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rlogical-operatorsr-faq

Logical operators (AND, OR) with NA, TRUE and FALSE

I cannot understand the properties of logical (boolean) values TRUE, FALSE and NA when used with logical OR (|) and logical AND (&). Here are some examples:

NA | TRUE
# [1] TRUE

NA | FALSE
# [1] NA

NA & TRUE
# [1] NA

NA & FALSE
# [1] FALSE

Can you explain these outputs?

Solution

  • To quote from ?Logic:

    NA is a valid logical object. Where a component of x or y is NA, the result will be NA if the outcome is ambiguous. In other words NA & TRUE evaluates to NA, but NA & FALSE evaluates to FALSE. See the examples below.

    The key there is the word "ambiguous". NA represents something that is "unknown". So NA & TRUE could be either true or false, but we don't know. Whereas NA & FALSE will be false no matter what the missing value is.