R：按在数据表中创建的新变量排序 [英] R: Order by new variable created in Data table

问题描述

我试图理解为什么不能按我在同一行中创建的新变量进行排序。

I am trying to understand why can't I order by a new variable that I create in the same line.

当前我需要写两行，一个

Currently I need to write two lines, one for creating the new variable and then for ordering it.

可以在 data.table 的同一行中完成此操作吗？

Can this be done in the same line in data.table:

DF <- data.table(ID = c(1,2,1,2,1,1,1,1,2), Value = c(1,1,1,1,1,1,1,1,1))
newDF <- DF[order(-Count), .(Count = .N), by = ID] 

# Gives error: Error in eval(v, x, parent.frame()) : object 'Count' not found


# Works Correctly
newDF <- DF[, .(Count = .N), by = ID]
newDF <- newDF[order(-Count)]

> newDF
    ID Count
1:  1     6
2:  2     3

推荐答案

您只需将两个操作链接在一行中即可，

You can simply chain both of the operations in a single line,

DF[, .(Count = .N), by = ID][order(-Count)]

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