子集设置列适用于data.frame，但不适用于data.table [英] Subsetting columns works on data.frame but not on data.table

问题描述

我可以从 data.frame 中选择几个列：

 > z [c（events，users）] 
事件用户
 1 26246016 201816 
 2 942767 158793 
 3 29211295 137205 
 4 30797086 124314 
  

，但不是 data.table ：

 >最好的[c（events，users）] 
开始二进制搜索...错误在`[.data.table`（best，c（events，users））：
 typeof x.pixel_id（integer）！= typeof i.V1（character）
 Calls：[ - > [.data.table 
  

我该怎么办？
有更好的方法比 turn data.table 返回 data.frame ？

c> c> c> c> $ c>。请改为：

  DT [，c（x，y），with = FALSE] 
   <= code>    j 不作为表达式求值。 
 
 
 
检查此演示文稿（幻灯片4）以了解如何阅读 data.table 语法（更像SQL）。这将有助于说服您在 j 中提供列 - 在SQL中等效于SELECT，这样更有意义。
 
I can select a few columns from a data.frame:
> z[c("events","users")]
     events  users
1  26246016 201816
2    942767 158793
3  29211295 137205
4  30797086 124314
but not from a data.table:
> best[c("events","users")]
Starting binary search ...Error in `[.data.table`(best, c("events", "users")) : 
  typeof x.pixel_id (integer) != typeof i.V1 (character)
Calls: [ -> [.data.table
What do I do?
Is there a better way than to turn the data.table back into a data.frame?
解决方案
Column subsetting should be done in j, not in i. Do instead:
DT[, c("x", "y"), with=FALSE]
The with=FALSE makes sure that j is not evaluated as an expression. This is basically the idiomatic way for column subset if you're looking for similar means as one would subset on a data.frame.

Check this presentation (slide 4) to get an idea of how to read a data.table syntax (more like SQL). That'll help convince you that it makes more sense for providing columns in j - equivalent of SELECT in SQL.

                        
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