逻辑运算符和字符串：函数错误

Question

这是一个产生错误的最小可重现示例：

 comb3 <- function(x) {
      if (x == "Unable to do") {
        x = 0
      } 
    } 

这是我原来的功能：

 comb <- function(x) {
      if (x == "Unable to do") {
        x = 0
      } else if (x == "Very difficult to do") {
        x = 1
      } else if (x == "Somewhat difficult to do") {
        x = 2
      } else if (x == "Not difficult") {
        x = 3
      } 
    }

我正在尝试在下面采样的列上使用此功能。我收到此错误：

Warning messages:
1: In if (x == "Unable to do") { :
  the condition has length > 1 and only the first element will be used
2: In if (x == "Very difficult to do") { :
  the condition has length > 1 and only the first element will be used

Here is a sample of what the data in one column looks like:
sample <- c("Unable to do", "Somewhat difficult to do", "Very difficult to do", "Unable to do", "Not difficult","Unable to do","Very difficult to do", "Not difficult", "Unable to do", "Not difficult")        

Answer 1

警告消息很好地描述了您的代码存在的问题。 if 是一个函数，它期望长度为一个逻辑向量作为输入。因此，要在向量上使用条件，您应该改用 ifelse 之类的东西，或者如 MrFlick 所说，使用 case_when 或 mutate_at。

使用ifelse 的函数的等效版本是这样的：

comb1 <- function(x) {
  ifelse(x == "Unable to do", 
    0,
    ifelse (x == "Very difficult to do",
      1,
      ifelse(x == "Somewhat difficult to do",
        2,
        ifelse(x == "Not difficult",
          3,
          ## If not match then NA
          NA
        )
      )
    )
  )
}

请注意，这很难阅读，因为 ifelse 调用被链接在一起。 因此，您可以通过在调用 sapply 时使用稍微修改过的函数版本来完成同样的事情来避免这种情况

comb2 <- function(x) {
  sapply(x, function(y) {
    if (y == "Unable to do") {
      0
    } else if (y == "Very difficult to do") {
      1
    } else if (y == "Somewhat difficult to do") {
      2
    } else if (y == "Not difficult") {
       3
    }
  ## USE.NAMES = FALSE means that the output is not named, and has no other effect
  }, USE.NAMES = FALSE)
}

您还可以使用因子，它们在内部编码为从 1 开始的整数，并且 (ab) 使用它从字符串转换为数字：

comb3 <- function(x) {
  fac <- factor(x, 
    levels = c(
      "Unable to do",
      "Very difficult to do",
      "Somewhat difficult to do",
      "Not difficult"
    )
  )
  as.numeric(fac) - 1
}

这 3 个版本的输出是相同的，并且是一个很好的例子，说明了在 R 中可以有多种方法来完成任务。这有时可能是一种诅咒而不是一种恩赐。

sample <- c("Unable to do", "Somewhat difficult to do", "Very difficult to do", "Unable to do", "Not difficult","Unable to do","Very difficult to do", "Not difficult", "Unable to do", "Not difficult")
comb1(sample)
# [1] 0 2 1 0 3 0 1 3 0 3
comb2(sample)
# [1] 0 2 1 0 3 0 1 3 0 3
comb3(sample)
# [1] 0 2 1 0 3 0 1 3 0 3