[R] Problem with parser and if/else

[Paul Lemmens]

Dear Peter,

--On donderdag 13 november 2003 15:05 +0100 Peter Dalgaard 
<p.dalgaard at biostat.ku.dk> wrote:

> "Brown, Simon" <simon.brown at metoffice.com> writes:
>
>> 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?
>
> Again? This has been hashed over several times before. The basic issue
> is whether a statement can be assumed to be syntactically complete at
> the end of a line. It is fairly obvious what happens when you type the
> same expressions at an interactive prompt:
>
>> x1 <- 1:6
>> t1 <- 5
>> if (length(x1) >= t1) {
> + cat("in the if 2\n")
> + }
> in the if 2
>> else {
> Error: syntax error
>> cat("in the else\n")
> in the else
>> }
> Error: syntax error
>
> Notice that the first right curly brace is seen as terminating the if
> construct. Otherwise, R would need to wait and check whether the
> *next* line starts with an "else" which would certainly be confusing
> in interactive use. So R assumes the expression is finished and
> evaluates it. Then it gets an "else" keyword that it doesn't know what
> to do with and barfs.
>
I'm trying to grasp this: if you're saying (or are you saying) that the 
only way to have if() know that an else will be present is to put it on the 
same line as the closing curly brace of the if() (compound) statement. But 
if I look at some code from, e.g., aov and lm, I see plenty violations of 
that rule.

regards,
Paul


-- 
Paul Lemmens
NICI, University of Nijmegen              ASCII Ribbon Campaign /"\
Montessorilaan 3 (B.01.03)                    Against HTML Mail \ /
NL-6525 HR Nijmegen                                              X
The Netherlands                                                 / \
Phonenumber    +31-24-3612648
Fax            +31-24-3616066