[R] Simple loop

[William Dunlap]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Petr Savicky
> Sent: Wednesday, May 04, 2011 12:51 AM
> To: r-help at r-project.org
> Subject: Re: [R] Simple loop
> 
> On Tue, May 03, 2011 at 12:04:47PM -0700, William Dunlap wrote:
> [...]
> > ave() can deal that problem:
> >   > cbind(x, newCol2 = with(x, ave(H, Site, Prof,
> > FUN=function(y)y-min(y))))
> >     Site Prof  H newCol2
> >   1    1    1 24       8
> >   2    1    1 16       0
> >   3    1    1 67      51
> >   4    1    2 23       0
> >   5    1    2 56      33
> >   6    1    2 45      22
> >   7    2    1 67      21
> >   8    2    1 46       0
> >   Warning message:
> >   In min(y) : no non-missing arguments to min; returning Inf
> > The warning is unfortunate: ave() calls FUN even for when
> > there is no data for a particular group (Site=2, Prof=2 in this
> > case).
> 
> The warning may be avoided using min(y, Inf) instead of min().

Yes, but the fact remains that ave() wastes time and causes
unnecessary warnings and errors by calling FUN when it knows
it will do nothing with the result (because there are no entries
in x with a given combination of the factor levels in the ...
arguments).

Using paste(Site,Prof) when calling ave() is ugly, in that it
forces you to consider implementation details that you expect
ave() to take care of (how does paste convert various types
to strings?).  It also courts errors  since paste("A B", "C")
and paste("A", "B C") give the same result but represent different
Site/Prof combinations.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

> 
>   cbind(x, newCol2 = with(x, ave(H, Site, Prof, 
> FUN=function(y)y-min(y,Inf))))
> 
>     Site Prof  H newCol2
>   1    1    1 24       8
>   2    1    1 16       0
>   3    1    1 67      51
>   4    1    2 23       0
>   5    1    2 56      33
>   6    1    2 45      22
>   7    2    1 67      21
>   8    2    1 46       0
> 
> Another approach is to combine Site, Prof to a single column
> in any way suitable for the application. For example
> 
>   cbind(x, newCol2 = with(x, ave(H, paste(Site, Prof), 
> FUN=function(y)y-min(y))))
> 
> Petr Savicky.
> 
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