Dictionary

Dictionaries in Python

A Python dictionary is a data structure that stores the value in key: value pairs. Values in a dictionary can be of any data type and can be duplicated, whereas keys can’t be repeated and must be immutable.

Example: Here, The data is stored in key: value pairs in dictionaries, which makes it easier to find values.

d = {1: 'Geeks', 2: 'For', 3: 'Geeks'}

print(d)

Create a Dictionary

In Python, a dictionary can be created by placing a sequence of elements within curly {} braces, separated by a ‘comma’.

# create dictionary using { }

d1 = {1: 'Geeks', 2: 'For', 3: 'Geeks'}

print(d1)

# create dictionary using dict() constructor

d2 = dict(a = "Geeks", b = "for", c = "Geeks")

print(d2)

• From Python 3.7 Version onward, Python dictionary are Ordered.
• Dictionary keys are case sensitive: the same name but different cases of Key will be treated distinctly.
• Keys must be immutable: This means keys can be strings, numbers, or tuples but not lists.
• Keys must be unique: Duplicate keys are not allowed and any duplicate key will overwrite the previous value.
• Dictionary internally uses Hashing. Hence, operations like search, insert, delete can be performed in Constant Time.

• Print the data type of a dictionary:

thisdict = {

  "brand": "Ford",

  "model": "Mustang",

  "year": 1964

} 

print(type(thisdict))

• The dict() Constructor

It is also possible to use the dict() constructor to make a dictionary.
Example

    Using the dict() method to make a dictionary:

    thisdict = dict(name = "John", age = 36, country = "Norway")

    print(thisdict)

• Accessing Dictionary Items

We can access a value from a dictionary by using the key within square brackets or get()method.

d = { "name": "Alice", 1: "Python", (1, 2): [1,2,4] }

# Access using key

print(d["name"])

# Access using get()

print(d.get("name"))

• Adding and Updating Dictionary Items

We can add new key-value pairs or update existing keys by using assignment.

d = {1: 'Geeks', 2: 'For', 3: 'Geeks'}

# Adding a new key-value pair

d["age"] = 22

# Updating an existing value

d[1] = "Python dict"

print(d)

• Removing Dictionary Items

We can remove items from dictionary using the following methods:

·        del: Removes an item by key.

·        pop(): Removes an item by key and returns its value.

·        clear(): Empties the dictionary.

·        popitem(): Removes and returns the last key-value pair.

d = {1: 'Geeks', 2: 'For', 3: 'Geeks', 'age':22}

# Using del to remove an item

del d["age"]

print(d)

# Using pop() to remove an item and return the value

val = d.pop(1)

print(val)

# Using popitem to removes and returns

# the last key-value pair.

key, val = d.popitem()

print(f"Key: {key}, Value: {val}")

# Clear all items from the dictionary

d.clear()

print(d)

• Iterating Through a Dictionary

We can iterate over keys [using keys() method] , values [using values() method] or both [using item() method] with a for loop.

d = {1: 'Geeks', 2: 'For', 'age':22}

# Iterate over keys

for key in d:

    print(key)

# Iterate over values

for value in d.values():

    print(value)

# Iterate over key-value pairs

for key, value in d.items():

    print(f"{key}: {value}")

• Nested Dictionaries

Example of nested Dictionaries

d = {1: 'Geeks', 2: 'For',

        3: {'A': 'Welcome', 'B': 'To', 'C': 'Geeks'}}

print(d)

 

Python Dictionary Problems

• Length of a Dictionary

To calculate the length of a dictionary, we can use Python built-in len() method. It method returns the number of keys in dictionary.

d ={'Name':'Steve', 'Age':30, 'Designation':'Programmer'}

print(len(d))

• Check if a Key Exists

Python dictionary can not contain duplicate keys, so it is very crucial to check if a key is already present in the dictionary. If you accidentally assign a duplicate key value, the new value will overwrite the old one.

To check if given Key exists in dictionary, you can use either in operator or get() method. Both are easy to use and work efficiently.

Using IN operator

# Example dictionary

d = {'a': 1, 'b': 2, 'c': 3}

# Key to check

key = 'b'

print(key in d)  # Output: True

key = 'g'

print(key in d)  # Output: False

• Access a Value by Key

Retrieving a value from a dictionary by its key is a common operation in many Python programs. In this article, we will explore five simple and commonly used methods to get a dictionary value by its key in Python.

# Creating a sample dictionary

d = {'name': 'geeks', 'age': 21, 'place': 'India'}

# Accessing value using bracket notation

val = d['name']

print("Name:", val)

• Remove a Key from a Dictionary

We are given a dictionary and our task is to remove a specific key from it. For example, if we have the dictionary d = {“a”: 1, “b”: 2, “c”: 3}, then after removing the key “b”, the output will be {‘a’: 1, ‘c’: 3}.

Using pop()

pop() method removes a specific key from the dictionary and returns its corresponding value.

a = {"name": "Nikki", "age": 25, "city": "New York"}

# Remove the key 'age'

rv = a.pop("age")

print(a) 

print(rv)

Using del()

del() statement deletes a key-value pair from the dictionary directly.

a = {"name": "Nikki", "age": 25, "city": "New York"}

# Delete the key 'city'

del a["city"]

print(a)

Using Dictionary Comprehension

Dictionary comprehension allows us to create a new dictionary without the key we want to remove.

a = {"name": "Nikki", "age": 25, "city": "New York"}

# Remove the key 'name' using dictionary comprehension

a = {k: v for k, v in a.items() if k != "name"}

print(a)

Using popitem() for Last Key Removal

If we want to remove the last key-value pair in the dictionary, popitem() is a quick way to do it.

a = {"name": "Nikki", "age": 25, "city": "New York"}

# Remove the last key-value pair

c = a.popitem()

print(a) 

print(c)