conditional - Evaluating an If condition to yield True/False

I would like to decide whether an option passed to my custom function has the value Automatic or something else. This is my attempt:

f[x_, OptionsPattern[{DataRange -> Automatic}]]:= 
    Module[{opt = OptionValue[DataRange]},{x, If[opt == Automatic, True, opt]}];

However,

f[x, DataRange -> 20]

produces

{x, If[20 == Automatic, True, opt$540]}

rather than the expected

{x, 20}

What do I need to change?

Answer

You need to use === (or SameQ) instead of == (or Equal) to test the condition. This is because === always returns True or False, whereas == can remain unevaluated. For example:

a === b
(* False *)

a == b
(* a == b *)

The fact that == remains unevaluated is why it is useful in Solve, Reduce and related functions, where you can write an expression such as a x^2 + b x + c == 0.

Now, == does evaluate in cases such as comparisons between numeric quantities and strings or when the objects being compared are identical. For example:

1 == 1
(* True *)


"abc" == "def"
(* False *)

2 == "a"
(* False *)

a == a
(* True *)

However, make note of the fact that comparison between machine numbers and exact numbers can give different results for == and ===:

1 === 1.
(* False *)

1 == 1.
(* True *)

This is because SameQ tests if the two expressions are exactly the same, down to the representation (which they're not), whereas for Equal (see link to docs above):

Approximate numbers with machine precision or higher are considered equal if they differ in at most their last seven binary digits (roughly their last two decimal digits).