Dictionaries.md

Dictionaries

Dictionaries are unordered collections of key-value pairs, providing a way to associate values with unique keys.

Creating and Accessing Dictionaries:

Dictionaries are created by enclosing key-value pairs in curly braces {} or by using the dict() function. Each key-value pair is separated by a colon :. Let's look at some examples:

# Creating dictionaries
student1 = {"name": "Alice", "age": 20, "major": "Computer Science"}
student2 = dict(name="Bob", age=21, major="Mathematics")

In the first example, we create a dictionary called student1 using curly braces and assigning values to keys. In the second example, we create a dictionary called student2 using the dict() function and specifying key-value pairs using the key=value syntax.

To access values in a dictionary, we use the corresponding keys:

# Accessing dictionary values
print(student1["name"])   # Output: Alice
print(student2["age"])    # Output: 21

In this example, we access and print the values associated with the keys "name" and "age" in the student1 and student2 dictionaries, respectively.

Dictionary Methods and Operations:

Python provides several methods and operations to work with dictionaries. Let's explore a few commonly used ones:

• keys(): Returns a view object containing the keys in the dictionary.

# Using the keys() method
print(student1.keys())  # Output: dict_keys(['name', 'age', 'major'])

• values(): Returns a view object containing the values in the dictionary.

# Using the values() method
print(student2.values())  # Output: dict_values(['Bob', 21, 'Mathematics'])

• items(): Returns a view object containing the key-value pairs in the dictionary.

# Using the items() method
print(student1.items())  # Output: dict_items([('name', 'Alice'), ('age', 20), ('major', 'Computer Science')])

• get(): Returns the value associated with a specified key. If the key is not found, it returns a default value (or None if not specified).

# Using the get() method
print(student2.get("major"))      # Output: Mathematics
print(student1.get("gpa", 3.5))   # Output: 3.5 (default value)

These are just a few examples of the many methods and operations available for dictionaries in Python. Feel free to explore the Python documentation to discover more!

Iterating Over Dictionaries:

To iterate over a dictionary, we can use a for loop combined with the items() method. The items() method returns a view object that contains the key-value pairs of the dictionary.

Let's see an example:

# Iterating over a dictionary
student = {"name": "Alice", "age": 20, "major": "Computer Science"}

for key, value in student.items():
    print(key, ":", value)

In this example, we use a for loop to iterate over the student dictionary. The items() method returns a view object containing the key-value pairs, which we unpack into the variables key and value. We then print each key-value pair.

The output will be:

name : Alice
age : 20
major : Computer Science

Iterating Over Keys or Values:

If you only need to iterate over the keys or values of a dictionary, you can use the keys() or values() methods, respectively. These methods return view objects that can be directly iterated over.

Let's see some examples:

# Iterating over keys
for key in student.keys():
    print(key)

# Iterating over values
for value in student.values():
    print(value)

In these examples, we use for loops to iterate over the keys and values of the student dictionary, respectively. The keys() method returns a view object with the keys, while the values() method returns a view object with the values.

Iterating Using Comprehensions:

Python also allows you to use comprehensions to create new dictionaries based on existing dictionaries while iterating over them. This can be done using dictionary comprehensions.

Let's see an example:

# Creating a new dictionary based on an existing dictionary
student = {"name": "Alice", "age": 20, "major": "Computer Science"}

new_student = {key: value for key, value in student.items() if key != "age"}

print(new_student)

In this example, we use a dictionary comprehension to create a new dictionary new_student based on the student dictionary. We iterate over the key-value pairs using items() and conditionally include only those pairs where the key is not "age". The result is a new dictionary that excludes the "age" key-value pair.

The output will be:

{'name': 'Alice', 'major': 'Computer Science'}