如何在 data.table 列中放置不同大小的向量

Question

我已经使用?stats::aggregate 函数实现了一个简单的分组操作。它在向量中收集每组的元素。我想使用 data.table 包让它更快。但是我无法使用 data.table 重现想要的行为。

样本数据集：

df <- data.frame(group = c("a","a","a","b","b","b","b","c","c"), val = c("A","B","C","A","B","C","D","A","B"))

使用 data.table 重现的输出：

by_group_aggregate <- aggregate(x = df$val, by = list(df$group), FUN = c)

我尝试过的：

data_t <- data.table(df)
# working, but not what I want
by_group_datatable <- data_t[,j = paste(val,collapse=","), by = group]
# no grouping done when using c or as.vector
by_group_datatable <- data_t[,j = c(val), by = group]
by_group_datatable <- data_t[,j = as.vector(val), by = group]
# grouping leads to error when using as.list
by_group_datatable <- data_t[,j = as.list(val), by = group]

data.table 列中是否可以有不同大小的向量？如果是，我该如何实现？

Answer 1

这是一种方法：

data_t[, list(list(val)), by = group]
#   group      V1
#1:     a   A,B,C
#2:     b A,B,C,D
#3:     c     A,B

使用第一个list() 是因为您要聚合结果。使用第二个 list 是因为您希望将 val 列聚合到每个组的单独列表中。

检查结构：

str(data_t[, list(list(val)), by = group])
#Classes ‘data.table’ and 'data.frame': 3 obs. of  2 variables:
# $ group: Factor w/ 3 levels "a","b","c": 1 2 3
# $ V1   :List of 3
#  ..$ : Factor w/ 4 levels "A","B","C","D": 1 2 3
#  ..$ : Factor w/ 4 levels "A","B","C","D": 1 2 3 4
#  ..$ : Factor w/ 4 levels "A","B","C","D": 1 2
# - attr(*, ".internal.selfref")=<externalptr> 

---

使用 dplyr，您可以执行以下操作：

library(dplyr)
df %>% group_by(group) %>% summarise(val = list(val))
#Source: local data frame [3 x 2]
#
#   group         val
#  (fctr)       (chr)
#1      a <S3:factor>
#2      b <S3:factor>
#3      c <S3:factor>

检查结构：

df %>% group_by(group) %>% summarise(val = list(val)) %>% str
#Classes ‘tbl_df’, ‘tbl’ and 'data.frame':  3 obs. of  2 variables:
# $ group: Factor w/ 3 levels "a","b","c": 1 2 3
# $ val  :List of 3
#  ..$ : Factor w/ 4 levels "A","B","C","D": 1 2 3
#  ..$ : Factor w/ 4 levels "A","B","C","D": 1 2 3 4
#  ..$ : Factor w/ 4 levels "A","B","C","D": 1 2

Answer 2

这是dplyr/tidyr的另一个选项

library(dplyr)
library(tidyr)
res <- df %>% 
        nest(-group)
str(res)
#'data.frame':   3 obs. of  2 variables:
# $ group: Factor w/ 3 levels "a","b","c": 1 2 3
# $ data :List of 3
#  ..$ :'data.frame':    3 obs. of  1 variable:
#  .. ..$ val: Factor w/ 4 levels "A","B","C","D": 1 2 3
#  ..$ :'data.frame':    4 obs. of  1 variable:
#  .. ..$ val: Factor w/ 4 levels "A","B","C","D": 1 2 3 4
#  ..$ :'data.frame':    2 obs. of  1 variable:
#  .. ..$ val: Factor w/ 4 levels "A","B","C","D": 1 2