10. Dictionary Comprehension

Dictionary comprehension provides a concise way to create dictionaries from iterables or other dictionaries.

Syntax:

new_dict = {key_expr: value_expr for item in iterable if condition}

Examples:

# Squares of numbers 1 to 5

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

print(squares)   # {1:1, 2:4, 3:9, 4:16, 5:25}

# Filter: only even squares

even_sq = {x: x**2 for x in range(1, 11) if x % 2 == 0}

print(even_sq)   # {2:4, 4:16, 6:36, 8:64, 10:100}

# Swap keys and values

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

swapped  = {v: k for k, v in original.items()}

print(swapped)   # {1:'a', 2:'b', 3:'c'}

# Convert two lists into a dictionary

keys   = ['name', 'age', 'city']

values = ['Alice', 21, 'Delhi']

d = {k: v for k, v in zip(keys, values)}

print(d)   # {'name':'Alice', 'age':21, 'city':'Delhi'}

11. Nested Dictionaries

A dictionary can contain another dictionary as a value, creating a nested structure. This is useful for representing complex, hierarchical data.

students = {

    'Alice': {'age': 21, 'marks': 95, 'grade': 'A'},

    'Bob'  : {'age': 22, 'marks': 88, 'grade': 'B'},

    'Carol': {'age': 20, 'marks': 73, 'grade': 'C'}

}

# Accessing nested values

print(students['Alice']['marks'])   # 95

print(students['Bob']['grade'])     # B

# Adding a new student

students['Dave'] = {'age': 23, 'marks': 80, 'grade': 'B'}

# Iterating over nested dictionary

for name, info in students.items():

    print(name, '->', info['marks'])

12. Linear Search in Dictionary

12.1 Search by Key

student = {'name': 'Alice', 'age': 21, 'grade': 'A'}

search_key = 'age'

found = False

for key in student:

    if key == search_key:

        found = True

        print('Key found! Value:', student[key])

        break

if not found:

    print('Key not found')

# Output: Key found! Value: 21

12.2 Search by Value

student = {'name': 'Alice', 'age': 21, 'grade': 'A'}

search_val = 21

found = False

for key, value in student.items():

    if value == search_val:

        found = True

        print('Value found! Key:', key)

        break

if not found:

    print('Value not found')

# Output: Value found! Key: age

13. Counting Frequency Using a Dictionary

Dictionaries are excellent for counting how many times each element appears in a sequence.

# Count frequency of characters in a string

text = 'hello world'

freq = {}

for ch in text:

    if ch in freq:

        freq[ch] += 1

    else:

        freq[ch]  = 1

print(freq)

# {'h':1, 'e':1, 'l':3, 'o':2, ' ':1, 'w':1, 'r':1, 'd':1}

# Shorter version using get()

freq2 = {}

for ch in text:

    freq2[ch] = freq2.get(ch, 0) + 1

# Count word frequencies in a sentence  ( we discuss about this after learning string functions)

sentence = 'apple banana apple cherry banana apple'

words     = sentence.split()

word_freq = {}

for word in words:

    word_freq[word] = word_freq.get(word, 0) + 1

print(word_freq)   # {'apple':3, 'banana':2, 'cherry':1}

14. Merging Dictionaries

d1 = {'a': 1, 'b': 2}

d2 = {'c': 3, 'd': 4}

d1.update(d2)

print(d1)   # {'a':1, 'b':2, 'c':3, 'd':4}

15. Dictionary vs List vs Tuple

Feature	Dictionary	List	Tuple
Syntax	{ key: value }	[ ]	( )
Ordered	Yes (3.7+)	Yes	Yes
Mutable	Yes	Yes	No
Indexed by	Key (any immutable)	Integer	Integer
Duplicates	Keys: No, Values: Yes	Yes	Yes
Use Case	Key-value mapping	Ordered items	Fixed data
Speed (lookup)	O(1) avg.	O(n)	O(n)

16. Common Errors with Dictionaries

Error	Cause	Example
KeyError	Accessing a key that does not exist	d['city'] when 'city' not in d
TypeError	Using a mutable type (list) as a key	d[[1,2]] = 'val'
AttributeError	Calling a wrong method name	d.append('x')  (no append in dict)
TypeError	Iterating with wrong assumption	for i in range(len(d)): d[i]

17. Practical Programs

17.1 Student Report Card

report = {}
n = int(input("Number of students: "))
for _ in range(n):
    name  = input("Name: ")
    marks = float(input("Marks: "))
    report[name] = marks
print("\n--- Report Card ---")
for name, marks in report.items():
    if marks >= 90:
        grade = "A"
    elif marks >= 75:
        grade = "B"
    else:
        grade = "C"
    print(f"{name}: {marks} -> Grade {grade}")
topper = max(report, key=report.get)
print(f"Topper: {topper} with {report[topper]} marks")

17.2 Phone Book Application

phonebook = {}

while True:
    print("\n--- Phonebook Menu ---")
    print("1. Add Contact")
    print("2. Search Contact")
    print("3. Delete Contact")
    print("4. Show All Contacts")
    print("5. Exit")
    choice = input("Enter choice (1-5): ")
    if choice == "1":
        name = input("Enter name: ")
        number = input("Enter number: ")
        phonebook[name] = number
        print(f"{name} added.")
    elif choice == "2":
        name = input("Enter name to search: ")
        if name in phonebook:
            print(f"{name}: {phonebook[name]}")
        else:
            print("Contact not found.")
    elif choice == "3":
        name = input("Enter name to delete: ")
        if name in phonebook:
            del phonebook[name]
            print(f"{name} deleted.")
        else:
            print("Contact not found.")
    elif choice == "4":
        print("\n--- All Contacts ---")
        for name, number in phonebook.items():
            print(f"{name}: {number}")
    elif choice == "5":
        print("Exiting Phonebook. Goodbye!")
        break
    else:
        print("Invalid choice. Try again.")