📖 Dictionaries

How does your phone find "Mom" among hundreds of contacts instantly? It doesn't scan position 0, 1, 2, 3… — it looks up the name and jumps straight to the number! Lists and tuples store values by position, but in real life we often want to find a value by a meaningful name: a roll number → a student, a word → its meaning, a country → its capital. Python's dictionary does exactly this. This chapter shows how to create a dictionary, the rules for its keys, how to add, change and delete items, what keys(), values(), items(), get(), update() and pop() do, and how to count the frequency of characters with a dictionary.

12.1 🏁 Introduction

A dictionary is a mutable collection of key : value pairs enclosed in curly braces { }. Each key is linked (mapped) to a value, and you use the key to get the value — not an index number.

student = {"roll": 101, "name": "Riya", "marks": 92.5}
print(student["name"])       # Riya

    student = {  "roll" : 101 ,   "name" : "Riya" ,   "marks" : 92.5  }
                   ↑       ↑
                  key    value      ← each key : value is ONE item

Analogy: A dictionary is exactly like an English dictionary 📕 — you look up a word (key) to find its meaning (value). You never say "give me the meaning at page 237, line 4"; you just look up the word!

12.2 🛠️ Creating a Dictionary

# 1. Using curly braces
capitals = {"India": "New Delhi", "Japan": "Tokyo", "France": "Paris"}

# 2. An empty dictionary, filled later
marks = {}
marks["Maths"] = 95
marks["Physics"] = 88
print(marks)                    # {'Maths': 95, 'Physics': 88}

# 3. Using dict() with keyword arguments (keys become strings)
emp = dict(name="Aman", age=30, city="Pune")
print(emp)                      # {'name': 'Aman', 'age': 30, 'city': 'Pune'}

# 4. Using dict() with a list of (key, value) pairs
d = dict([(1, "one"), (2, "two")])
print(d)                        # {1: 'one', 2: 'two'}

# 5. Using dict.fromkeys() — same value for many keys
scores = dict.fromkeys(["Riya", "Aman", "Zoya"], 0)
print(scores)                   # {'Riya': 0, 'Aman': 0, 'Zoya': 0}

12.2.1 Rules for Keys 🔑

d = {"a": 1, "b": 2, "a": 3}
print(d)                    # {'a': 3, 'b': 2}   ← duplicate key: last value wins

points = {(0, 0): "origin", (2, 3): "P"}   # ✅ tuple keys are allowed
print(points[(2, 3)])       # P

# bad = {[1, 2]: "list key"}   # ❌ TypeError: unhashable type: 'list'

12.3 🔍 Accessing Values

12.3.1 Using Square Brackets d[key]

student = {"roll": 101, "name": "Riya", "marks": 92.5}
print(student["marks"])      # 92.5
# print(student["age"])      # ❌ KeyError: 'age'

12.3.2 Using get() — The Safe Way

student = {"roll": 101, "name": "Riya", "marks": 92.5}
print(student.get("name"))          # Riya
print(student.get("age"))           # None         ← no error!
print(student.get("age", "N/A"))    # N/A          ← your own default value

d[key] vs d.get(key) — The Critical Comparison:

12.3.3 Checking a Key with in

The membership operator in checks KEYS only, never values.

student = {"roll": 101, "name": "Riya"}
print("name" in student)                 # True
print("Riya" in student)                 # False  ← "Riya" is a VALUE, not a key
print("Riya" in student.values())        # True   ← check the values explicitly

12.4 ✏️ Adding, Updating and Deleting Items

12.4.1 Adding and Updating — Same Syntax! 🔄

d[key] = value updates the value if the key exists, and adds a new item if it doesn't.

stock = {"pen": 50, "pencil": 30}
stock["pen"] = 45          # key exists → value UPDATED
stock["eraser"] = 20       # new key → item ADDED
print(stock)               # {'pen': 45, 'pencil': 30, 'eraser': 20}

   d[key] = value
        │
        ├── key already present? ──► value is REPLACED   (update)
        └── key not present?     ──► new item is ADDED   (insert)

