data.table聚合到列表列 [英] data.table aggregation to list column

问题描述

我正在尝试从data.table中聚合数据，以创建一个新列，该列是以前行的列表。通过示例更容易看到：

  dt < -  data.table（id = c ，2,2,3,3,3），letter = c（'a'，'a'，'b'，'c'，'a'，'c'，'b'，'b' '））
  

我想以这样一种方式聚合这个结果：

  id letter 
 1：1 a，a，b，c 
 2：2 a，c 
 3：3 b，b，a 
  

直观地我尝试了

  dt [，j = list（list（letter）），by = id] 
   
 
但这不工作。奇怪的是，当我逐个案例，例如：
 > dt [id == 1，j = list（list（letter）），by = id] 
 
 id V1 
 1：1 a，a，b，c 
  
结果很好...我觉得我缺少一个 .SD 某处或类似的东西... 
 
 
 
任何人都能指向正确的方向？ 
 
 
 
谢谢！
解决方案
 更新：行为 DT [，list（list（。）），由=。] 有时会导致R version> = 3.1.0中的错误结果。现在，在提交＃1280 中修正了 data.table  v1.9.3。从新闻：
 
 
 
 
  DT [，list（list（。）），by =。] 返回正确的结果在R> = 3.1.0。该错误是由于R v3.1.0中最近（欢迎）更改，其中 list（。）不会导致复制。关闭＃481 。
 
 
 
 
使用此更新，不再需要 I（）。你可以这样做： DT [，list（list（。）），by =。] 。
 
 
 
 
 
 
这似乎是一个类似的问题，像已知的错误＃5585 。在你的情况下，我想你可以使用
  dt [，粘贴（letter，collapse =，） id] 
  
 
 
 
 
正如@ilir指出，如果实际上希望获取列表（而不是显示的字符），可以使用错误报告中建议的解决方法： / p> 
 
 
  dt [，list（list（I（letter））），by = id] 
  
 
I'm trying to aggregate a data from a data.table to create a new column which is a list of previous rows. It's easier to see by example:
dt <- data.table(id = c(1,1,1,1,2,2,3,3,3), letter = c('a','a','b','c','a','c','b','b','a'))
I would like to aggregate this in such a ways that the result should be
   id  letter
1:  1 a,a,b,c
2:  2     a,c
3:  3   b,b,a  
Intuitively I tried
dt[,j = list(list(letter)), by = id]
but that doesn't work. Oddly enough when I go case by case, for example:
> dt[id == 1,j = list(list(letter)), by = id]

   id      V1
1:  1 a,a,b,c
the result is fine... I feel like I'm missing an .SD somewhere or something like that... 

Can anybody point me in the right direction? 

Thanks!
解决方案
Update: The behaviour DT[, list(list(.)), by=.] sometimes resulted in wrong results in R version >= 3.1.0. This is now fixed in commit #1280 in the current development version of data.table v1.9.3. From NEWS:

  

  
DT[, list(list(.)), by=.] returns correct results in R >=3.1.0 as well. The bug was due to recent (welcoming) changes in R v3.1.0 where list(.) does not result in a copy. Closes #481.
  
With this update, it's not necessary for I() anymore. You can just do: DT[, list(list(.)), by=.] as before.



This seems to be a similar issue as the known bug #5585. In your case, I think you could just use 
dt[, paste(letter, collapse=","), by = id] 
to fix your problem. 

As @ilir pointed out, if it is actually desirable to get a list (rather than the displayed character), you could use the workaround suggested in the bug report: 
dt[, list(list(I(letter))), by = id]


                        
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