r - if-else 与带列表的 ifelse

Question

为什么 if-else 构造和函数 ifelse() 的行为不同？

mylist <- list(list(a=1, b=2), list(x=10, y=20))

l1 <- ifelse(sum(sapply(mylist, class) != "list")==0, mylist, list(mylist))

l2 <-
if(sum(sapply(mylist, class) != "list") == 0){  # T: all list elements are lists
  mylist
} else {
  list(mylist)
}

all.equal(l1,l2)
#  [1] "Length mismatch: comparison on first 1 components"

Answer 1

从 ifelse 文档:

 ‘ifelse’ returns a value with the same shape as ‘test’ which is
 filled with elements selected from either ‘yes’ or ‘no’ depending
 on whether the element of ‘test’ is ‘TRUE’ or ‘FALSE’.

所以你的输入长度为 1，所以输出被截断为长度 1。

您还可以通过一个更简单的示例来说明这一点:

ifelse(TRUE, c(1, 3), 7)
# [1] 1