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[ Automatic>]]:= Module[,];

f[x, DataRange -> 20]

rather than the expected What do I need to change? 125k 11 11 gold badges 404 404 silver badges 579 579 bronze badges asked Jul 27, 2012 at 23:36 groovybaby groovybaby 415 3 3 silver badges 5 5 bronze badges

2 Answers 2

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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).