正确设置多个if语句 [英] Setting multiple if statements correctly

问题描述

我很难在用户定义的函数中设置适当的嵌套 if语句。

I have difficulty to set proper nested if statement in a user-defined function.

我的示例数据就像这样

test <- data.frame(x=rev(0:10),y=10:20)

if_state <- function(x,y) {
  if (x==min(x) && y==max(y)) {
    "good"
  } else if (max(x)/2==y[which(y==15)]/3) {  # to find when x=5 and y=5 condition if it is true set class to "y==5"
    "y==5"
  }
    NA
}

   > test
    x  y
1  10 10
2   9 11
3   8 12
4   7 13
5   6 14
6   5 15
7   4 16
8   3 17
9   2 18
10  1 19
11  0 20

library(dplyr)
test %>%
  mutate(class = if_state(x,y))

    x  y class
1  10 10    NA
2   9 11    NA
3   8 12    NA
4   7 13    NA
5   6 14    NA
6   5 15    NA
7   4 16    NA
8   3 17    NA
9   2 18    NA
10  1 19    NA
11  0 20    NA

我不知道为什么if语句不能正常工作？
问题是与dplyr的 case_when 相同的基本R函数是什么？请参阅下面的注释。

I don't know why the if statement is not working correctly? The question is what is the base R function that work same as dplyr's case_when ? please see the comments below.

因此预期输出

    x  y class
1  10 10    NA
2   9 11    NA
3   8 12    NA
4   7 13    NA
5   6 14    NA
6   5 15    y==5
7   4 16    NA
8   3 17    NA
9   2 18    NA
10  1 19    NA
11  0 20    good

推荐答案

R函数返回在调用期间评估的最后一个值，即使没有显式调用返回（请参见此答案以了解更多信息）；因此，其中 NA 是 if_state 函数中最后求值的值（因为它位于之外） if-else if 控制流，因此将始终对其进行评估），即使，它也会始终返回 NA if 和 else如果条件为真。为了使功能按预期工作，您需要将 NA 移至else语句：

R functions return the last value evaluated evaluated during their invocation, even without an explicit call to return (see this answer for more detail); so, where NA is the last value evaluated in your if_state function (as it's outside the if-else if control flow, and so will always be evaluated), it will always return NA, even when the if and else if conditions are true. For your function to work as you expect, you need to move NA into an else statement:

if_state <- function(x,y) {
  if (x == min(x) && y == max(y)) {
    "good"
  } else if (max(x)/2 == y[which(y == 15)]/3) {
    "y==5"
  } else {
    NA 
  }
}

请注意，使用dplyr时，请测试多个确定返回值的条件通常使用 case_when 可以更简洁地完成：

Note that when using dplyr, testing for multiple conditions to determine a return value is often more succinctly accomplished with case_when:

test %>% mutate(class = case_when(
  x == min(x) && y == max(y) ~ "good",
  max(x)/2 == y[which(y == 15)]/3 ~ "y == 5",
  TRUE ~ NA_character_
))

编辑：基于OP的说明和eipi10的帮助，这是最终功能：

based on OP's clarification and eipi10's help, here is the final function:

if_state = function(x, y) {
  case_when(x == min(x) && y == max(y) ~ "good", 
            x == max(x)/2 & y/3 == 5 ~ "y==5", 
            TRUE ~ NA_character_)
}

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