Lecture 5 – Loops in Python (for, while, range, nested loops)

Learn loops in Python with clear examples for beginners. Lecture 5 covers for loops, while loops, range(), nested loops, and real Data Science use-cases, complete with MCQs and practice questions.

What You Will Learn

By the end of this lecture, you will understand:

• What loops are and why we use them
• for loop
• while loop
• range() function
• Looping through lists & dictionaries
• Nested loops
• Real-world data science applications
• MCQs + Practice Questions

1) What Are Loops?

A loop repeats a block of code multiple times.

Real-life examples:

• You check emails every morning → repeated
• You count your steps → repeated
• You scroll Instagram → repeated until you stop

Python does the same using loops.

2) FOR LOOP

Used when you know how many times you want to repeat.

Basic Structure

for variable in sequence:
     do something

Example 1 – Print numbers 1 to 5

for i in range(1, 6):
    print(i)

Example 2 – Loop through a list

fruits = ["apple", "banana", "mango"]

for fruit in fruits:
    print(fruit)

xample 3 – Loop through characters

for ch in "Python":
    print(ch)

Example 4 – Print even numbers

for num in range(2, 21, 2):
    print(num)

Example 5 – Sum of numbers

total = 0

for x in range(1, 6):
    total += x

print(total)

3) range() Function – VERY IMPORTANT

range(start, stop, step)

Examples:

Default (0 to 4)

for i in range(5):
    print(i)

Custom range

for i in range(10, 21):
    print(i)

With step

for i in range(1, 20, 3):
    print(i)

4) WHILE LOOP

Runs until a condition becomes False.

Basic Structure

while condition:
     do something

Example 1 – Print 1 to 5

i = 1

while i <= 5:
    print(i)
    i += 1

Example 2 – Countdown

count = 5

while count > 0:
    print(count)
    count -= 1

Example 3 – Ask user until correct password

password = ""

while password != "python123":
    password = input("Enter password: ")

print("Access Granted")

Example 4 – Sum until input is 0

total = 0
num = int(input("Enter number (0 to stop): "))

while num != 0:
    total += num
    num = int(input("Enter number (0 to stop): "))

print("Total:", total)

Main Python Course Page

5) Looping Through DICTIONARIES

Example 1 – Loop through keys

student = {"name": "Ali", "age": 21, "grade": "A"}

for key in student:
    print(key)

Example 2 – Loop through values

for value in student.values():
    print(value)

Example 3 – Loop through items

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

Lecture 4 – Conditional Statements

6) Nested Loops (Loop inside Loop)

Example 1 – Multiplication Table

for i in range(1, 6):
    for j in range(1, 6):
        print(i, "*", j, "=", i*j)

Example 2 – Printing a Square Pattern

for i in range(5):
    for j in range(5):
        print("*", end=" ")
    print()

Example 3 – Looping Students & Courses

students = ["Ali", "Sara"]
courses = ["Math", "AI"]

for s in students:
    for c in courses:
        print(s, "is enrolled in", c)

More Control Flow Tools

7) Real Data Science Examples

Example 1 – Counting missing values

data = [10, None, 20, None, 30]
missing = 0

for x in data:
    if x is None:
        missing += 1

print("Missing values:", missing)

Example 2 – Cleaning dataset values

raw_scores = ["85", "90", "N/A", "75"]
cleaned = []

for score in raw_scores:
    if score != "N/A":
        cleaned.append(int(score))

print(cleaned)

Example 3 – Finding maximum temperature

temps = [32, 36, 30, 38, 35]
max_temp = temps[0]

for t in temps:
    if t > max_temp:
        max_temp = t

print("Max temp:", max_temp)

Example 4 – Building frequency table

nums = [1, 2, 2, 3, 3, 3]
freq = {}

for n in nums:
    if n in freq:
        freq[n] += 1
    else:
        freq[n] = 1

print(freq)

What does a loop do?

a) Runs code once
b) Runs code repeatedly
c) Stops code
d) Deletes variables

Answer: b) Runs code repeatedly

What will this print?

for i in range(3):
    print(i)

a) 1 2 3
b) 0 1 2
c) 0 1 2 3
d) Error

Answer: b) 0 1 2

Which loop runs until a condition is false?

a) for
b) while
c) until
d) loop

Answer: b) while

What is the output?

i = 5
while i > 3:
    print(i)
    i -= 1

Answer:

5
4
3

for i in range(10, 0, -1):
    print(i)

total = 0
for i in range(1, 51):
    total += i
print(total)

for i in range(2, 101, 2):
    print(i)

nums = []

for i in range(5):
    n = int(input("Enter number: "))
    nums.append(n)

print(nums)

for i in range(1, 5):
    for j in range(i):
        print("#", end="")
    print()