R Tip: use isTRUE().
A lot of R functions are type unstable, which means they return different types or classes depending on details of their values.
For example consider all.equal(), it returns the logical value TRUE when the items being compared are equal:
However, when the items being compared are not equal all.equal() instead returns a message:
This can be inconvenient in using functions similar to all.equal() as tests in if()-statements and other program control structures.
The saving functions is isTRUE(). isTRUE() returns TRUE if its argument value is equivalent to TRUE, and returns FALSE otherwise. isTRUE() makes R programming much easier.
Some examples of isTRUE() are given below:
Using isTRUE() one can write safe and legible code such as the following:
R now also has isFALSE(), but by design it does not mean the same thing as !isTRUE(). The ideas is: for a value v at most of one of isTRUE() or isFALSE() is set, and both are non-NA unnamed scalar logical values. (example: isTRUE(5), isFALSE(5)).
Or as help(isTRUE) puts it:
… if(isTRUE(cond)) may be preferable to if(cond) …
Note: prior to R 3.5 isTRUE (the current version!) was defined as “isTRUE <- function(x) identical(x, TRUE)” (please see change-log here). This seemed clever, but failed on named logical values (violating a principle of least surprise):
This caused a lot of problems (example taken from R3.5.0 NEWS):
Like this:
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