From Ted.Harding at manchester.ac.uk  Sun Jun 14 22:21:24 2009
From: Ted.Harding at manchester.ac.uk ( (Ted Harding))
Date: Sun, 14 Jun 2009 21:21:24 +0100 (BST)
Subject: [Rd] read.csv
In-Reply-To: <971536df0906141156j78b5570ajf08257c88098d7d8@mail.gmail.com>
Message-ID: <XFMail.090614212124.Ted.Harding@manchester.ac.uk>

On 14-Jun-09 18:56:01, Gabor Grothendieck wrote:
> 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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Date: 14-Jun-09                                       Time: 21:21:22
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[Rd] read.csv
 (Ted Harding