r - 使用 mean() 和 sum()/.N 时，按组的 data.table 平均值提供不同的结果

Question

在 data.table 中按组计算平均值时，我得到了不同的结果:

qty <- c(1:6)
name <- c("a", "b","a", "a", "c","b" )
type <- c("i", "i", "i", "f", "f", "f")

DT <- data.table(qty,name,type) 

DT[, avg_mean  := mean(qty)   , by = .(name, type)]
DT[, avg_sum_N := sum(qty)/.N , by = .(name, type)]

 > DT
     qty   name   type avg_mean avg_sum_N
   <int> <char> <char>    <num>     <num>
1:     1      a      i        2         2
2:     2      b      i        4         2
3:     3      a      i        2         2
4:     4      a      f        2         4
5:     5      c      f        6         5
6:     6      b      f        5         6

我希望 avg_mean 和 avg_sum_N 完全相同，例如 avg_sum_N。为什么它们不同？谢谢。

请在下面找到 session 信息。

> packageVersion('data.table')
[1] ‘1.14.3’
> sessionInfo()
R version 4.1.0 (2021-05-18)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19044)

Matrix products: default

locale:
[1] LC_COLLATE=Portuguese_Brazil.1252  LC_CTYPE=Portuguese_Brazil.1252    LC_MONETARY=Portuguese_Brazil.1252
[4] LC_NUMERIC=C                       LC_TIME=Portuguese_Brazil.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] zoo_1.8-10        lubridate_1.8.0   RPostgres_1.4.3   DBI_1.1.2         stringi_1.7.6     readxl_1.4.0     
 [7] gsubfn_0.7        proto_1.0.0       stringr_1.4.0     magrittr_2.0.3    stringdist_0.9.8  fuzzyjoin_0.1.6  
[13] data.table_1.14.3

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.8.3     pillar_1.7.0     compiler_4.1.0   cellranger_1.1.0 tools_4.1.0      bit_4.0.4       
 [7] lattice_0.20-44  lifecycle_1.0.1  tibble_3.1.6     pkgconfig_2.0.3  rlang_1.0.2      cli_3.2.0       
[13] rstudioapi_0.13  writexl_1.4.0    parallel_4.1.0   dplyr_1.0.8      hms_1.1.1        generics_0.1.2  
[19] vctrs_0.4.1      grid_4.1.0       bit64_4.0.5      tidyselect_1.1.2 glue_1.6.2       R6_2.5.1        
[25] fansi_1.0.3      tcltk_4.1.0      blob_1.2.3       purrr_0.3.4      ellipsis_0.3.2   assertthat_0.2.1
[31] utf8_1.2.2       crayon_1.5.1

Answer 1

问题是 dev data.table 版本中的错误。data.table::update.dev.pkg() 修复了问题。