Ruby's equivalent to C#'s ?? operator

The name of the operator is the null-coalescing operator. The original blog post I linked to that covered the differences in null coalescing between languages has been taken down. A newer comparison between C# and Ruby null coalescing can be found here.

In short, you can use ||, as in:

a_or_b = (a || b)

If you don't mind coalescing false, you can use the || operator:

a = b || c

If false can be a valid value, you can do:

a = b.nil? ? c : b

Where b is checked for nil, and if it is, a is assigned the value of c, and if not, b.

Be aware that Ruby has specific features for the usual null coalescing to [] or 0 or 0.0.

Instead of

x = y || [] # or...
x = y || 0

...you can (because NilClass implements them) just do...

x = y.to_a # => [] or ..
x = y.to_i # or .to_f, => 0

This makes certain common design patterns like:

(x || []).each do |y|

...look a bit nicer:

x.to_a.each do |y|