是否有可能對數據進行修改?

[英]Is it possible to modify a data.frame in-place (destructively)?


It appears that one can add/delete a column to a data.table in-place, i.e., without copying all the other columns over to a new table.

似乎可以向數據添加/刪除列。表就地,即。,而無需將所有其他列復制到新表中。

Is it possible to do that with a vanilla data.frame?

有可能用一個普通的數據。

PS. I know how to add/delete columns "functionally", i.e., creating a new frame without modifying the original one.

我知道如何“功能性地”添加/刪除列。,創建一個新的框架而不修改原來的框架。

1 个解决方案

#1


33  

You can delete or modify an existing column from a data.frame by reference with data.table::set. I doubt you can add a column without making a copy. The reason that you can add a column to a data.table without making a copy is that data.table over allocates memory. See ?alloc.col for more.

可以使用data.table::set從data.frame中刪除或修改現有列。我懷疑你不復制就能添加一列。您可以向數據添加列的原因。沒有復制的表就是數據。表分配內存。看到了什么? alloc。坳。

R> library(data.table)
R> data(mtcars)
R> tracemem(mtcars)
[1] "<0x59fef68>"
R> set(mtcars, j="mpg", value=NULL)        # remove a column
R> set(mtcars, j="cyl", value=rep(42, 32)) # modify a column
R> untracemem(mtcars)
R> str(mtcars)
'data.frame':   32 obs. of  10 variables:
 $ cyl : num  42 42 42 42 42 42 42 42 42 42 ...
 $ disp: num  160 160 108 258 360 ...
 $ hp  : num  110 110 93 110 175 105 245 62 95 123 ...
 $ drat: num  3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...
 $ wt  : num  2.62 2.88 2.32 3.21 3.44 ...
 $ qsec: num  16.5 17 18.6 19.4 17 ...
 $ vs  : num  0 0 1 1 0 1 0 1 1 1 ...
 $ am  : num  1 1 1 0 0 0 0 0 0 0 ...
 $ gear: num  4 4 4 3 3 3 3 4 4 4 ...
 $ carb: num  4 4 1 1 2 1 4 2 2 4 ...

Compare that with normal data.frame operations

與普通的data.frame操作進行比較

R> data(mtcars)
R> tracemem(mtcars)
[1] "<0x6b3ec30>"
R> mtcars[, "mpg"] <- NULL
tracemem[0x6b3ec30 -> 0x84de0c8]: 
tracemem[0x84de0c8 -> 0x84de410]: [<-.data.frame [<- 
tracemem[0x84de410 -> 0x84de6b0]: [<-.data.frame [<- 
R> tracemem(mtcars)
[1] "<0x84dca30>"
R> mtcars[, "cyl"] <- rep(42, 32)
tracemem[0x84dca30 -> 0x84dcc28]: 
tracemem[0x84dcc28 -> 0x84dd018]: [<-.data.frame [<- 
tracemem[0x84dd018 -> 0x84dff70]: [<-.data.frame [<- 
R> untracemem(mtcars)
R> data(mtcars)

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