What evaluates to True/False in R?

Question

For example, in Ruby, only nil and false are false. What is what in R?

e.g.: 5==TRUE and 5==FALSE both evaluate to FALSE. However, 1==TRUE is TRUE. Is there any general rule as to what (objects, numbers, etc.) evaluate to?

Answer 1

This is documented on ?logical. The pertinent section of which is:

Details:       ‘TRUE’ and ‘FALSE’ are reserved words denoting logical constants      in the R language, whereas ‘T’ and ‘F’ are global variables whose      initial values set to these.  All four are ‘logical(1)’ vectors.       Logical vectors are coerced to integer vectors in contexts where a      numerical value is required, with ‘TRUE’ being mapped to ‘1L’,      ‘FALSE’ to ‘0L’ and ‘NA’ to ‘NA_integer_’. 

The second paragraph there explains the behaviour you are seeing, namely 5 == 1L and 5 == 0L respectively, which should both return FALSE, where as 1 == 1L and 0 == 0L should be TRUE for 1 == TRUE and 0 == FALSE respectively. I believe these are not testing what you want them to test; the comparison is on the basis of the numerical representation of TRUE and FALSE in R, i.e. what numeric values they take when coerced to numeric.

However, only TRUE is guaranteed to the be TRUE:

> isTRUE(TRUE) [1] TRUE > isTRUE(1) [1] FALSE > isTRUE(T) [1] TRUE > T <- 2 > isTRUE(T) [1] FALSE 

isTRUE is a wrapper for identical(x, TRUE), and from ?isTRUE we note:

Details: ....       ‘isTRUE(x)’ is an abbreviation of ‘identical(TRUE, x)’, and so is      true if and only if ‘x’ is a length-one logical vector whose only      element is ‘TRUE’ and which has no attributes (not even names). 

So by the same virtue, only FALSE is guaranteed to be exactly equal to FALSE.

> identical(F, FALSE) [1] TRUE > identical(0, FALSE) [1] FALSE > F <- "hello" > identical(F, FALSE) [1] FALSE 

If this concerns you, always use isTRUE() or identical(x, FALSE) to check for equivalence with TRUE and FALSE respectively. == is not doing what you think it is.