dplyr汇总data.table：引用刚刚创建的列

Question

引用刚刚在dplyr中创建的列在tbl_dfs上工作但在data.tables上没有。这是预期的吗？我在文档中没有看到关于此的说明。

以下是可重现的例子：

library("hflights")
library("plyr")
library("dplyr")
library("data.table")

hflights_df <- tbl_df(hflights)

summarise(hflights_df,
      delay = mean(DepDelay, na.rm = TRUE),
      delay2 = 2*delay)
## Source: local data frame [1 x 2]
## 
##      delay  delay2
## 1 9.444951 18.8899

hflights_dt <- data.table(hflights_df)

summarise(hflights_dt,
      delay = mean(DepDelay, na.rm = TRUE),
      delay2 = 2*delay)
## Error in eval(expr, envir, enclos) : object 'delay' not found

这是我的sessionInfo：

sessionInfo()
## R version 3.0.2 (2013-09-25)
## Platform: x86_64-apple-darwin10.8.0 (64-bit)
## 
## locale:
## [1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/C
## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## 
## other attached packages:
## [1] data.table_1.9.2  dplyr_0.2         plyr_1.8.1        hflights_0.1     
## [5] andRstuff_1.0     devtools_1.5.0.99 tikzDevice_0.7.0  filehash_2.2-2   
## 
## loaded via a namespace (and not attached):
##  [1] assertthat_0.1 compiler_3.0.2 digest_0.6.4   evaluate_0.5.3 grid_3.0.2    
##  [6] httr_0.3       memoise_0.1    parallel_3.0.2 Rcpp_0.11.1    RCurl_1.95-4.1
## [11] reshape2_1.2.2 stringr_0.6.2  tools_3.0.2    whisker_0.3-2 

编辑： 这有效（类似于jazzurro和KFB的建议）：

summarise(tbl_df(hflights_dt),
      delay = mean(DepDelay, na.rm = TRUE),
      delay2 = 2*delay)
## Source: local data frame [1 x 2]
## 
##      delay  delay2
## 1 9.444951 18.8899

但这不起作用：

summarise(tbl_dt(hflights_dt),
      delay = mean(DepDelay, na.rm = TRUE),
      delay2 = 2*delay)
## Error in eval(expr, envir, enclos) : object 'delay' not found

Answer 1

改为使用as.data.table：

> hflights_df <- tbl_df(hflights)
> hflights_dt <- as.data.table(hflights_df)
> summarise(hflights_dt,
+           delay = mean(DepDelay, na.rm = TRUE),
+           delay2 = 2*delay)
Source: local data frame [1 x 2]

     delay  delay2
1 9.444951 18.8899