Test If a Value is One of a List

Category: Coding

Problem

You need to check if a value is one of several different choices.

Ingredients

if, Regular Expressions, object.Has?, object.Member?, switch

Recipe

The most obvious solution is to simply use if and or, for example:

if (value is 'red' or value is 'green' or value is 'blue')

However, if you have more than a few choices, or if you need to do this test several times, it soon becomes awkward.

Another option is to use regular expressions:

if (value =~ "^(red|green|blue)$")

The ^(...)$ ensure that you are matching the entire string, not just a portion.

This option will only work for strings. For other types of values, you can use an object to contain the list:

if (#('red', 'green', 'blue').Has?(value))

For simple values (e.g. strings, numbers, dates), it's faster to make the values the members (keys) of the object:

if (#('red':, 'green':, 'blue':).Member?(value))

This is faster because it takes advantage of the hash lookup that object use to find members. Notice that we didn't supply any values for the members - the default value is true.

Another option is to use a switch:

switch (value)
    {
case 'red', 'green', 'blue':
    ...
    }

This is usually not the best choice unless you need to test for several different cases.

Discussion

For a small number of string values I would use a regular expression. My next choice for simple values would be .Member?. Otherwise, the most general solution is to use .Has?.