[R] as.integer question

[Marc Schwartz]

Erin Hodgess wrote:
> Hi R People:
>
> I'm reading "Statistical Computing with R", by Maria Rizzo, and it's
> really good.
>
> Anyhow, I have a question about something in there.
>
>> u<- runif(5)
>> u
> [1] 0.1177041 0.4271790 0.4601597 0.2204846 0.4051473
>> #in the book
>> sum(as.integer(u>  0.4))
> [1] 3
>> #what I would do
>> sum(u>  0.4)
> [1] 3
>
> Is one way better than the other, please?
>
> Thanks,
> Erin

There is additional coercion overhead in the first approach, since 
as.integer() is called separately:

set.seed(1)

Vec <- sample(c(TRUE, FALSE), 1000000, replace = TRUE)

 > system.time(sum(Vec))
    user  system elapsed
   0.004   0.000   0.025

 > system.time(sum(as.integer(Vec)))
    user  system elapsed
   0.013   0.019   0.050


To paraphrase a financial quote: A microsecond here, a microsecond there 
and pretty soon you are talking about a serious amount of time...  ;-)

HTH,

Marc Schwartz