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

Question

I received the error

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

or

Error in while (condition) { : argument is          


        

Answer 1

What causes this error message, and what does it mean?

if statements take a single logical value (technically a logical vector of length one) as an input for the condition.

The error is thrown when the input condition is of length zero. You can reproduce it with, for example:

if (logical()) {}
## Error: argument is of length zero

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

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Common mistakes that lead to this error

It is easy to accidentally cause this error when using $ indexing. For example:

l <- list(a = TRUE, b = FALSE, c = NA)
if(l$d) {}   
## Error in if (l$d) { : argument is of length zero

Also using if-else when you meant ifelse, or overriding T and F.

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Note these related errors and warnings for other bad conditions:

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

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

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

if ("not logical") {}
## Error: argument is not interpretable as logical

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

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How do I test for such values?

NULL values can be tested for using is.null. See GSee's answer for more detail.

To make your calls to if safe, a good code pattern is:

if(!is.null(condition) && 
   length(condition) == 1 && 
   !is.na(condition) && 
   condition) {
  # do something
}

You may also want to look at assert_is_if_condition from assertive.code.