On 06-May-2014 18:09:12 Göran Broström wrote:
> A thread on r-devel ("Historical NA question") went (finally) off-topic,
> heading towards "Precedence". This triggered a question that I think is
> better put on this list:
>
> I have been more or less regularly been writing programs since the
> seventies (Fortran, later C) and I early got the habit of using
> parentheses almost everywhere, for two reasons. The first is the
> obvious, to avoid mistakes with precedences, but the second is almost
> as important: Readability.
>
> Now, I think I have seen somewhere that unnecessary parentheses in R
> functions may slow down execution time considerably. Is this really
> true, ant should I consequently get rid of my faiblesse for parentheses?
> Or are there rules for when it matters and doesn't matter?
>
> Thanks, Göran
I have much sympathy with your motives for using parentheses!
(And I have a similar computing history).
My general encouragement would be: continue using them when you
feel that each usage brings a benefit.
Of course, you would avoid silliness like
x <- (1*(2*(3*(4*(5)))))
and indeed, that does slow it down somewhat (it takes about twice
as long as a <- 1*2*3*4*5):
system.time(for(i in (1:10000)) 1*2*3*4*5)
# user system elapsed
# 0.032 0.000 0.032
system.time(for(i in (1:10000)) (1*(2*(3*(4*(5))))))
# user system elapsed
# 0.056 0.000 0.054
The main reason, I suppose, is that a "(" forces a new level
on a stack, which is not popped until the matching ")" arrives.
Interestingly, the above silliness takes a little longer when
the nesting is the other way round:
system.time(for(i in (1:10000)) 1*2*3*4*5)
# user system elapsed
# 0.028 0.000 0.029
system.time(for(i in (1:10000)) (((((1)*2)*3)*4)*5) )
# user system elapsed
# 0.052 0.000 0.081
(though in fact the times are somwhat variable in both cases,
so I'm not sure of the value of the relationship).
Best wishes,
Ted.
-------------------------------------------------
E-Mail: (Ted Harding) <ted.hard...@wlandres.net>
Date: 06-May-2014 Time: 19:41:13
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