12.4.2 Deleting Items 🗑️

12.5 🚶 Traversing a Dictionary

A for loop over a dictionary gives you its KEYS one by one.

marks = {"Maths": 95, "Physics": 88, "Chemistry": 91}

for sub in marks:                       # 1. loop over keys
    print(sub, "→", marks[sub])

for sub, m in marks.items():            # 2. loop over (key, value) pairs
    print(f"{sub}: {m}")

total = 0
for m in marks.values():                # 3. loop over values only
    total += m
print("Total:", total)                  # Total: 274

Maths → 95
Physics → 88
Chemistry → 91
Maths: 95
Physics: 88
Chemistry: 91
Total: 274

12.6 🧰 Dictionary Functions and Methods

12.6.1 Viewing Keys, Values and Items 👀

d = {"x": 10, "y": 20}
print(list(d.keys()))      # ['x', 'y']
print(list(d.values()))    # [10, 20]
print(list(d.items()))     # [('x', 10), ('y', 20)]

12.6.2 Other Useful Methods and Functions 🔧

prices = {"apple": 120, "banana": 40, "mango": 90}
print(max(prices))                          # mango   ← max KEY (alphabetical)
print(max(prices.values()))                 # 120     ← max VALUE
print(sorted(prices))                       # ['apple', 'banana', 'mango']
print(sorted(prices.values()))              # [40, 90, 120]

A = {"x": 1}
B = A                # alias — same dictionary
C = A.copy()         # a separate copy
B["y"] = 2
print(A)             # {'x': 1, 'y': 2}   ← changed through the alias
print(C)             # {'x': 1}

12.7 🪆 Nested Dictionaries

A dictionary's values can themselves be dictionaries (or lists), which is perfect for storing records.

students = {
    101: {"name": "Riya", "marks": 92},
    102: {"name": "Aman", "marks": 85},
}
print(students[102]["name"])            # Aman
students[101]["marks"] = 95             # update a nested value
for roll, info in students.items():
    print(roll, info["name"], info["marks"])

Aman
101 Riya 95
102 Aman 85

12.8 ⚖️ List vs Dictionary

12.9 💻 Important Dictionary Programs

Code Example: char_frequency.py

# Count how many times each character appears in a string
text = input("Enter a string: ")
freq = {}
for ch in text:
    if ch in freq:
        freq[ch] += 1
    else:
        freq[ch] = 1
print(freq)
# Input: banana → {'b': 1, 'a': 3, 'n': 2}

The same program using get() — shorter!

text = "mississippi"
freq = {}
for ch in text:
    freq[ch] = freq.get(ch, 0) + 1
print(freq)          # {'m': 1, 'i': 4, 's': 4, 'p': 2}

Code Example: word_frequency.py

# Count the frequency of each word in a sentence
sentence = "the cat and the hat and the bat"
count = {}
for word in sentence.split():
    count[word] = count.get(word, 0) + 1
for word, c in count.items():
    print(word, ":", c)

the : 3
cat : 1
and : 2
hat : 1
bat : 1

Code Example: employee_salary.py

# Create a dictionary of employees and their salaries, then use it
n = int(input("How many employees? "))
emp = {}
for i in range(n):
    name = input("Enter name: ")
    salary = float(input("Enter salary: "))
    emp[name] = salary

print("\nEmployee   Salary")
for name in emp:
    print(name, "  ", emp[name])

search = input("Enter a name to search: ")
if search in emp:
    print(search, "earns", emp[search])
else:
    print(search, "not found")

How many employees? 2
Enter name: Aman
Enter salary: 45000
Enter name: Riya
Enter salary: 52000

Employee   Salary
Aman    45000.0
Riya    52000.0
Enter a name to search: Riya
Riya earns 52000.0

Code Example: topper.py

# Find the student with the highest marks
marks = {"Riya": 92, "Aman": 85, "Zoya": 97, "Kabir": 88}
topper = ""
highest = -1
for name, m in marks.items():
    if m > highest:
        highest = m
        topper = name
print("Topper:", topper, "with", highest, "marks")
# Topper: Zoya with 97 marks

