Class Notes on Loops in Python

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Introduction: Why Loops?

In everyday life, we often need to repeat tasks multiple times. Consider these examples:

• Multiplying a number from 1 to 10 to create a table.
• Calculating factorial by multiplying a number from n down to 1.
• Repeating daily tasks like preparing your school bag or attending school.

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Pseudocode Example of Repetition (Multiplying Table)

• Step 1: Start
• Step 2: Input number n to print its table
• Step 3: Set count = 1
• Step 4: Display n * count
• Step 5: Increment count by 1
• Step 6: If count == 10, stop
• Step 7: Else, repeat from Step 4
• Step 8: Stop

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Python Loops

Python provides two types of loops to handle repetition:

1. While Loop (Conditional loop)
2. For Loop (Counting loop)

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Understanding range() Function

The range() function is often used with the for loop to repeat a task a certain number of times.

• Syntax:

  range(lower_limit, upper_limit)

This generates values from lower_limit to upper_limit – 1.

Examples:

• range(1, 10) → generates numbers: [1, 2, 3, 4, 5, 6, 7, 8, 9]
• range(0, 7) → generates numbers: [0, 1, 2, 3, 4, 5, 6]

By default, the step value is 1. You can also specify a different step:

• range(1, 10, 2) → generates: [1, 3, 5, 7, 9]

You can even use a negative step to generate numbers in reverse:

• range(10, 0, -1) → generates: [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

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Using in and not in in Python

The operators in and not in are used in loops to check whether a value is present in a range or a list.

Examples:

>>> 5 in [1, 2, 3, 4, 5]    # True
>>> 5 in [1, 2, 3, 4]      # False
>>> 'a' in 'apple'          # True
>>> 'national' in 'international'   # True

Program Example: Check if a Word is Part of a Sentence

line = input("Enter a statement: ")
word = input("Enter a word: ")
if word in line:
    print(f"Yes, '{word}' is a part of '{line}'")
else:
    print(f"No, '{word}' is not a part of '{line}'")

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For Loop in Python

A for loop in Python processes items in any sequence like a list, tuple, dictionary, or string. It can also repeat a task a fixed number of times using the range() function.

Examples:

# For loop with a list
School = ["Principal", "PGT", "TGT", "PRT"]
for sm in School:
    print(sm)

# For loop with a tuple
Code = (10, 20, 30, 40, 50, 60)
for cd in Code:
    print(cd)

Printing Natural Numbers from 1 to 100:

for i in range(1, 101):
    print(i, end='\t')

Multiplication Table Program:

num = int(input("Enter a number: "))
for i in range(1, 11):
    print(f"{num} x {i} = {num * i}")

Sum of Numbers Divisible by 7 (Between 1 and 100):

sum = 0
for i in range(1, 101):
    if i % 7 == 0:
        sum += i
print("Sum of numbers divisible by 7 is:", sum)

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While Loop in Python

The while loop is a conditional loop that repeats as long as the condition remains True. It checks the condition first, then enters the loop if the condition is true.

Components of a While Loop:

1. Initialization: Starting value of the loop variable.
2. Test Condition: Loop continues until this condition becomes false.
3. Body of the Loop: Statements to execute repeatedly.
4. Update Statement: Updates loop variables for the next iteration.

Example:

i = 1
while i <= 10:
    print(i)
    i += 1

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Practice Questions:

1. Write a program to enter a number and find its factorial.
2. Write a program to print the Fibonacci series up to n terms.
3. Write a program to enter 10 numbers and find their sum and average.
4. Write a program to enter a range (Lower_Limit, Upper_Limit) and find the sum of all odd and even numbers within the range.

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Convert for Loop Programs to while Loop

Example (Multiplication Table with While Loop):

num = int(input("Enter a number: "))
i = 1
while i <= 10:
    print(f"{num} x {i} = {num * i}")
    i += 1

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Break and Continue Statements in Loops

• Break: Terminates the loop when a specific condition is met.
• Continue: Skips the current iteration and moves to the next iteration.

Example – Break Statement:

for i in range(1, 20):
    if i % 6 == 0:
        break
    print(i)

Output:

1
2
3
4
5

Example – Continue Statement:

for i in range(1, 20):
    if i % 6 == 0:
        continue
    print(i, end=' ')

Output:

1 2 3 4 5 7 8 9 10 11 13 14 15 16 17 19

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Else Clause with Loops

Both for and while loops in Python can have an else clause. It runs only when the loop terminates normally (not using break).

Example:

i = 1
while i <= 10:
    print(i)
    i += 1
else:
    print("Loop Over")

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Random Number Generation

Python allows generating random numbers using the random library:

import random
print(random.randint(1, 10))  # Generates a random number between 1 and 10

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These notes introduce you to the basic concepts of loops, covering both for and while loops in Python, with practical examples to solidify your understanding. Happy coding!