Annachi kadai

1. Create and print a dictionary Concept: A dictionary stores data in key–value pairs.

student = {
    "name": "Alice",
    "age": 21,
    "major": "Computer Science"
}
print(student)

Explanation:
Each key is linked to a value. This helps organize related data.

2.Access specific values
Concept: Access values using keys.

print(student["name"])
print(student["major"])

Explanation:
We use the key inside square brackets to get the value.

3.Add and update values
Concept: Dictionaries are mutable.

student["gpa"] = 3.8
student["age"] = 22
print(student)

Explanation:
New key-value pairs can be added, and existing ones can be updated.

4.Remove a key using del
Concept: Delete elements.

del student["major"]
print(student)

Explanation:
Removes the key and its value permanently.

5.Check if a key exists
Concept: Use in operator.

print("age" in student)

Explanation:
Returns True if the key exists, otherwise False.

6.Iterate through a dictionary
Concept: Loop through key-value pairs.

prices = {"apple": 0.5, "banana": 0.3, "orange": 0.7}

for key, value in prices.items():
    print(key, value)

Explanation:
items() returns both keys and values together.

1. Count key-value pairs Concept: Use len().

print(len(prices))

Explanation:
Gives total number of entries.

8.Use get() method
Concept: Safe access.

print(student.get("gpa"))
print(student.get("graduation_year", 2025))

Explanation:
If key doesn’t exist, default value is returned.

9.Merge dictionaries
Concept: Combine dictionaries.

extra_info = {
    "graduation_year": 2025,
    "hometown": "Springfield"
}

student.update(extra_info)
print(student)

Explanation:
update() adds all key-value pairs from another dictionary.

10.Dictionary comprehension
Concept: Create dictionary in one line.

squares = {x: x**2 for x in range(1, 6)}
print(squares)

Explanation:
Keys are numbers, values are their squares.

1. Get keys and values separately Concept: Use keys() and values().

print(list(prices.keys()))
print(list(prices.values()))

Explanation:
Returns keys and values as lists.

12.Nested dictionary
Concept: Dictionary inside dictionary.

school = {
    "student1": {"name": "Alice", "age": 21},
    "student2": {"name": "Bob", "age": 22}
}
print(school["student2"]["age"])

Explanation:
Access nested values step by step.

1. Use setdefault() Concept: Add key only if it doesn’t exist.

student.setdefault("advisor", "Dr. Smith")
print(student)

Explanation:
Prevents overwriting existing values.

1. Use pop() Concept: Remove and store value.

value = student.pop("hometown")
print(value)

Explanation:
Removes key and returns its value.

15.Clear dictionary
Concept: Remove all items.

prices.clear()
print(prices)

Explanation:
Dictionary becomes empty {}.

16.Copy dictionary
Concept: Create duplicate.

copy_student = student.copy()
copy_student["name"] = "Charlie"
print(student)
print(copy_student)

Explanation:
Original dictionary remains unchanged.

17.Create dictionary using zip()
Concept: Combine two lists.

keys = ["name", "age", "major"]
values = ["Eve", 20, "Mathematics"]
new_dict = dict(zip(keys, values))
print(new_dict)

Explanation:
Pairs elements from both lists.

18.Iterate using items()
Concept: Loop through dictionary.

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

Explanation:
Useful for accessing both key and value together.

19.Count occurrences (manual method)
Concept: Frequency counting.

fruits = ["apple", "banana", "apple", "orange", "banana", "banana"]
fruit_count = {}
for fruit in fruits:
    fruit_count[fruit] = fruit_count.get(fruit, 0) + 1
print(fruit_count)

Explanation:
Counts how many times each item appears.

20.Using defaultdict
Concept: Simplifies counting.

from collections import defaultdict
words = ["hello", "world", "hello", "python"]
word_count = defaultdict(int)
for word in words:
    word_count[word] += 1
print(dict(word_count))

Explanation:
Automatically initializes missing keys with default value