R数据表的复制和修改更改了原始数据 [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，原始的表被更改了，我无法弄清楚。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

在创建新的data.table时我没有任何问题： dtt = data.table（dt）

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

推荐答案

将新变量分配给已存在的变量时，R不会创建副本，而只是指向新变量，很好，因为除非您绝对需要复制，否则您不想复制。

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.

此后，由于使用了：= 运算符，该运算符可就地修改 （通过引用），由于目前两个对象都指向相同的位置，因此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.

解决方法是使用<$ c $显式复制 data.table c> copy（）函数，然后按如下所示进行引用分配：

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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