总结一个嵌套列表

Question

如果我有如下列表：

pets = list(list("Shoji", "cat", 18), list("Hanako", "dog", 15), list("Sachiko", "alligator", 7), list("Toby", "dog", "12"))

如何将动物的名称归入不同的动物类别，并将相同的动物归入相同的类别？

$cat
[1] "Shoji"

$dog
[1] "Hanako" "Toby"  

$alligator
[1] "Sachiko"

Answer 1

你可以使用tapply:

tapply(X = sapply(pets, `[[`, 1), 
       INDEX = sapply(pets, `[[`, 2), 
       FUN = paste)
# $alligator
# [1] "Sachiko"
# 
# $cat
# [1] "Shoji"
# 
# $dog
# [1] "Hanako" "Toby"