R 数据表复制和修改更改原始数据表 [英] R Data table copy and modification alters original one

问题描述

当我复制一个 data.table 并修改新的表时，原来的表会被更改，我无法找出一个.这是正常行为吗?

When I copy a data.table and modify the new one the orginal one gets altered and I cannot figure out one. Is this a normal behaviour?

dt = data.table(zone=1:5, pc=11:15)
dtt = dt
dtt[, pc := pc*2 ]
dtt

       zone pc
    1:    1 22
    2:    2 24
    3:    3 26
    4:    4 28
    5:    5 30

dt

       zone pc
    1:    1 22
    2:    2 24
    3:    3 26
    4:    4 28
    5:    5 30

更明确地创建新的data.table时我没有问题:dtt = data.table(dt)

I have no problem when creating the new data.table more explicitely: dtt = data.table(dt)

推荐答案

当你将一个新变量分配给一个已经存在的变量时，R 不会创建一个副本，而只是指向新的变量，这很好你不想复制，除非你绝对需要 - 修改时复制.

When you assign a new variable to an already existing variable, R doesn't create a copy, but just points to the new variable, which is very nice as you don't want to make copies unless you absolutely need to - copy on modify.

在此之后，由于您使用 := 运算符，它修改 in-place(通过引用)，并且因为此时，两个对象都指向相同的位置，它会反映在两个对象上.

After this, since you use the:= operator, which modifies in-place (by reference), and since at the moment, both objects are pointing to the same location, it gets reflected on both the objects.

解决方法是显式复制 data.table 使用copy() 函数，然后通过引用分配如下:

The fix is to explicitly copy the data.table using copy() function and then assign by reference as follows:

dtt = copy(dt)     ## dt and dtt are not pointing to same locations anymore
dtt[, pc := pc*2]  ## assignment by reference doesn't affect dt

HTH

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