Default Map Value

In this post, I'll explain how to provide a default value when querying an absent key in a hash map in different programming languages.

Java

Let's start with Java, my first professional programming language.

In older versions, retrieving a value from a map required using the get() method:

Map map = new HashMap();                                 //1
Object value = map.get(new Object());                    //2
if (value == null) {
    value = "default";                                   //3
}

1. Initialize an empty map.
2. Attempt to retrieve a non-existent key.
3. Assign a default value if the key is absent.

With Java 1.8, the Map interface introduced a more concise way to handle absent keys:

var map = new HashMap<Object, String>();
var value = map.getOrDefault(new Object(), "default");   //1

1. Retrieve the value with a default in one step.

Kotlin

Kotlin provides several approaches to retrieve values from a map:

• get() and getOrDefault() function just like their Java counterparts.
• getValue() throws an exception if the key is missing.
• getOrElse() accepts a lambda to provide a default value lazily.

val map = mapOf<Any, String>()
val default = map.getOrDefault("absent", "default")      //1
val lazyDefault = map.getOrElse("absent") { "default" }  //2

1. Retrieve the default value.
2. Lazily evaluate the default value.

Python

Python is less forgiving than Java when handling absent keys — it raises a KeyError:

map = {}
value = map['absent']                                    #1

1. Raises a KeyError

To avoid this, Python offers the get() method:

map = {}
value = map.get('absent', 'default')                     #1

Alternatively, Python's collections.defaultdict allows setting a default for all absent keys:

from collections import defaultdict
map = defaultdict(lambda: 'default')                     #1
value = map['absent']

1. Automatically provide a default value for any absent key.

Ruby

Ruby's default behavior returns nil for absent keys:

map = {}
value = map['absent']

For a default value, use the fetch method:

map = {}
value = map.fetch('absent', 'default')                  #1

1. Provide a default value for the absent key.

Ruby also supports a more flexible approach with closures:

map = {}
value = map.fetch('absent') { |key| key }               #1

1. Return the queried key instead of a constant.

Lua

My experience with Lua is relatively new, having picked it up for Apache APISIX. Let's start with Lua's map syntax:

map = {}                                                --1
map["a"] = "A"
map["b"] = "B"
map["c"] = "C"
for k, v in pairs(map) do                               --2
  print(k, v)                                           --3
end

1. Initialize a new map.
2. Iterate over key-value pairs.
3. Print each key-value pair.

Fun fact: the syntax for tables is the same as for maps:

table = {}                                              --1
table[0] = "zero"
table[1] = "one"
table[2] = "two"
for k,v in ipairs(table) do                             --2
  print(k, v)                                           --3
end

1. Initialize a new map
2. Loop over the pairs of key values

3. Print the following:

   1   one
   2   two

   
   Lua arrays start at index 0!

We can mix and match indices and keys. The syntax is similar, but there's no difference between a table and a map. Indeed, Lua calls the data structure a table:

something = {}                                              
something["a"] = "A"
something[1] = "one"
something["b"] = "B"
for k,v in pairs(something) do
  print(k, v)
end

The result is the following:

1	one
a	A
b	B

In Lua, absent keys return nil by default:

map = {}
value = map['absent']

To provide a default, Lua uses metatables and the __index metamethod:

Metatables allow us to change the behavior of a table. For instance, using metatables, we can define how Lua computes the expression a+b, where a and b are tables. Whenever Lua tries to add two tables, it checks whether either of them has a metatable and whether that metatable has an __add field. If Lua finds this field, it calls the corresponding value (the so-called metamethod, which should be a function) to compute the sum.

- Metatables and Metamethods

Each table in Lua may have its own metatable.

As I said earlier, when we access an absent field in a table, the result is nil. This is true, but it is not the whole truth. Such access triggers the interpreter to look for an __index metamethod: if there is no such method, as usually happens, then the access results in nil; otherwise, the metamethod will provide the result.

- The __index Metamethod

Here's how to use it:

table = {}                                              --1
mt = {}                                                 --2
setmetatable(table, mt)                                 --3
mt.__index = function (table, key)                      --4
  return key
end
default = table['absent']                               --5

1. Create the table.
2. Create a metatable.
3. Associate the metatable with the table.
4. Define the __index function to return the absent key.
5. The __index function is called because the key is absent.

Summary

This post explored how to provide default values when querying absent keys across various programming languages. Here's a quick summary: