Dictionary

In programming, a dictionary (or associative container, symbol table, map) is a data type that has a collection of key-value pairs. Items within dictionaries are unordered.

myDict = { # declaration in Python
		  'name': 'Joe!',
		  'height': '170',
		  'mood': 'terrible'
}

Designing efficient data structures to implement dictionaries is difficult (called the dictionary problem), but can be done with hash tables and search trees. Language support of dictionaries is mixed. In Python, they are called dictionaries. In C++ and Java, they’re called maps.

Python

Dictionaries in Python are declared with curly brackets (see above for an example). Some associated methods are:

• keys() and values() return a “list” of associated data.
• get('key', fallback) grabs a key’s value. If it doesn’t exist, it returns the second argument.
• setdefault('key', value) sets a default value for a given key, even if it doesn’t exist.

C++

The C++ map is a part of the Standard Template Library as std::map, implemented with a red-black tree (as a balancing BST).

The std::map iterator advances as an in-order traversal of the