Hash Tables and Hashmaps in Python

Hash table or a Hashmap in Python

In computer science,a Hash table or a Hashmap is a type of data structure that maps keys to its value pairs (implement abstract array data types). It basically makes use of a function that computes an index value that in turn holds the elements to be searched, inserted, removed, etc. This makes it easy and fast to access data. In general, hash tables store key-value pairs and the key is generated using a hash function.

Hash tables or has maps in Python are implemented through the built-in dictionary data type. The keys of a dictionary in Python are generated by a hashing function. The elements of a dictionary are not ordered and they can be changed.

An example of a dictionary can be a mapping of employee names and their employee IDs or the names of students along with their student IDs.

Difference between Hashtable and Hashmap in Python

Hash Table	Hashmap
Synchronized	Non-Synchronized
Fast	Slow
Allows one null key and more than one null values	Does not allows null keys or values

Creating Dictionaries

Dictionaries can be created in two ways:

• Using curly braces ({})
• Using the dict() function

Using curly braces

Dictionaries in Python can be created using curly braces as follows:

For example:

Output:

Using dict() function

Python has a built-in function, dict() that can be used to create dictionaries in Python. 

For example:

Output:

In the above example, an empty dictionary is created since no key-value pairs are supplied as a parameter to the dict() function. In case you want to add values, you can do as follows:

For example:

Output:

Performing Operations on Hash tables using Dictionaries

There are a number of operations that can be performed on has tables in Python through dictionaries such as:

• Accessing Values
• Updating Values
• Deleting Element

Accessing Values

The values of a dictionary can be accessed in many ways such as:

• Using key values
• Using functions
• Implementing the for loop

Using key values

Dictionary values can be accessed using the key values as follows:

For example:

Output:

Using functions

There are a number of built-in functions that can be used such as get(), keys(), values(), etc.

For example:

Output:

Implementing the for loop

The for loop allows you to access the key-value pairs of a dictionary easily by iterating over them. 

For example:

Output:

Updating Values

Dictionaries are mutable data types and therefore, you can update them as and when required. 

If I want to change the ID of the employee named Dave from ‘001’ to ‘004’ and if I want to add another key-value pair to my dictionary, I can do as follows:

For example:

Output:

Deleting items from a dictionary

There are a number of functions that allow you to delete items from a dictionary such as del(), pop(), popitem(), clear(), etc. 

For example:

Output:

The above output shows that all the elements except ‘Joe: 003’ have been removed from the dictionary using the various functions.