R copies for no apparent reason

Question

Some function R will make R copy the object AFTER calling the function, for example, nrow, while some others will not, for example, 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

The call to foo is NOT slow because x is not copied. However, changing x again happens very slowly, as x is copied. I assume that calling foo will leave a link to x, so by changing it, R will make another copy.

Does anyone know why R does this? Even if the function does not change x at all? Thanks.

Answer 1

Hadley Advanced R , , , , . (, @joran @lmo), , --.

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