[R] R multiline expression grief

[Thomas Lumley]

On Fri, 13 Mar 2009, Paul Suckling wrote:

> Dear all.
>
> After much grief I have finally found the source of some weird
> discrepancies in results generated using R. It turns out that this is
> due to the way R handles multi-line expressions. Here is an example
> with R version 2.8.1:
>
> ----------------------------------------------------
> # R-script...
>
> r_parse_error <- function ()
> {
>  a <- 1;
>  b <- 1;
>  c <- 1;
>  d <- a + b + c;
>  e <- a +
>    b +
>    c;
>  f <- a
>    + b
>    + c;
>  cat('a',a,"\n");
>  cat('b',b,"\n");
>  cat('c',c,"\n");
>  cat('d',d,"\n");
>  cat('e',e,"\n");
>  cat('f',f,"\n");
> }
> ----------------------------------------------------
>> r_parse_error();
> a 1
> b 1
> c 1
> d 3
> e 3
> f 1
> ----------------------------------------------------
>
> As far as I am concerned f should have the value 3.

That is most unfortunate for you.

> Is this behaviour a bug?

No.

>  If not, is it
> possible to get R to generate a warning that several lines of an
> expression are potentially being ignored, perhaps by turning on a
> strict mode which requires the semi-colons?

No.

R is not ignoring several lines of an expression.
  f <- a
     + b
     + c;

is three perfectly legitimate expressions over three lines. R evaluates
   f<-a
then evaluates
    +b
then evaluates
    +c

For people who like semicolons, it's the same as if you had
   f <- a;
   +b;
   +c;

The semicolons are just an alternative to a newline, so a semicolon at the end of a line is purely cosmetic.  Modifying the parser to require a semicolon to terminate a statement would break essentially every piece of R code and documentation in existence, so it's probably easier to change your house style.

You could fairly easily write a tool that parsed your scripts and checked that all your expressions were either assignments or function calls and that the top-level expressions did not include unary plus or minus.


       -thomas

Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle