Question

通常，ifelse需要3个参数（test，yes，no）：

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仅在收到测试参数时工作正常：

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

任何人都可以解释为什么第三块代码工作正常，但第二块会导致错误吗？

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

为什么ifelse在与％＆gt;％

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