R devel - Mar 2007 - parse error with if else (PR#9551)

Full_Name: Stephanie MAHEVAS
Version: 2.4.1
OS: Windows NT
Submission from: (NULL) (134.246.55.50)



the two following instructions provide a synthax  error :

if ( 5 > 4 ) cat("ok1")
else cat("ok2")

and

if ( 5 > 4 ){ cat("ok1")}
else cat("ok2")

whereas these ones don't

if ( 5 > 4 ) cat("ok1") else cat("ok2")

and

if ( 5 > 4 ){ cat("ok1")
}else cat("ok2")

It looks like a parser problem. If else is not on the same line as if or if the
end of block statement of if } is not paste to else, else does not seem linked
with if

Stephanie.Mahevas at ifremer.fr wrote:
> Full_Name: Stephanie MAHEVAS
> Version: 2.4.1
> OS: Windows NT
> Submission from: (NULL) (134.246.55.50)
> 
> 
> 
> the two following instructions provide a synthax  error :
> 
> if ( 5 > 4 ) cat("ok1")
> else cat("ok2")
This is not a bug!

Since you are allowed to omit the else, R cannot know whether you want 
to provide it and thinks you have finished, since
     if ( 5 > 4 ) cat("ok1")
already is a valid and complete expression.
Therefore
     else cat("ok2")
is a *new* expression which obviously is not valid without if() before.

If you want that R looks at the whole at first, either do as below or 
make it a whole expression by putting it into braces as in:

{
     if ( 5 > 4 ) cat("ok1")
     else cat("ok2")
}


Uwe Ligges



> and
> 
> if ( 5 > 4 ){ cat("ok1")}
> else cat("ok2")
> 
> whereas these ones don't
> 
> if ( 5 > 4 ) cat("ok1") else cat("ok2")
> 
> and
> 
> if ( 5 > 4 ){ cat("ok1")
> }else cat("ok2")
> 
> It looks like a parser problem. If else is not on the same line as if or if
the
> end of block statement of if } is not paste to else, else does not seem
linked
> with if
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

On 3/6/2007 12:15 PM, Stephanie.Mahevas at ifremer.fr
wrote:
> Full_Name: Stephanie MAHEVAS
> Version: 2.4.1
> OS: Windows NT
> Submission from: (NULL) (134.246.55.50)
> 
> 
> 
> the two following instructions provide a synthax  error :
> 
> if ( 5 > 4 ) cat("ok1")
> else cat("ok2")
> 
> and
> 
> if ( 5 > 4 ){ cat("ok1")}
> else cat("ok2")
> 
> whereas these ones don't
> 
> if ( 5 > 4 ) cat("ok1") else cat("ok2")
> 
> and
> 
> if ( 5 > 4 ){ cat("ok1")
> }else cat("ok2")
> 
> It looks like a parser problem. If else is not on the same line as if or if
the
> end of block statement of if } is not paste to else, else does not seem
linked
> with if
This is a documented "feature" of the language, not a bug.  (See the 
language guide, section 3.2.1 "if", around the 4th paragraph.)

It arises because R tries to evaluate a statement as soon as it is 
complete, and

if ( 5 > 4 ) cat("ok1")

is a complete statement.  You can get the behaviour you want by wrapping 
the whole think in curly brackets so that R doesn't start evaluating too 
early, e.g.

 > {
+  if ( 5 > 4 ) cat("ok1")
+  else cat("ok2")
+ }
ok1>

Duncan Murdoch