R help - Mar 2010 - Why doesn't vec[-real.number] give an error or warning? Kids do the darndest things!

Hi all...

My students were conflating grepping for a value in a vector to get the
index, and then removing it with [-index], for instance like this:

set.seed(17)
v <- rnorm(20)
s <- v[-1.18]

They were trying to remove the 12th value in v, which is -1.18 or so.

But the result is, as documented in ?Extract, to coerce 1.18 to the next
lowest integer, and remove the 1st value of v, not the 12th value.

This strikes me as something that ought to give an error or warning, or at
least be in the R Inferno, but I couldn't find it there.

Under what circumstances would the documented behavior be valuable?  I can
see that it avoids doing floor or as.integer in computations where an index
is needed but real numbers are generated for whatever reason, but is that
common?

Thanks for any insight.  Bryan
*************
Bryan Hanson
Acting Chair
Professor of Chemistry & Biochemistry
DePauw University, Greencastle IN USA

That notation  'v[-1.18]' is not going to remove the 12th entry if it
was
1.18.  They should have used:

v[-which(v == 1.18)]

to remove it, but it probably would not have worked and we would have
refered them to FAQ 7.31.

R is doing exactly what you are asking it to do.  There are cases where I
have used a real number and would not have expected an error.

As you say, kids do the darnest things.  I use the comment that "users
always find new and unusual ways to use a system that the developers never
envisioned".  All we can do it tell them how it does work (for better or
for
worse) and even if they think it is wrong, we can not change it as this
point in time to adapt to their way that they think it should work.

R is a great learning experience for people and it allows you to experiment
without breaking things.  I still have to write little test cases every so
often to understand how things work, especially if you have NAs in your data
and you are trying to do some equality test or do a calculation on it.

On Sat, Mar 13, 2010 at 9:39 PM, Bryan Hanson <hanson@depauw.edu> wrote:
> Hi all...
>
> My students were conflating grepping for a value in a vector to get the
> index, and then removing it with [-index], for instance like this:
>
> set.seed(17)
> v <- rnorm(20)
> s <- v[-1.18]
>
> They were trying to remove the 12th value in v, which is -1.18 or so.
>
> But the result is, as documented in ?Extract, to coerce 1.18 to the next
> lowest integer, and remove the 1st value of v, not the 12th value.
>
> This strikes me as something that ought to give an error or warning, or at
> least be in the R Inferno, but I couldn't find it there.
>
> Under what circumstances would the documented behavior be valuable?  I can
> see that it avoids doing floor or as.integer in computations where an index
> is needed but real numbers are generated for whatever reason, but is that
> common?
>
> Thanks for any insight.  Bryan
> *************
> Bryan Hanson
> Acting Chair
> Professor of Chemistry & Biochemistry
> DePauw University, Greencastle IN USA
>
> ______________________________________________
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> PLEASE do read the posting guide
>
http://www.R-project.org/posting-guide.html<http://www.r-project.org/posting-guide.html>
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

	[[alternative HTML version deleted]]

>>> Bryan Hanson <hanson at depauw.edu> 03/14/10 2:39 AM
>>>
>Under what circumstances would the documented behavior be valuable?  
<Inserts tongue in cheek:>
It would be useful in all the circumstances in which it would be
sensible to use a real number as a vector index.

Not that I can think of any.

Somebody else might, though. And it is possible that someone might
calculate intended integer values from reals, leading to a real vector
of indices. Under those circs, returning an error or warning wouldn't be
helpful. So in this case R is kind of treating users like grown-ups who
are assumed to know what they're doing. 

It's also pedagogically (is that a word?) useful. Users who confuse
reals with integers may remember the lesson better if it fouls up their
entire script with a problem that takes hours to track down than if it
generates a polite warning they don't understand but accommodate by
changing the script. And that kind of difference is occasionally
terribly important in statistics and in general computing. So maybe it
is useful to be allowed to foul up occasionally.

<removes tongue from cheek>

Steve E

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