if(cond) expr if(cond) cons.expr else alt.expr

?"if"

Note that it is a common mistake to forget to put braces ('{ .. }') around your statements, e.g., after 'if(..)' or 'for(....)'. In particular, you should not have a newline between '}' and 'else' to avoid a syntax error in entering a 'if ... else' construct at the keyboard or via 'source'. For that reason, one (somewhat extreme) attitude of defensive programming is to always use braces, e.g., for 'if' clauses.

Это-то я читал, да только не всё понял. Вроде, newline перед else считается за ошибку синтаксиса, а вот смысл последнего предложения от меня как-то ускользает. Translate, please?

в догонку

>>>>> "D" == Dani <danicyber@gmail.com>
>>>>> >>>>> on Tue, 24 Feb 2009 14:09:36 -0800 writes:

D> Hi list,
D> I don't know if somebody has spent a lot of time debugging strange
D> problems with if else positioning - the parser seems to recognize only
D> the syntax bellow - this is the only way of making these pieces of
D> code to work.

D> As far as i'm concerned, no examples were available (it would be so
D> awesome to have them in the introductory manual!)

D> #Try to change the placement of the keywords and you are dead!
["dead"?]

Oh dear...
Note this has nothing to do with if( ) .. else ..
but indeed with how things are parsed.

I think this is FAQ (or should become one):

?if [the help page you really should read before spending too
much time or even post to R-help]
has the following section

> Note that it is a common mistake to forget to put braces ('{ .. }')
> around your statements, e.g., after 'if(..)' or 'for(....)'.
> In particular, you should not have a newline between '}' and
> 'else' to avoid a syntax error in entering a 'if ... else'
> construct at the keyboard or via 'source'. For that reason, one
> (somewhat extreme) attitude of defensive programming is to always
> use braces, e.g., for 'if' clauses.

Regards,
Martin Maechler, ETH Zurich


D> Ex1:
D> if (1==1){
D> print('if')
D> print('if again')
D> }else
D> print('else')

D> Ex2:
D> if (2==2) print('if') else print('else')

D> Ex3:
D> if (2==2){
D> print('if')
D> print('if again')
D> }else
D> {
D> print('else')
D> print('else2')
D> }

D> Ex4:
D> if (2==2){
D> print('if')
D> print('if again')
D> }else print('else')



D> cheers,
D> -------------------------------------
D> Daniela

D> ______________________________________________
D> R-help@r-project.org mailing list
D> https://stat.ethz.ch/mailman/listinfo/r-help
D> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
D> and provide commented, minimal, self-contained, reproducible code.

______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.


I too have had many problems with if-else.

My solution is to always always always
use the line

"} else {"

in any if-else construction. This guarantees that
there won't be problems of the sort discussed here.

HTH

rksh

рекомендуют всегда использовать { } вокруг блоков, поскольку они могут растянуться впоследствии на несколько строк. И более экстремально, употреблять "} else {" везде где есть else.

} else

, значит... Вот незадача-то. Ну, всё лучше, чем на одной строчке. Спасибо! :-)