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

I received this error message:

Error in if (co开发者_StackOverflow社区ndition) { : missing value where TRUE/FALSE needed

or

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

What does it mean, and how do I prevent it?

The evaluation of condition resulted in an NA. The if conditional must have either a TRUE or FALSE result.

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

This can happen accidentally as the results of calculations:

if(TRUE && sqrt(-1)) {}
## Error in if (TRUE && sqrt(-1)) { : missing value where TRUE/FALSE needed

To test whether an object is missing use is.na(x) rather than x == NA.

See also the related errors:

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

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

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

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

I ran into this when checking on a null or empty string

if (x == NULL || x == '') {

changed it to

if (is.null(x) || x == '') {

this works with "NA" not for NA

comments = c("no","yes","NA")
  for (l in 1:length(comments)) {
    #if (!is.na(comments[l])) print(comments[l])
    if (comments[l] != "NA") print(comments[l])
  }

I was getting this same error in my forloops with complex if statements. I fixed this issue by just wrapping my condition with isTRUE.

if(isTRUE(condition)==TRUE) {do something}