r - 为什么 ifelse 与 %>% 一起使用时只能接收一个参数

Question

通常 ifelse 需要 3 个参数（测试，是，否）：

ifelse(c(1,2) == 1, T, F)

仅提供测试参数会导致错误（因为没有默认的 yes 或 no 字段）：

ifelse(c(1,2) == 1)

在 magrittr 中使用时，ifelse 在仅接收测试参数时工作正常：

c(1:2) %>% ifelse(. == 1)

谁能解释为什么第三段代码可以正常工作，但第二段代码会出错？

Answer 1

# the standard use
ifelse(c(1,2) == 1, T, F)  # [1]  TRUE FALSE

# here it crashes because 1st argument is FALSE for 2 and there's no `no` argument
ifelse(c(1,2) == 1, T)     # Error in ifelse(c(1, 2) == 1, T) : argument "no" is missing, with no default

# but `no` is not needed if the test is always TRUE
ifelse(c(1,1) == 1, T)     # [1] TRUE TRUE

# These are equivalent,
library(magrittr)
c(1:2) %>% ifelse(. == 1)   # [1]  TRUE FALSE
c(1:2) %>% ifelse(.,. == 1) # [1]  TRUE FALSE

# the dot is a vector of non-zero numeric so it evaluates to TRUE both times, the value returned is `yes`, and that is
c(1:2) == 1 # [1]  TRUE FALSE