Unit-2: Python Program Flow Control

1. Conditional Blocks

if Statement

Syntax:

if condition:
    # code block executes if condition is True
    statement(s)

Example:

age = 18

if age >= 18:
    print("You are eligible to vote")
    print("Welcome to voting booth")

print("Program continues")

Output:

You are eligible to vote
Welcome to voting booth
Program continues

if-else Statement

Syntax:

if condition:
    # executes if condition is True
    statement(s)
else:
    # executes if condition is False
    statement(s)

Example:

age = 16

if age >= 18:
    print("You can vote")
else:
    print("You cannot vote yet")
    print(f"Wait for {18 - age} more years")

Output:

You cannot vote yet
Wait for 2 more years

if-elif-else Statement

Syntax:

if condition1:
    # executes if condition1 is True
elif condition2:
    # executes if condition2 is True
elif condition3:
    # executes if condition3 is True
else:
    # executes if all conditions are False

Example:

marks = 75

if marks >= 90:
    grade = 'A'
elif marks >= 80:
    grade = 'B'
elif marks >= 70:
    grade = 'C'
elif marks >= 60:
    grade = 'D'
else:
    grade = 'F'

print(f"Marks: {marks}, Grade: {grade}")

Output:

Marks: 75, Grade: C

Nested if Statements

num = 15

if num > 0:
    print("Positive number")
    if num % 2 == 0:
        print("Even number")
    else:
        print("Odd number")
else:
    print("Non-positive number")

Output:

Positive number
Odd number

Practical Example: Calculator

num1 = 10
num2 = 5
operator = '+'

if operator == '+':
    result = num1 + num2
elif operator == '-':
    result = num1 - num2
elif operator == '*':
    result = num1 * num2
elif operator == '/':
    if num2 != 0:
        result = num1 / num2
    else:
        result = "Error: Division by zero"
else:
    result = "Invalid operator"

print(f"{num1} {operator} {num2} = {result}")

Output:

10 + 5 = 15

Ternary Operator (Conditional Expression)

# Syntax: value_if_true if condition else value_if_false

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)  # Adult

# Traditional if-else equivalent:
if age >= 18:
    status = "Adult"
else:
    status = "Minor"

# Practical use
a = 10
b = 20
max_value = a if a > b else b
print(f"Maximum: {max_value}")  # Maximum: 20

Output:

Adult
Maximum: 20

2. for Loops in Python

Simple for Loop

Syntax:

for variable in sequence:
    # code block
    statement(s)

Example:

# Loop through list
fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

Output:

apple
banana
cherry

for Loop with range()

range(stop)

# range(5) generates 0, 1, 2, 3, 4
for i in range(5):
    print(i, end=" ")
print()  # newline

Output:

0 1 2 3 4

range(start, stop)

# range(2, 8) generates 2, 3, 4, 5, 6, 7
for i in range(2, 8):
    print(i, end=" ")
print()

Output:

2 3 4 5 6 7

range(start, stop, step)

# range(0, 10, 2) generates 0, 2, 4, 6, 8
for i in range(0, 10, 2):
    print(i, end=" ")
print()

# Countdown
for i in range(10, 0, -1):
    print(i, end=" ")
print()

Output:

0 2 4 6 8
10 9 8 7 6 5 4 3 2 1

for Loop with Strings

# Iterate through characters
text = "Python"

for char in text:
    print(char, end="-")
print()

# With index using enumerate()
for index, char in enumerate(text):
    print(f"Index {index}: {char}")

Output:

P-y-t-h-o-n-
Index 0: P
Index 1: y
Index 2: t
Index 3: h
Index 4: o
Index 5: n

for Loop with Lists

numbers = [10, 20, 30, 40, 50]

# Simple iteration
for num in numbers:
    print(num * 2, end=" ")
print()

# With index
for i in range(len(numbers)):
    print(f"Index {i}: {numbers[i]}")

# Using enumerate (better way)
for index, value in enumerate(numbers):
    print(f"numbers[{index}] = {value}")

Output:

