.5
    Year Mmath Fmath
 1    67   514   467
 2    68   512   470
 3    69   513   470
 4    70   509   465
 5    71   507   466
 6    72   505   461
 7    73   502   460
 8    74   501   459
 9    75   495   449
 10   76   497   446
 11   77   497   445
 12   78   494   444
 R> data.class( T10.5 )
 [1] "data.frame"

If you find your data are in a matrix and you wish to convert them to
a data frame, you can use the "as.data.frame" function.  

 R> matSAT <- as.matrix( T10.5 )
 R> matSAT
    Year Mmath Fmath
 1    67   514   467
 2    68   512   470
 3    69   513   470
 4    70   509   465
 5    71   507   466
 6    72   505   461
 7    73   502   460
 8    74   501   459
 9    75   495   449
 10   76   497   446
 11   77   497   445
 12   78   494   444
 R> data.class( matSAT )
 [1] "matrix"
 R> dfSAT <- as.data.frame( matSAT )
 R> data.class( dfSAT )
 [1] "data.frame"

Generally data frames are preferred to matrices for storing data sets.
If your data are in a data frame then you can create a new variable by
assigning a component of that name in the data frame.  It looks like

 R> dfSAT$ID <- 1:nrow(dfSAT)
 R> dfSAT
    Year Mmath Fmath ID
 1    67   514   467  1
 2    68   512   470  2
 3    69   513   470  3
 4    70   509   465  4
 5    71   507   466  5
 6    72   505   461  6
 7    73   502   460  7
 8    74   501   459  8
 9    75   495   449  9
 10   76   497   446 10
 11   77   497   445 11
 12   78   494   444 12

A more complete discussion is provided in section 2.1 of Venables and
Ripley (1997), "Modern Applied Statistics with S-PLUS (second
edition)".  Please see also the "R Complements" file for the book as
mentioned in
	   http://www.stats.ox.ac.uk/pub/MASS2/Compl.shtml
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