Page 420 - Computer Science V2.0 Class 11

14 DICTIONARIES

Chapter Outline

14.1 Dictionary in Python
14.2 Aggregate operations min, max, and sum that can be applied on dictionaries
14.3 Nested Dictionary 14.4 Traversing a Dictionary
14.5 Dictionary Methods 14.6 Sorting Keys/ Values of a Dictionary
14.7 Building a Thesaurus

Introduction

We are already familiar with objects of type tuple, list, and str (strings). An str object defines a mapping of
integer indexes to characters. For example, the str object 'beautiful' maps integers 0, 1, 2, …, 8 to characters
'b', 'e', 'a', 'u', 't', 'i', 'f', 'u', and 'l'. Similarly, lists and tuples map integer indexes
to objects of arbitrary types. The Python dictionaries take the generalisation a step further, by mapping objects of
arbitrary immutable types to objects of arbitrary types, including mutable types. Thus, a dictionary may be thought
of as a collection of key-value pairs, i.e., a set of keys of immutable types that map to a set of values. For example, a
dictionary of synonyms might comprise (word, synonyms)pairs, where a word is an str object, and synonyms
is a list object comprising the synonyms of the word. In this chapter, we will learn about dictionaries and their
applications.

14.1 Dictionary in Python

A dictionary is an unordered set of key:value pairs. A dictionary (an object of type dict) is defined in Python by
enclosing the comma-separated key: value pairs in braces. An empty pair of braces denotes an empty dictionary. The
keys in a dictionary are required to be unique. However, the same value may be associated with multiple keys. As keys
are used to search for a key-value pair, given a key-value pair, Python disallows modification of keys. So, objects of
the immutable types, strings, numbers, and tuples are used as keys. However, the values associated with keys may be
objects of any type. The syntax for defining and naming a dictionary is as follows:
{key1:value1, key2:value2, ... }
For example,
>>> subjects = {'Sanskrit':78, 'English':85, 'Maths':88, 'Hindi':90}
>>> subjects
{'Sanskrit': 78, 'English':85, 'Maths':88, 'Hindi':90}

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