20 40 60 80 100
Index 0: 10
Index 1: 20
Index 2: 30
Index 3: 40
Index 4: 50
numbers[0] = 10
numbers[1] = 20
numbers[2] = 30
numbers[3] = 40
numbers[4] = 50

for Loop with Dictionaries

student = {
    "name": "Alice",
    "age": 20,
    "grade": "A"
}

# Iterate through keys
print("Keys:")
for key in student:
    print(key)

# Iterate through values
print("\nValues:")
for value in student.values():
    print(value)

# Iterate through key-value pairs
print("\nKey-Value pairs:")
for key, value in student.items():
    print(f"{key}: {value}")

Output:

Keys:
name
age
grade

Values:
Alice
20
A

Key-Value pairs:
name: Alice
age: 20
grade: A

Nested for Loops

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i*j}", end="  ")
    print()  # newline after inner loop

# Pattern printing
for i in range(1, 6):
    for j in range(i):
        print("*", end="")
    print()

Output:

1 x 1 = 1  1 x 2 = 2  1 x 3 = 3  
2 x 1 = 2  2 x 2 = 4  2 x 3 = 6  
3 x 1 = 3  3 x 2 = 6  3 x 3 = 9  
*
**
***
****
*****

3. while Loops in Python

Basic while Loop

Syntax:

while condition:
    # code block executes while condition is True
    statement(s)

Example:

count = 1

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

print("Loop ended")

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Loop ended

while Loop with Input

# Password verification
password = ""

while password != "python123":
    password = input("Enter password: ")
    if password != "python123":
        print("Wrong password. Try again.")

print("Access granted!")

Output:

Enter password: hello
Wrong password. Try again.
Enter password: test
Wrong password. Try again.
Enter password: python123
Access granted!

Infinite while Loop

# Infinite loop (use with caution)
# while True:
#     print("This runs forever")

# Controlled infinite loop with break
count = 0
while True:
    print(count)
    count += 1
    if count >= 5:
        break

Output:

while vs for Loop

# Using for loop
print("For loop:")
for i in range(5):
    print(i, end=" ")
print()

# Using while loop (same result)
print("While loop:")
i = 0
while i < 5:
    print(i, end=" ")
    i += 1
print()

Output:

For loop:
0 1 2 3 4
While loop:
0 1 2 3 4

4. Loop Manipulation

pass Statement

Empty placeholder
Does nothing
Used when syntax requires a statement but you don't want to execute anything

# pass in loop
for i in range(5):
    pass  # TODO: implement later

# pass in conditional
x = 10
if x > 5:
    pass  # will implement logic later
else:
    print("x is small")

# pass in function
def my_function():
    pass  # empty function

print("Program continues")

Output:

Program continues

continue Statement

Skip current iteration
Jump to next iteration
Rest of the loop code is skipped

# Skip even numbers
print("Odd numbers from 1 to 10:")
for i in range(1, 11):
    if i % 2 == 0:
        continue  # skip even numbers
    print(i, end=" ")
print()

# Skip specific values
fruits = ["apple", "banana", "cherry", "date"]
for fruit in fruits:
    if fruit == "banana":
        continue  # skip banana
    print(fruit)

Output:

Odd numbers from 1 to 10:
1 3 5 7 9
apple
cherry
date

break Statement

Exit loop immediately
No further iterations
Control moves to statement after loop

# Find first even number
numbers = [1, 3, 5, 8, 9, 10]

for num in numbers:
    if num % 2 == 0:
        print(f"First even number: {num}")
        break
    print(f"Checking {num}")

# Search in list
fruits = ["apple", "banana", "cherry", "date"]
search = "cherry"

for fruit in fruits:
    if fruit == search:
        print(f"Found {search}!")
        break
else:
    print(f"{search} not found")

Output:

Checking 1
Checking 3
Checking 5
First even number: 8
Found cherry!

else with Loops

else executes when loop completes normally
else doesn't execute if loop exits with break

