# Re: [R] How do I avoid a loop?

From: Martin Becker <martin.becker_at_mx.uni-saarland.de>
Date: Tue, 19 Jun 2007 15:42:00 +0200

Gabor Grothendieck wrote:
> xx is 1 in every position of the first run of TRUE, 2 in every
> position in the 2nd run of TRUE and so on. The parenthesized
> expression in the second line converts those to increasing
> values and multiplying it by x zaps the garbage in the positions
> that correspond to FALSE in x.
>
> xx <- cumsum(diff(c(FALSE, x)) > 0)
> (seq_along(x) - match(xx, xx) + 1) * x
>
>
>

If speed is a critical issue, there is another possibility. Thanks to Oleg Sklyar's "inline"-package, embedding C code is now quite easy:

library(inline)
SEXP res;
PROTECT(res=allocVector(INTSXP,LENGTH(a)));  int i,j=0;
int *result = INTEGER(res);
int *input = INTEGER(a);
for (i=0;i<LENGTH(a);i++) {
if (input[i]) j = j+1; else j = 0;
result[i] = j;
}
UNPROTECT(1);
return res;
"))
myfastfunc <- cfunction(signature(a="logical"), code)

This solution is about ten times faster than Gabor's on my machine (time to compile the C code excluded!).

Regards,

Martin

> On 6/19/07, Feng, Ken <ken.feng@citi.com> wrote:
>
>> Hi,
>>
>>
>> x <- c( TRUE, TRUE, TRUE, FALSE, FALSE, TRUE, TRUE, FALSE, TRUE );
>>
>> I want to define an y <- f(x) such that:
>>
>> y <- c( 1, 2, 3, 0, 0, 1, 2, 0, 1 );
>>
>> In other words, do a cumsum when I see a TRUE, but reset to 0 if I see a FALSE.
>>
>> I know I can do this with a very slow and ugly loop or maybe use apply,
>> but I was hoping there are some R experts out there who can show me
>> a cleaner/more elegant solution?
>>
>>
>> - Ken
>>
>> ______________________________________________
>> R-help_at_stat.math.ethz.ch mailing list
>>
https://stat.ethz.ch/mailman/listinfo/r-help
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>
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