Is it possible to set multiple type to a variable?

Question

Consider this

f[x_Real] := 2 x

f[1] 

f[1]

f[1.0]

2.0

I want f to be evaluated if x is an integer or real. One way can be define it twice

f[x_Real] := 2 x
f[x_Integer] := 2 x

which might not be a good idea while dealing with big functions with several variables.

Is it possible to assign multiple type (for example Real and Integer) to a single variable in a function?

Answer 1

You may specify multiple alternative patterns (originally mentioned in a comment):

f[x:(_Real|_Integer)] := 2 x;

You may specify multiple alternative matches simultaneously:

f[(x_Real|x_Integer)] := 2 x;

You may specify by set-membership tests (Element):

f[x_?(Element[#, Reals] === True &)] := 2 x;

Note, however, that while this option will work for exceptional cases like f[Sqrt[2]], is also likely going to be quite a bit slower than any of the above.

If you are concerned not with membership in the reals but rather numeric quantities, you may use:

f[x_?NumericQ] := 2 x;

Note, however, that this will allow complex quantities. This is frequently used to prevent excessive symbolic computation with difficult functions implemented inside of other complex operations, such as Integrate.