In Ruby, to assign a value if it doesn't exist, we can use the ||= operator or the fetch() method.
The ||= operator assigns a value to a variable only if that variable is nil or false. Here's an example:
my_var ||= "default value"
In this example, my_var is assigned the value "default value" only if my_var is nil or false.
The fetch() method can also be used to assign a default value to a variable if it doesn't exist. Here's an example:
my_hash = {a: 1, b: 2}
my_var = my_hash.fetch(:c, "default value")
In this example, we're trying to fetch the value of the key :c from the hash my_hash. Since the key :c doesn't exist in the hash, fetch() returns the second argument, "default value", which is then assigned to my_var.
Both of these methods can be helpful for setting default values in a program and avoiding errors caused by uninitialized variables.