Py11: Dictionary

Dictionary is a data structure that stores information about an object in key and value pairs. Dictionary is also called hash or map in other programming languages.

Introduction

Dictionary is an ordered list of items. Instead of an index, it uses a key to refer to stored value. Dictionary is mutable, we can add new pair-value pairs and update existing values. Its entries are unique, so no two entries are the same.

Creating Dictionary

A dictionary can be created by using the curly brackets { and }.

dict_name = {} # empty dictionary

to create a dictionary with pairs of keys and values.

dict_name = {key1: value1, key2:value2,...., keyN:valueN}

The key can be an integer or a string, usually, it's a string. The value can be numbers, strings, lists, tuples, or dictionaries.

Example

a_dict = {'title':'Peter Pan', 'year':1911, 'author':'J.M.Barrie'}

print(a_dict)

Output

{'title': 'Peter Pan', 'year': 1911, 'author': 'J.M.Barrie'}
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Display using Loop

Display using loop using the items() function. It returns the key and value of each item in the dictionary.

a_dict = {'title':'Peter Pan', 'year':1911, 'author':'J.M.Barrie'}

for item in a_dict.items(): # iterate dictionary items()
    print(item)             # print current item

Output

('title', 'Peter Pan')
('year', 1911)
('author', 'J.M.Barrie')
>

To assign key and value obtained from items() function in a separate variable.

for key, value in a_dict.items():# iterate dictionary items.
    print(key,"=", value)

Output

title = Peter Pan
year = 1911
author = J.M.Barrie
>

Other than this, the values() and the keys() function can be used to only get a list of keys or values. And, the len() function returns how many items are there in a dictionary.

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Accessing Dictionary

Instead of an index number, the key is used to refer to an element in the dictionary. The dict_name[key] code returns a value corresponding to the key.

a_dict = {'title':'Peter Pan', 'year':1911, 'author':'J.M.Barrie'}

print(a_dict['year'])     # get value of 'year' key

Output

1911
>

And, dict_name.get(key) returns the value corresponding to the key, None if the key is not in the dictionary.

a_dict = {'title':'Peter Pan', 'year':1911, 'author':'J.M.Barrie'} 

print(a_dict.get('year'))     # get value of year 

print(a_dict.get('pages'))    # get value of pages

Output

1911 
None
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Modify Element

The following example shows how to modify a dictionary element.

a_dict = {'title':'Peter Pan', 'year':1911, 'author':'J.M.Barrie'}

a_dict['author'] = 'James Matthew Barrie'    # update author key

print(a_dict)           # print key:value in dictionary

Output

{'title': 'Peter Pan', 'year': 1911, 'author': 'James Matthew Barrie'}
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Add Element

The following example shows how to add an element to a dictionary. An element with a key named "pages" with a value of 267 is added to the dictionary.

a_dict = {'title':'Peter Pan', 'year':1911, 'author':'J.M.Barrie'}

a_dict['pages'] = 267   # add a new element the key:value pair

print(a_dict)           # print key:value in dictionary

Output

{'title': 'Peter Pan', 'year': 1911, 'author': 'J.M.Barrie', 'pages': 267}
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Remove element

The following example shows how to delete an element from a dictionary. Element with a key value of "year" is removed from the dictionary.

a_dict = {'title':'Peter Pan', 'year':1911, 'author':'J.M.Barrie'}

del a_dict['year'] # delete the 'year' key: value pair
print(a_dict)      # print key:value in dictionary

Output

{'title': 'Peter Pan', 'author': 'J.M.Barrie'}
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A post by Cuber

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