Code Example: invert_dictionary.py

# Swap keys and values
country_code = {"IN": "India", "JP": "Japan", "FR": "France"}
code_of = {}
for code, country in country_code.items():
    code_of[country] = code
print(code_of)
# {'India': 'IN', 'Japan': 'JP', 'France': 'FR'}

⚠️ Common Errors and Misconceptions

🔑 Quick Revision

Dictionary Basics:

• A mutable collection of key : value pairs in { }
• Keys → unique and immutable (int, str, tuple); values → anything
• Access by key: d[key] (KeyError if missing) or d.get(key, default) (no error)
• d[key] = value → updates if the key exists, adds if not
• in checks keys · a for loop gives keys

Methods at a Glance:

Frequency pattern: freq[x] = freq.get(x, 0) + 1

🎯 Sample Exam Questions

Q1: Very Short Answer [1 mark each]

a) What is the output of print(len({"a": 1, "b": 2, "a": 3}))? → 2 (the duplicate key "a" is stored only once)

b) Which method returns the value for a key without raising an error if the key is missing? → get()

c) What does popitem() return? → The last inserted item as a (key, value) tuple

d) What will sorted({"b": 2, "c": 1, "a": 3}) return? → ['a', 'b', 'c']

e) Name any two data types that can be used as dictionary keys. → int and str (also tuple, float, bool)

Q2: Output Based [2 marks]

Q: Write the output of the following code:

d = {1: "one", 2: "two", 3: "three"}
d[2] = "TWO"
d[4] = "four"
print(d)
print(d.get(5, "not found"), len(d))

Answer:

{1: 'one', 2: 'TWO', 3: 'three', 4: 'four'}
not found 4

Q3: Output Based [2 marks]

Q: Write the output of the following code:

stock = {"pen": 10, "ink": 5, "pad": 8}
x = stock.pop("ink")
y = stock.popitem()
print(x, y)
stock.update({"pen": 12, "clip": 20})
print(stock)

Answer:

5 ('pad', 8)
{'pen': 12, 'clip': 20}

Q4: Output Based [3 marks]

Q: Write the output of the following code:

word = "APPLE"
d = {}
for ch in word:
    d[ch] = d.get(ch, 0) + 1
print(d)
for k in sorted(d):
    print(k * d[k], end=" ")
print()
print("P" in d, 2 in d, 2 in d.values())

Answer:

{'A': 1, 'P': 2, 'L': 1, 'E': 1}
A E L PP
True False True

Q5: Program [3 marks]

Q: Write a program to input the names and marks of n students into a dictionary and print the names of the students who scored more than 75.

n = int(input("Enter number of students: "))
result = {}
for i in range(n):
    name = input("Name: ")
    marks = int(input("Marks: "))
    result[name] = marks
print("Students scoring above 75:")
for name, marks in result.items():
    if marks > 75:
        print(name, marks)

✏️ Practice Problems

1. Differentiate between a list and a dictionary (any three points).

2. Write the output of: (a) dict.fromkeys([1, 2, 3]) (b) dict([("a", 1), ("b", 2)]) (c) {"x": 1, "x": 2} (d) len({})

3. Explain the difference between del d[key], d.pop(key) and d.popitem() with examples.

4. Write a program to input a sentence and print the frequency of each vowel in it using a dictionary.

5. Write a program to create a dictionary with roll numbers as keys and names as values, then delete a record whose roll number is entered by the user (display a message if it doesn't exist).

6. Write a program to input a dictionary of items and prices, and print the items whose price is greater than the average price.

7. Write a program to merge two dictionaries; if a key appears in both, add their values.

8. Write a program to create a dictionary from two lists — one of country names and one of their capitals — and then search for the capital of a country entered by the user.

9. Why can a tuple be used as a dictionary key but a list cannot? Give an example of each.

10. Write a program that stores the phone numbers of friends in a dictionary and lets the user (a) add a contact, (b) search for a contact, and (c) display all contacts in alphabetical order of names.