Unordered Collection

Python provides two data structures for unordered collection: Dictionaries and Sets.

Dictionaries

Dictionary is an unordered set of key:value pairs in which the keys are unique. A pair of braces creates an empty dictionary: { }

Here is an example of dictionary with values:

country_codes = {'US':'United States', 'UK':'United Kingdom', 
                'CA':'Canada', 'MX':'Mexico'}

Sequences are indexed by range of numbers but dictionaries are indexed by keys. A key can be of any immutable type although using a string or a number is most common. Tuples can be used as keys as long as the elements of the tuples are immutable.

Here are some of common operations on Dictionaries

Set

Set data structure is an unordered collection with no duplicate elements. Using curly braces { } you can create a set. There is also a set() function you can use to create a Set. Since empty curly brace { } is used to create both Set and Dictionary, to create an empty set you have to use set() function.

Here is an example:

americas = {'Canada', 'United States', 'Mexico', 'Mexico'}
print(americas)

Output: {'United States', 'Canada', 'Mexico'}

You will notice that duplicate 'Mexico' is not added. Also please note that your output may be different from the one shown. It may or may not be in the same order. Print output order is not predictable. Hence the term unordered collection. If you want the sequence order to be maintained then you should use one of the ordered collection types.

You can also construct using set() function. Here we create a set out of list elements.

americas = set(['Canada', 'United States', 'Mexico'])
print(americas)

Output: {'United States', 'Canada', 'Mexico'}

If you do not put the square braces and also remove comma between words, then a set is formed with all the letters in the words.

americas = set('Canada' 'United States' 'Mexico')
print(americas)

Output: {'c', 'e', ' ', 'i', 'd', 'n', 'M', 'o', 'S', 't', ',', 's', 'a', 'U', 'C', 'x'}

You will notice that the result is not only case sensitive but also duplicate letters are eliminated. Notice only one letter 'a' is present in the set and similarly other duplicate letters are eliminated.

Set operations

Let us consider another set of three countries which were English colonies:

old_eng_col = {'Canada', 'India', 'Australia', 'United States'}

Then, here are some of the common set operations you can perform on both the sets and their results: