Question

考虑一个整数数组

> a <- 1:12
> tracemem(a)

设置dim属性不会复制对象

> dim(a) <- c(3, 2, 2)
> a
, , 1

     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6

, , 2

     [,1] [,2]
[1,]    7   10
[2,]    8   11
[3,]    9   12

> dim(a) <- c(dim(a)[1L], prod(dim(a)[-1L]))
> a
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12

但是如果在函数中设置属性会怎么样？

> f <- function(x) { dim(x) <- c(dim(x)[1L], prod(dim(x)[-1L])); x}
> dim(a) <- c(3, 2, 2)
> f(a)
tracemem[0xb99678 -> 0x1b7aac30]: f 
     [,1] [,2] [,3] [,4]
[1,]    1    4    7   10
[2,]    2    5    8   11
[3,]    3    6    9   12

现在它已经复制了。我理解a和f(a)是不同的对象，但是这些对象是指向相同的数据，还是数据也是如此复制？

Answer 1

object_size包中pryr的输出表明数据被复制。

> object_size(list(a))
304 B
> object_size(list(a, a))
312 B
> object_size(list(a, a, a))
328 B
> b <- f(a)
tracemem[0x1fa7660 -> 0xbadae8]: f 
> object_size(list(a, b))
504 B
>

R在复制对象时是否始终复制数据？

1 个答案: