[R] Problem with parser and if/else

[Prof Brian D Ripley]

On Thu, 13 Nov 2003, Ben Bolker wrote:

>
>   In the second case, R stops when it has a syntactically complete clause:
>
> if (...) {
>   ...
> }
>
> is complete since an else{} clause is not required.  R evaluates it, then
> moves onto
>
> else { ... }
>
> which is a syntax error (since it "doesn't have an if {} in front of it,
> since that has already been evaluated)
>
>   One way to see this illustrated is to enter these commands a line at a
> time in interactive mode.
>
>   I don't know exactly where this appears in the documentation, probably
> someone will point to it in another message.

help("if"), for example (the most obvious place to look?)
MASS4, p.58
....


> On Thu, 13 Nov 2003, Brown, Simon wrote:
>
> > Dear r-help people,
> >
> > could you confirm that this is correct behaviour for R?  I am using RH9.
> >
> > the code:
> > x1 <- 1:6
> > t1 <- 5
> > if (length(x1) >= t1) {
> > 	cat("in the if \n")
> > } else {
> > 	cat("in the else\n")
> > }
> >
> > runs fine:
> > > source("test_if_else.R")
> > in the if
> > >
> >
> > but the code:
> > x1 <- 1:6
> > t1 <- 5
> > if (length(x1) >= t1) {
> > 	cat("in the if 2\n")
> > }
> > else {
> > 	cat("in the else\n")
> > }
> >
> > fails with the error:
> > > source("test_if_else2.R")
> > Error in parse(file, n, text, prompt) : syntax error on line 6
> > >
> >
> > Could someone explain this to me please?
> >
> > Thanks,
> >
> > Simon.
> >
>
> --
> 620B Bartram Hall                            bolker at zoo.ufl.edu
> Zoology Department, University of Florida    http://www.zoo.ufl.edu/bolker
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>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595