read.csv

If read.csv's colClasses= argument is NOT used then read.csv accepts
double quoted numerics:

1: > read.csv(stdin())
0: A,B
1: "1",1
2: "2",2
3:
  A B
1 1 1
2 2 2

However, if colClasses is used then it seems that it does not:

read.csv(stdin(), colClasses = "numeric")

0: A,B
1: "1",1
2: "2",2
3:
Error in scan(file, what, nmax, sep, dec, quote, skip, nlines,
na.strings,  :
  scan() expected 'a real', got '"1"'

Is this really intended?  I would have expected that a csv file
in which each field is surrounded with double quotes is acceptable
in both cases. This may be documented as is yet seems undesirable
from both a consistency viewpoint and the viewpoint that it should
be possible to double quote fields in a csv file.

Well, the default for colClasses is NA, for which ?read.csv says:
  [...]
  Possible values are 'NA' (when 'type.convert' is used),
  [...]
and then ?type.convert says:
  This is principally a helper function for 'read.table'. Given a
  character vector, it attempts to convert it to logical, integer,
  numeric or complex, and failing that converts it to factor unless
  'as.is = TRUE'.  The first type that can accept all the non-missing
  values is chosen.

It would seem that type 'logical' won't accept integer (naively one
might expect 1 --> TRUE, but see experiment below), so the first
acceptable type for "1" is integer, and that is what happens.
So it is indeed documented (in the R[ecursive] sense of "documented" :))

However, presumably when colClasses is used then type.convert() is
not called, in which case R sees itself being asked to assign a
character entity to a destination which it has been told shall be
integer, and therefore, since the default for as.is is
  as.is = !stringsAsFactors
but for this ?read.csv says that stringsAsFactors "is overridden
bu [sic] 'as.is' and 'colClasses', both of which allow finer
control.", so that wouldn't come to the rescue either.

Experiment:
  X <-logical(10)
  class(X)
  # [1] "logical"
  X[1]<-1
  X
  # [1] 1 0 0 0 0 0 0 0 0 0
  class(X)
  # [1] "numeric"
so R has converted X from class 'logical' to class 'numeric'
on being asked to assign a number to a logical; but in this
case its hands were not tied by colClasses.

Or am I missing something?!!

Ted.



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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 14-Jun-09                                       Time: 21:21:22
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