[R] Efficiency of for-loop in R

[jim holtman]

I will venture a guess in that in the case of the 'for' loop, you are
calling 'sin' 100,000 times incurring the cost of individual function
calls at the interpreter level plus evaluating 'for' loop.  In the
vectorized case, you are only interpreting a single 'sin' call and
then internally evaluating the 'sin' function, which is a lot faster.
Also the results are different in the two cases.  In case 1) you get
100000 values back in 'y' and in 2) you only get the value of the last
loop through the 'for' loop.

> system.time(for (i in 1:100000) y <- sin(i))
   user  system elapsed
   0.17    0.00    0.28
> str(y)
 num 0.0357
> system.time(y <- sin(1:100000))
   user  system elapsed
   0.01    0.00    0.02
> str(y)
 num [1:100000]  0.841  0.909  0.141 -0.757 -0.959 ...
>

R is typically optimized for vector type operations.

On Dec 27, 2007 8:39 PM, Tong Wang <wangtong at usc.edu> wrote:
> HI,
>   The question is meant to be a general one, I am trying to find out if there is new development in R that I might have missed.
>
> but here's a trivial example,
>
> To compute  y=sin(x) , x = 1,2,... 100000
>  x=1:100000,
> 1.  y =sin(x)
> 2.  for(i in 1:100000) y=sin(x[i])
>
> 1 is much faster than 2.
> Old Matlab also had this problem, but in new versions, 1 and 2 are mostly the same.
> I am just wondering if the same improvement has happened or will happen to R.
>
>
> Thanks .
>
>
>
>
> ----- Original Message -----
> From: jim holtman <jholtman at gmail.com>
> Date: Thursday, December 27, 2007 4:39 pm
> Subject: Re: [R] Efficiency of for-loop in R
> To: Tong Wang <wangtong at usc.edu>
> Cc: R help <r-help at stat.math.ethz.ch>
>
> > Exactly "what is the problem you are trying to solve"?  Could you
> > "provide commented, minimal, self-contained, reproducible code"?
> >
> > A lot depends on what you are trying to do,  There might be other
> > ways, in R, than a 'for' loop to solve your problems.
> >
> > On Dec 27, 2007 6:44 PM, Tong Wang <wangtong at usc.edu> wrote:
> > > Hi,
> > >   I just realized that in Matlab, as long as memory is pre-
> > allocated, doing for-loop doesn't cost more time than doing things
> > in vector form.
> > >   But it seems in R, it still cost a lot to do for-loop.  Is
> > there any improvement in R that I missed. Thanks a lot.
> > >
> > > Merry Xmas Everyone !
> > >
> > > ______________________________________________
> > > R-help at 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.
> > >
> >
> >
> >
> > --
> > Jim Holtman
> > Cincinnati, OH
> > +1 513 646 9390
> >
> > What is the problem you are trying to solve?
> >
>



-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?