The Dictionary Data structure


This page briefly reviews a dictionary, also known as an associative array, a map or a symbol table. A dictionary is composed of a collection of keys and values, and each key appears only once in a collection. JSON files, human-readable files that are commonly used for web applications, contains data objects in the form of a dictionary.

Let's demonstrate how a dictionary is defined in python. Open a python repl by typing on the command line: python. Once in your repl type:

a = { 'Hello': 'How are you?'}

'a' is a dictionary. Notice how it was defined with the curly brackets. This dictionary contains one key on the left of the colon, and one value on the right side of the colon. You can retrieve the value by typing in the repl:


and pressing enter, which returns 'How are you?'. You can retreive the list of keys by typing:


and pressing enter, which returns 'Hello'. Now let's make this dictionary a little more complicated:

a = { 'Hello': 'How are you?', 'Goodbye': 'See you Later!' }

Here you now have two keys that have their own associated values. Here you can still run:


which will still return 'How are you?', but now you can type:


which returns 'See you Later!' A list of keys can be obtained by typing:


which returns the keys in a list form: ['Hello', 'Goodbye']. These keys can be retrieved individually by specifying their element indices starting at 0. For example:


returns 'Hello'. Making sense? OK, let's add one more layer of abstraction which is that the values (of the key/value pair) can be a dictionary. Changing our dictionary again, type:

a = { 'Hello': 'How are you?', 'Goodbye': {'a':'See you Later!','b':'Later Gator' }}

into the python repl. Notice that the key 'Goodbye' now points to a dictionary. If you type into the python repl:

a ['Goodbye']

you now get the dictionary {'a': 'See you Later!', 'b': 'Later Gator'}. You can choose a particular value of this sub-dictionary by specifying:

a ['Goodbye']['b']

which returns 'Later Gator'.

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