difftime的分组平均值在data.table中失败 [英] Grouped mean of difftime fails in data.table

问题描述

我在difftime值的data.table中有一列，单位设置为天。我正在尝试创建另一个data.table来总结这些值，

I have a column in a data.table of difftime values with units set to days. I am trying to create another data.table summarizing the values with

dt2 <- dt[, .(AvgTime = mean(DiffTime)), by = Group]

在打印新data.table时，我看到诸如

When printing the new data.table, I see values such as

1.925988e+00 days
1.143287e+00 days
1.453975e+01 days

我只想限制此列的小数位数（即不设置 options （），除非我可以这样专门针对difftime值执行此操作）。当我尝试使用上述方法进行修改时，例如

I would like to limit the decimal place values for this column only (i.e. not setting options() unless I can do this specifically for difftime values this way). When I try to do this using the method above, modified, e.g.

dt2 <- dt[, .(AvgTime = round(mean(DiffTime)), 2), by = Group]

我剩下NA值，基本的 round（）和 format（）函数都返回警告：

I am left with NA values, with both the base round() and format() functions returning the warning:

在mean（DiffTime）中：参数不是数字或逻辑。

In mean(DiffTime) : argument is not numeric or logical.

奇怪的是，如果我在数字字段上执行相同的操作，则运行不会有任何问题。另外，如果我运行两行代码，则可以完成我想做的事情：

Oddly enough, if I perform the same operation on a numeric field, this runs with no problems. Also, if I run the two separate lines of code, I can accomplish what I am looking to do:

dt2 <- dt[, .(AvgTime = mean(DiffTime)), by = Group]
dt2[, AvgTime := round(AvgTime, 2)]

可重现的示例：

Reproducible Example:

library(data.table)
set.seed(1)
dt <- data.table(
  Date1 = 
    sample(seq(as.Date('2017/10/01'), 
               as.Date('2017/10/31'), 
               by="days"), 24, replace = FALSE) +
    abs(rnorm(24)) / 10,
  Date2 = 
    sample(seq(as.Date('2017/10/01'), 
               as.Date('2017/10/31'), 
               by="days"), 24, replace = FALSE) +
    abs(rnorm(24)) / 10,
  Num1 =
    abs(rnorm(24)) * 10,
  Group = 
    rep(LETTERS[1:4], each=6)
)
dt[, DiffTime := abs(difftime(Date1, Date2, units = 'days'))]

# Warnings/NA:
class(dt$DiffTime) # "difftime"
dt2 <- dt[, .(AvgTime = round(mean(DiffTime), 2)), by = .(Group)]

# Works when numeric/not difftime:
class(dt$Num1) # "numeric"
dt2 <- dt[, .(AvgNum = round(mean(Num1), 2)), by = .(Group)]

# Works, but takes an additional step:
dt2<-dt[,.(AvgTime = mean(DiffTime)), by = .(Group)]
dt2[,AvgTime := round(AvgTime,2)]

# Works with base::mean:
class(dt$DiffTime) # "difftime"
dt2 <- dt[, .(AvgTime = round(base::mean(DiffTime), 2)), by = .(Group)]

问题：

当类为 difftime 时，为什么为什么我不能一步一步完成转换（均值舍入）？我在执行过程中缺少什么吗？是 data.table 中的某种错误，无法正确处理 difftime ？

Question:

Why am I not able to complete this conversion (rounding of the mean) in one step when the class is difftime? Am I missing something in my execution? Is this some sort of bug in data.table where it can't properly handle the difftime?

在 github 上添加的问题。

更新：从data.table版本 1.10.4 更新为后，问题似乎已清除。 1.12.8 。

Update: Issue appears to be cleared after updating from data.table version 1.10.4 to 1.12.8.

推荐答案

此问题已通过更新＃3567 on 2019/05/15，data.table版本 1.12.4 已发布 2019/10/03

This was fixed by update #3567 on 2019/05/15, data.table version 1.12.4 released 2019/10/03

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