data.table对象在从函数返回后不打印 [英] data.table objects not printed after returned from function

问题描述

我想在函数中修改 data.table 。如果我在函数中使用：= 功能，结果只会打印第二次调用。

I would like to modify a data.table within a function. If I use the := feature within the function, the result is only printed for the second call.

在下面的例子中：

library(data.table)
mydt <- data.table(x = 1:3, y = 5:7)

myfunction <- function(dt) {
    dt[, z := y - x]
    dt
}

当我只调用函数时，这是标准的行为但是，如果我将返回的 data.table 保存到一个新对象，它不会在第一次调用时打印，只有第二次调用。 / p>

When I call only the function, the table is not printed (which is the standard behaviour. However, if I save the returned data.table into a new object, it is not printed at the first call, only for the second one.

myfunction(mydt)  # nothing is printed   
result <- myfunction(mydt) 
result  # nothing is printed
result  # for the second time, the result is printed
mydt                                                                     
#    x y z
# 1: 1 5 4
# 2: 2 6 4
# 3: 3 7 4 

你能解释为什么会发生这种情况， / p>

Could you explain why this happens and how to prevent it?

推荐答案

由于 David Arenburg 在评论中提及，可以在此处找到答案。

As David Arenburg mentions in a comment, the answer can be found here. There was a bug fixed in the version 1.9.6 but the fix introduced this downside.

应该调用 DT []

myfunction <- function(dt) {
    dt[, z := y - x][]
}
myfunction(mydt)  # prints immediately
#    x y z
# 1: 1 5 4
# 2: 2 6 4
# 3: 3 7 4 

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