從列中提取列表，對於另一列中的每個組

Question

我有一個長格式的數據框，其中有兩個因子變量作爲列，第一列代表一系列唯一類，第二列代表分組變量（在這種情況下，班級發生的城市）。

class <- c("A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K") 
city <- c("Atlanta", "Atlanta", "Atlanta", "Detroit", "Detroit", "Milwaukee", "Milwaukee", "Milwaukee", "Milwaukee", "Portland", "Juneau") 
samp <- data.frame(class, city) 

samp 
# class  city 
# A  Atlanta 
# B  Atlanta 
# C  Atlanta 
# D  Detroit 
# E  Detroit 
# F  Milwaukee 
# G  Milwaukee 
# H  Milwaukee 
# I  Milwaukee 
# J  Portland 
# K  Juneau 

對於每個城市，我想要列出所有在那裏發生的課程。理想情況下，輸出將如下所示：

class.list <- list(Atlanta = c("A", "B", "C"), Detroit = c("D", "E"), Milwaukee = c("F", "G", "H", "I"), Portland = "J", Juneau = "K") 

class.list 
# $Atlanta 
# [1] "A" "B" "C" 
# 
# $Detroit 
# [1] "D" "E" 
# 
# $Milwaukee 
# [1] "F" "G" "H" "I" 
# 
# $Portland 
# [1] "J" 
# 
# $Juneau 
# [1] "K" 

我試過各種解決方案並失敗。可以說我來最接近的是通過dcast，但它是不是很我的目標是爲輸出：

library(reshape2) 
class.list <-dcast(samp, city ~ class) 

class.list 
# place  A   B   C  D  E   F ... 
# Atlanta Atlanta Atlanta Atlanta <NA>  <NA>  <NA> ... 
# Detroit <NA>  <NA>  <NA> Detroit Detroit <NA> ... 
# ... 

Answer 1

我們可以使用split

split(as.character(samp$class), samp$city) 
#$Atlanta 
#[1] "A" "B" "C" 

#$Detroit 
#[1] "D" "E" 

#$Juneau 
#[1] "K" 

#$Milwaukee 
#[1] "F" "G" "H" "I" 

#$Portland 
#[1] "J" 

---

或者另一種選擇是unstack

unstack(samp, class~city)