if/while（条件）中的错误：参数不能解释为逻辑

Posted 2023-02-19

【中文标题】if/while（条件）中的错误：参数不能解释为逻辑

【英文标题】：Error in if/while (condition) : argument is not interpretable as logical

【发布时间】：2015-02-23 22:13:17

【问题描述】：

我收到了错误

Error in if (condition)  : argument is not interpretable as logical

或

Error in while (condition)  : argument is not interpretable as logical

这是什么意思，我该如何预防？

【参考方案1】：

condition 的评估导致了 R 无法解释为合乎逻辑的事情。例如，您可以使用

if("not logical") 
Error in if ("not logical")  : argument is not interpretable as logical

---

在if 和while 条件下，R 会将零解释为FALSE，将非零数字解释为TRUE。

if(1) 

  "1 was interpreted as TRUE"

## [1] "1 was interpreted as TRUE"

但这很危险，因为返回 NaN 的计算会导致此错误。

if(sqrt(-1)) 
## Error in if (sqrt(-1))  : argument is not interpretable as logical
## In addition: Warning message:
## In sqrt(-1) : NaNs produced

最好始终将逻辑值作为if 或while 条件传递。这通常表示包含comparison operator（== 等）或logical operator（&& 等）的表达式。

使用 isTRUE 有时有助于防止此类错误，但请注意，例如，isTRUE(NaN) 是 FALSE，这可能是也可能不是您想要的。

if(isTRUE(NaN)) 

  "isTRUE(NaN) was interpreted as TRUE"
 else

  "isTRUE(NaN) was interpreted as FALSE"

## [1] "isTRUE(NaN) was interpreted as FALSE"

---

类似地，字符串"TRUE"/"true"/"T"和"FALSE"/"false"/"F"可以用作逻辑条件。

if("T") 

  "'T' was interpreted as TRUE"

## [1] "'T' was interpreted as TRUE"

同样，这有点危险，因为其他字符串会导致错误。

if("TRue") 
Error in if ("TRue")  : argument is not interpretable as logical

---

另见相关错误：

Error in if/while (condition) : argument is of length zero

Error in if/while (condition) : missing Value where TRUE/FALSE needed

if (NULL) 
## Error in if (NULL)  : argument is of length zero

if (NA) 
## Error: missing value where TRUE/FALSE needed

if (c(TRUE, FALSE)) 
## Warning message:
## the condition has length > 1 and only the first element will be used

【讨论】：

防止执行错误的最防御措施是始终使用if(isTRUE(cond))。