R help - Nov 2006 - (no subject)

From: Martin Maechler <maechler at stat.math.ethz.ch>
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Message-ID: <17739.46076.735981.117358 at stat.math.ethz.ch>
Date: Fri, 3 Nov 2006 22:26:20 +0100
To: Barry Rowlingson <B.Rowlingson at lancaster.ac.uk>
Cc: Sarah Goslee <sarah.goslee at gmail.com>,
    r-help at stat.math.ethz.ch,
    Sarah White <swhite at sgul.ac.uk>
Subject: Re: [R] Translation of R code required
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Reply-To: Martin Maechler <maechler at stat.math.ethz.ch>
>>>>> "BaRow" == Barry Rowlingson <B.Rowlingson at
lancaster.ac.uk>
>>>>>     on Fri, 03 Nov 2006 18:38:38 +0000 writes:
    BaRow> Sarah Goslee wrote:
    >> Since this step works,
    >> 
    >> mental$Rx <- factor(mental$Rx,
levels=c("VS","IPS"))
    >> 
    >> I think that some of the names are right, but perhaps
    >> that one is spelled differently (Centre vs centre, maybe,
    >> since R is case-sensitive?).

    BaRow>   How do you know that step works? If the dataframe
    BaRow> 'mental' doesnt have an 'Rx' column then mental$Rx
    BaRow> will be 'NULL', and the factor() function will make a
    BaRow> factor out of that...

    >> x=data.frame(foo=1:10)

    BaRow>   so x is a data frame with one column, called
    BaRow> 'foo'. Now lets make levels from a non-existent
    BaRow> column, 'bar':

    >> x$bar=factor(x$bar,levels=c('x','y'))

    BaRow>   No complaints... But try printing 'x' now (by
    BaRow> typing x at the command line)... Ick!

Hmm, what version of R would that be?

In any recent versions,
I get

  > x <- data.frame(foo = 1:10)
  > x$bar <- factor(x$bar,levels=c('x','y'))
  Error in as.vector(x, mode) : invalid argument 'mode'

But I want to make another point:

For about a year now, for "serious" data-analysis using data frames,
I've been advocating to use the slightly more clumsy but much
more error-prone ``column indexing by name'' instead of the
quick-and-dirty
"$" selection :

  > x$bar
  NULL

  > x[,"bar"]
  Error in "[.data.frame"(x, , "bar") : undefined columns
selected


    BaRow>   Try things one line at a time and check the objects
    BaRow> created or modified are sensible.

Indeed!

On 03-Nov-06 Martin Maechler wrote:
> [...]
> But I want to make another point:
> 
> For about a year now, for "serious" data-analysis using
> data frames, I've been advocating to use the slightly more
> clumsy but much more error-prone ``column indexing by name''
> instead of the quick-and-dirty "$" selection :
> 
>   > x$bar
>   NULL
> 
>   > x[,"bar"]
>   Error in "[.data.frame"(x, , "bar") : undefined
columns selected
Your point is well made, Martin, but I'd like to present a
different view of the same issue.

Suppose you are working on a somewhat flexibly structured
computation in which columns get adjoined to a dataframe
(by name) according (e.g.) as they become available at
some (unforeseeable) point in the pathway through the
computation. This could also arise in "real time" if "bar"
is the output from some measuring device, not yet available,
yet the computation can get on with other things until it
is available.

Then a test based on

  is.null(x$bar)

could divert the computation round some path suitable for
a case where x$bar has not yet been defined and adjoined,
as opposed to a case where x$bar has already been adjoined.

The use of x[,"bar"], however, would generate an error and
cause the computation to slide through a hole in the floor.

"Horses for courses", I suppose!

Best wishes,
Ted.

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E-Mail: (Ted Harding) <Ted.Harding at nessie.mcc.ac.uk>
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Date: 04-Nov-06                                       Time: 00:29:03
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