使用 := 从函数内部分配的 data.table 对象未打印 [英] data.table objects assigned with := from within function not printed

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，则只为第二次调用打印结果.

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.

看下图:

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，不会打印表格(这是标准行为.但是，如果我将返回的 data.table 保存到新对象中，则不会在第一次调用，只为第二次调用.

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 

您能解释一下为什么会发生这种情况以及如何预防吗?

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

推荐答案

正如 David Arenburg 在一篇 comment，可以找到答案这里.在 1.9.6 版本中修复了一个错误，但该修复引入了这个缺点.

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.

应该在函数末尾调用 DT[] 来防止这种行为.

One should call DT[] at the end of the function to prevent this behaviour.

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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