R copies for no apparent reason

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


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