Home Computer Science Class 11Conditional and Looping Constructs

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Chapter 5: Conditional and Looping Constructs

Introduction

The order in which statements are executed in a Python program is called the flow of execution. Python provides different types of execution flows to control how your program runs.

This chapter covers two important control structures: conditional statements for making decisions and looping constructs for repeating actions.

Types of Execution Flow

1. Sequential Execution

Statements are executed one after another in the order they appear, from top to bottom without any jumps.

age = 25
print("Hello")
print("Welcome")

2. Selection/Conditional

Execution jumps to different blocks based on whether conditions are true or false.

if age >= 18:
    print("Adult")
else:
    print("Minor")

3. Iteration/Looping

Execution repeats a block of code multiple times based on a condition.

for i in range(3):
    print(i)
# Prints: 0 1 2

Conditional Statements

1. if Statement

The simplest form of conditional statement. Executes a block of code only if a condition is true.

Syntax:

if condition:
    Statements

Example:

age = int(input("Enter your Age: "))
if age >= 18:
    print("Eligible to Vote")

2. if-else Statement

Provides an alternative path when the condition is false. The else block executes if the condition is not met.

Syntax:

if condition:
    Statements
else:
    Statements

Example:

age = int(input("Enter your Age: "))
if age >= 18:
    print("Eligible to Vote")
else:
    print("Not Eligible to vote")

3. if-elif-else Statement

Allows multiple conditions to be checked in sequence. Useful when you have more than two possibilities.

Syntax:

if condition:
    Statements
elif condition:
    Statements
else:
    Statements

Example 1: Check if number is Positive or Negative

a = int(input("enter a Number: "))
if a > 0:
    print("Number is Positive")
elif a < 0:
    print("Number is Negative")
else:
    print("Number is Zero")

Example 2: Grading System

marks = int(input("Enter Your Mark: "))
if marks >= 90:
    print("Grade A+")
elif marks >= 80:
    print("Grade A")
elif marks >= 70:
    print("Grade B")
elif marks >= 60:
    print("Grade C")
elif marks >= 50:
    print("Grade D")
else:
    print("Grade F - Fail")

4. Nested if Statements

An if statement inside another if statement (or other conditional statements). Used for complex decision-making.

Syntax:

if condition:
    Statements
    if condition:
        Statements
    elif condition:
        Statements
    else:
        Statements
elif Condition:
    Statements
else:
    Statements

Example:

mark = int(input("Enter Your Mark: "))
if mark >= 80:
    print("First Class") 
    if mark >= 90:
        print("Rank Holder ")
elif mark <= 70 and mark >= 36:
    print("Second Class")
else:
    print("Fail")

Looping Constructs

1. for Loop

Used to iterate over a sequence (list, tuple, string, range, etc.).

Syntax:

for variable in sequence:
    statement(s)

Examples:

# 1. Iterating over a range
for i in range(5):
    print(i)  # Prints 0, 1, 2, 3, 4

# 2. Iterating over a string
for char in "Python":
    print(char)

# 3. Iterating over a list
fruits = ["apple", "banana", "orange"]
for fruit in fruits:
    print(fruit)

2. while Loop

Repeats a block of code as long as a condition is true.

Syntax:

while condition:
    statement(s)

Example:

count = 1
while count <= 5:
    print(f"Count: {count}")
    count += 1

# Example: Input validation
password = ""
while password != "secret":
    password = input("Enter password: ")

3. range() Function

Generates a sequence of numbers, commonly used with for loops.

Different forms of range():

# 1. range(stop)
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

# 2. range(start, stop)
for i in range(2, 8):
    print(i)  # 2, 3, 4, 5, 6, 7

# 3. range(start, stop, step)
for i in range(1, 10, 2):
    print(i)  # 1, 3, 5, 7, 9

# Reverse range
for i in range(10, 0, -1):
    print(i)  # 10, 9, 8, 7, 6, 5, 4, 3, 2, 1

4. Nested Loops

Loops inside other loops for handling multi-dimensional data or complex patterns.

Example:

# Example: Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i*j}")
    print()  # Empty line after each table

# Example: Pattern printing
for i in range(1, 6):
    for j in range(i):
        print("*", end="")
    print()  # New line after each row

Loop Control Statements

break Statement

Terminates the loop immediately when encountered.

for i in range(10):
    if i == 5:
        break
    print(i)  # Prints 0,1,2,3,4

continue Statement

Skips the rest of the current iteration and moves to the next.

for i in range(5):
    if i == 2:
        continue
    print(i)  # Prints 0,1,3,4

Key Points to Remember

Python uses indentation to define code blocks (not braces)
The if statement executes code only when the condition is True
The else block is optional and executes when condition is False
elif allows checking multiple conditions
for loop is best for iterating over sequences
while loop is best when number of iterations is unknown
break exits the loop completely
continue skips to the next iteration
Always ensure while loops have a terminating condition to avoid infinite loops

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