R help - Jan 2005 - zero index and lazy evaluation in ifelse()

I don't understand this behavior:
> a <- c(0, 1, 2, 3)
> b <- c(1, 2, 3, 4)
> ifelse (a == 0, 0, b[a])
[1] 0 2 3 1

rather than the desired 0 1 2 3.  Thanks for any explanation.

> Date: Thu, 13 Jan 2005 12:42:59 -0800
> From: White.Denis at epamail.epa.gov
> 
> I don't understand this behavior:
> 
> > a <- c(0, 1, 2, 3)
> > b <- c(1, 2, 3, 4)
> > ifelse (a == 0, 0, b[a])
> [1] 0 2 3 1
> 
> rather than the desired 0 1 2 3.  Thanks for any explanation.
> 
Look at b[a].

Ray

Dear Denis,

b[a] gives you c(1, 2, 3) (try it), which is recycled as c(1, 2, 3, 1); the
elements in positions 2, 3, and 4 of this vector (i.e., where a != 0) are 2,
3, 1.

Regards,
 John

--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox 
-------------------------------- 
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of 
> White.Denis at epamail.epa.gov
> Sent: Thursday, January 13, 2005 3:43 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] zero index and lazy evaluation in ifelse()
> 
> I don't understand this behavior:
> 
> > a <- c(0, 1, 2, 3)
> > b <- c(1, 2, 3, 4)
> > ifelse (a == 0, 0, b[a])
> [1] 0 2 3 1
> 
> rather than the desired 0 1 2 3.  Thanks for any explanation.
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html

White.Denis at epamail.epa.gov writes:
> I don't understand this behavior:
> 
> > a <- c(0, 1, 2, 3)
> > b <- c(1, 2, 3, 4)
> > ifelse (a == 0, 0, b[a])
> [1] 0 2 3 1
> 
> rather than the desired 0 1 2 3.  Thanks for any explanation.
b[a] is c(1,2,3), recycling to length 4 gives c(1,2,3,1), get it?

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907

Thanks to all for clarifying that it isn't an element by element test
and evaluation, rather an "any" test as the code says.


Peter Dalgaard <p.dalgaard at biostat.ku.dk> wrote on 2005-01-13 14:22:50:
> White.Denis at epamail.epa.gov writes:
>
> > This seems to contradict the help file.
> >
> > "... 'yes' will be evaluated if and only if any element
of 'test' is
> > true,
> > and analogously for 'no'..."
> >
>
> It doesn't.  Read closer.  Both 'yes' and 'no' is
evaluated. The
> former is c(0, 0, 0, 0) the latter is c(1, 2, 3, 1).
>
>
> > Peter Dalgaard <p.dalgaard at biostat.ku.dk> wrote on 2005-01-13
13:24:31:
> >
> > > White.Denis at epamail.epa.gov writes:
> > >
> > > > I don't understand this behavior:
> > > >
> > > > > a <- c(0, 1, 2, 3)
> > > > > b <- c(1, 2, 3, 4)
> > > > > ifelse (a == 0, 0, b[a])
> > > > [1] 0 2 3 1
> > > >
> > > > rather than the desired 0 1 2 3.  Thanks for any
explanation.
> > >
> > > b[a] is c(1,2,3), recycling to length 4 gives c(1,2,3,1), get it?
> > >