複製data.tables

Question

的名單我有以下情況：

1）數據表

2）出於測試目的的名單我故意想（深）複製整個列表，包括數據表

3）我想從複製列表中取一些元素並添加一個新列。

下面是代碼：

library(data.table) 
x = data.table(aaa = c(1,2)) 
y = data.table(bbb = c(1,2)) 
z = list(x,y) 
zz = copy(z) 

v = zz[[1]] 
v = v[, newColumn := 1] 

現在即時得到以下錯誤：

Error in `[.data.table`(res, , `:=`(xxx, TRUE)) : 
(converted from warning) Invalid .internal.selfref detected and fixed 
by taking a copy of the whole table so that := can add this new column 
by reference. At an earlier point, this data.table has been copied by R 
(or been created manually using structure() or similar). Avoid key<-, 
names<- and attr<- which in R currently (and oddly) may copy the whole 
data.table. Use set* syntax instead to avoid copying: ?set, ?setnames 
and ?setattr. Also, in R<=v3.0.2, list(DT1,DT2) copied the entire DT1 
and DT2 (R's list() used to copy named objects); please upgrade to 
R>v3.0.2 if that is biting. If this message doesn't help, please report 
to datatable-help so the root cause can be fixed. 

我不明白的副本呼叫系統精確的用R處理的，它們是如何傳遞給數據。表，但不是這樣的：（？）

如果有人明確地使用複製功能，然後他/她意識到「按價值」和「按引用」之間存在差異的事實。所以他/她應該分發對象的真實副本。

因此，我認爲不應該有任何錯誤，我認爲這是一個錯誤發生的'錯誤'。那是對的嗎？

FW

Answer 1

copy()是用於複製data.table的。您正在使用它來複制list。嘗試..

zz <- lapply(z,copy) 
zz[[1]][ , newColumn := 1 ] 

---

使用你原來的代碼，你會看到，申請copy()到list不會使原有data.table的副本。它們仍被內存中的相同位置引用：

library(data.table) 
x = data.table(aaa = c(1,2)) 
y = data.table(bbb = c(1,2)) 
z = list(x,y) 
zz = copy(z) 

# Both zz$x and z$x are the same object: 
.Internal(inspect(zz$x)) 
# @7fd58a079778 00 NILSXP g1c0 [MARK,NAM(2)] 
.Internal(inspect(z$x)) 
# @7fd58a079778 00 NILSXP g1c0 [MARK,NAM(2)]