After strings and lists, let\u2019s talk about dictionaries.<\/p>\n
Like a real-life dictionary has words and meanings, Python dictionaries have keys<\/strong> and values<\/strong>. They are an important data structure in Python and today, we will learn how to create<\/strong>, access<\/strong>, manipulate<\/strong> and use<\/strong> them.<\/p>\n Dictionaries in Python are collections<\/strong> that are unordered<\/strong>, indexed<\/strong> and mutable<\/strong>. They hold keys<\/strong> and values<\/strong>. This is a dictionary:<\/p>\n Code:<\/strong><\/p>\n Dictionaries are in curly brackets<\/strong>.<\/p>\n Key-value<\/strong> pairs<\/strong> are separated by commas<\/strong> and keys<\/strong> and values<\/strong> are separated by colons<\/strong>. Keys in dictionaries are unique<\/strong> and immutable<\/strong>.<\/p>\n Unlike other data structures<\/strong>, dictionaries hold two items<\/strong> in pairs<\/strong> instead of a single item<\/strong>. This is like a real-life dictionary. You can search for a value<\/strong> if you know the key<\/strong>. One key<\/strong> cannot have two values<\/strong>.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n The value of 1 is 9, not 4.<\/p>\n There are many ways to create a dictionary<\/strong>.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n After you have defined a dictionary, how do you access its items? There are two ways to do this- indexing<\/strong> and the get() method<\/strong>.<\/p>\n Dictionaries are unordered<\/strong> but we can index<\/strong> them with their keys<\/strong>.<\/p>\n Let\u2019s take an example.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n The get() method<\/strong> does the same thing as indexing<\/strong>. It takes a parameter<\/strong> for the key<\/strong> for which we want to find the value<\/strong>.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n If the key does not exist<\/strong> in the dictionary, it returns None<\/strong>.<\/p>\n Code:<\/strong><\/p>\n If you have a dictionary (empty or non-empty<\/strong>) and want to add<\/strong> more key-value<\/strong> pairs<\/strong> to it, you can:<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Code:<\/strong><\/p>\n squares<\/strong> is this dictionary now:<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Dictionaries are mutable<\/strong> and so we can remove items<\/strong> from them.<\/p>\n Let\u2019s look at different ways to do this:<\/p>\n The pop() method<\/strong> removes the item with the key we specify<\/strong>.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n If the key is not present<\/strong> in the dictionary, it raises a KeyError<\/strong>.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n This method removes<\/strong> the last inserted<\/strong> item<\/strong> from the dictionary and returns<\/strong> it.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n We can use the del keyword<\/strong> to delete dictionary items too.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n You can also delete the complete<\/strong> dictionary with this del keyword<\/strong>.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n The clear() method<\/strong> does not delete a dictionary, it removes<\/strong> all the key-value pairs<\/strong> from it.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n Again, dictionaries are mutable<\/strong> and you can change values<\/strong> for different keys.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n In this example, we change the value of key 3 to 27.0.<\/p>\n Like strings <\/strong>and lists<\/strong>, we can loop<\/strong> on dictionaries<\/strong>. If you want to perform an operation for all values<\/strong> in a dictionary, you can use it in a for loop<\/strong>.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n We use the items() method<\/strong> for this to get the key-value pairs<\/strong> for this dictionary.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n These are the keys<\/strong> in the dictionary.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n And these are the values<\/strong> in this dictionary.<\/p>\n We can check a dictionary\u2019s length<\/strong> with the len() function<\/strong>.<\/p>\n Code:<\/strong><\/p>\n Output:<\/strong><\/p>\n If you want to see whether a key is present<\/strong> in a dictionary, you can use the membership operators<\/strong> in<\/strong> and not in<\/strong>.<\/p>\nWhat are Dictionaries in Python?<\/h3>\n
>>> animals={'dog': 'mammal', 'cat': 'mammal', 'snake': 'reptile', 'frog': 'amphibian'}<\/pre>\n>>> dict1={1:4, 1:9, 2:7}\n>>> dict1<\/pre>\nCreating Dictionaries in Python<\/h3>\n
>>> squares={1:1, 2:4, 3:9}\n\n>>> squares={}\n>>> squares[1]=1\n>>> squares[2]=4\n>>> squares[3]=9\n>>> squares<\/pre>\n>>> squares=dict()\n>>> squares[1]=1\n>>> squares[2]=4\n>>> squares[3]=9\n>>> squares<\/pre>\n
>>> squares={num:num**2 for num in range(1,4)}\n>>> squares<\/pre>\nAccessing Items in Python Dictionaries<\/h3>\n
1. Indexing<\/h4>\n
>>> squares<\/pre>\n
>>> squares[2]<\/pre>\n
>>> squares={'one':1, 'two':4, 'three':9}\n>>> squares['two']<\/pre>\n2. get()<\/h4>\n
>>> squares.get('one')<\/pre>\n>>> squares.get('four')\n>>><\/pre>\nAdding Items in Python Dictionaries<\/h3>\n
>>> squares<\/pre>\n
>>> squares['four']=16\n>>> squares['five']=25\n>>> squares['six']=36<\/pre>\n
>>> squares<\/pre>\n
Removing Items in Python Dictionaries<\/h3>\n
1. pop()<\/h4>\n
>>> squares={1:1, 2:4, 3:9, 4:16, 5:25, 6:36, 7:49, 8:64, 9:81, 10:100}\n>>> squares.pop(7)<\/pre>\n>>> squares.pop(11)<\/pre>\n
\n\u00a0\u00a0<\/b>File “<pyshell#32>”, line 1, in <module>
\n\u00a0\u00a0\u00a0\u00a0<\/b>squares.pop(11)
\nKeyError: 11<\/div>\n2. popitem()<\/h4>\n
>>> squares.popitem()<\/pre>\n
>>> squares<\/pre>\n
3. del<\/h4>\n
>>> del squares[4]\n>>> squares<\/pre>\n
>>> del squares[10]<\/pre>\n
\n\u00a0\u00a0<\/b>File “<pyshell#37>”, line 1, in <module>
\n\u00a0\u00a0\u00a0\u00a0<\/b>del squares[10]
\nKeyError: 10<\/div>\n>>> cubes={1:1, 2:8, 3:27, 4:64}\n>>> del cubes\n>>> cubes<\/pre>\n
\n\u00a0\u00a0<\/b>File “<pyshell#41>”, line 1, in <module>
\n\u00a0\u00a0\u00a0\u00a0<\/b>cubes
\nNameError: name ‘cubes’ is not defined<\/div>\n4. clear()<\/h4>\n
>>> cubes={1:1, 2:8, 3:27, 4:64}\n>>> cubes.clear()\n>>> cubes<\/pre>\nChanging Values in Python Dictionaries<\/h3>\n
>>> cubes={1:1, 2:8, 3:27, 4:64}\n>>> cubes[3]=27.0\n>>> cubes<\/pre>\nLooping on Dictionaries<\/h3>\n
for key,value in squares.items():\n print(key,value)<\/pre>\n
\n2 4
\n3 9
\n5 25
\n6 36
\n8 64
\n9 81<\/div>\nfor key in squares:\n print(key)<\/pre>\n
\n2
\n3
\n5
\n6
\n8
\n9<\/div>\nfor value in squares.values():\n print(value)<\/pre>\n
\n4
\n9
\n25
\n36
\n64
\n81<\/div>\nLength and Membership<\/h3>\n
>>> len(squares)<\/pre>\n