[R] R copies for no apparent reason

[Bert Gunter]

Your assumptions are wrong.

a full discussion and answer to your question can be found here:
http://adv-r.had.co.nz/memory.html

This *is* complex and probably off topic for this list.

Cheers,
Bert


Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Thu, Mar 2, 2017 at 7:35 AM, Jiucang Hao <jiucang at goraltrading.com> wrote:
> Hi Everyone,
>
>
>
> Some R function will make R copy the object AFTER the function call, like
> nrow, while some others don't, like sum. For example the following code:
>
> x = as.double(1:1e8)
>
> system.time(x[1] <- 100)
>
> y = sum(x)
>
> system.time(x[1] <- 200)     ## Fast (takes 0s), after calling sum
>
> foo = function(x) {
>
>     return(sum(x))
>
> }
>
> y = foo(x)
>
> system.time(x[1] <- 300)     ## Slow (takes 0.35s), after calling foo
>
> Calling foo is NOT slow, because x isn't copied. However, changing x again
> is very slow, as x is copied. My guess is that calling foo will leave a
> reference to x, so when changing it after, R makes another copy.
>
> Any one knows why R does this? Even when the function doesn't change x at
> all? Thanks.
>
>
>
>
>
> Regards,
>
> JiuCang Hao
>
>
>         [[alternative HTML version deleted]]
>
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