# else with for loop
print("Example 1: Loop completes normally")
for i in range(5):
    print(i, end=" ")
else:
    print("\nLoop completed successfully")

# else with break
print("\nExample 2: Loop exits with break")
for i in range(5):
    if i == 3:
        print(f"\nBreaking at {i}")
        break
    print(i, end=" ")
else:
    print("\nThis won't print")

# Practical use: Search
print("\nExample 3: Search")
numbers = [1, 3, 5, 7, 9]
search = 4

for num in numbers:
    if num == search:
        print(f"Found {search}")
        break
else:
    print(f"{search} not found in list")

Output:

Example 1: Loop completes normally
0 1 2 3 4
Loop completed successfully

Example 2: Loop exits with break
0 1 2
Breaking at 3

Example 3: Search
4 not found in list

Combining break, continue, and pass

print("Numbers from 1 to 10:")
for i in range(1, 11):
    if i == 5:
        pass  # placeholder, do nothing
    elif i % 2 == 0:
        continue  # skip even numbers
    elif i == 9:
        break  # stop at 9
    print(i, end=" ")
print()

Output:

Numbers from 1 to 10:
1 3 5 7

5. Programming Examples

Example 1: Prime Number Check

num = int(input("Enter a number: "))

if num < 2:
    print(f"{num} is not prime")
else:
    is_prime = True
    for i in range(2, int(num ** 0.5) + 1):
        if num % i == 0:
            is_prime = False
            break
    
    if is_prime:
        print(f"{num} is prime")
    else:
        print(f"{num} is not prime")

Output:

Enter a number: 17
17 is prime

Example 2: Factorial Calculation

# Using for loop
n = 5
factorial = 1

for i in range(1, n + 1):
    factorial *= i

print(f"Factorial of {n} = {factorial}")

# Using while loop
n = 5
factorial = 1
i = 1

while i <= n:
    factorial *= i
    i += 1

print(f"Factorial of {n} = {factorial}")

Output:

Factorial of 5 = 120
Factorial of 5 = 120

Example 3: Fibonacci Series

n = 10  # number of terms
a, b = 0, 1

print("Fibonacci series:")
for i in range(n):
    print(a, end=" ")
    a, b = b, a + b
print()

Output:

Fibonacci series:
0 1 1 2 3 5 8 13 21 34

Example 4: Sum of Digits

num = 12345
sum_digits = 0

while num > 0:
    digit = num % 10
    sum_digits += digit
    num //= 10

print(f"Sum of digits: {sum_digits}")

Output:

Sum of digits: 15

Example 5: Reverse a Number

num = 12345
reverse = 0

while num > 0:
    digit = num % 10
    reverse = reverse * 10 + digit
    num //= 10

print(f"Reversed number: {reverse}")

Output:

Reversed number: 54321

Example 6: Pattern Printing

# Pattern 1: Right triangle
n = 5
for i in range(1, n + 1):
    for j in range(i):
        print("*", end="")
    print()

print()

# Pattern 2: Number pyramid
for i in range(1, n + 1):
    for j in range(1, i + 1):
        print(j, end="")
    print()

print()

# Pattern 3: Inverted triangle
for i in range(n, 0, -1):
    for j in range(i):
        print("*", end="")
    print()

Output:

*
**
***
****
*****

1
12
123
1234
12345

*****
****
***
**
*

Example 7: Menu-Driven Calculator

while True:
    print("\n=== Calculator ===")
    print("1. Addition")
    print("2. Subtraction")
    print("3. Multiplication")
    print("4. Division")
    print("5. Exit")
    
    choice = int(input("Enter choice (1-5): "))
    
    if choice == 5:
        print("Exiting...")
        break
    
    if choice < 1 or choice > 5:
        print("Invalid choice!")
        continue
    
    num1 = float(input("Enter first number: "))
    num2 = float(input("Enter second number: "))
    
    if choice == 1:
        print(f"Result: {num1 + num2}")
    elif choice == 2:
        print(f"Result: {num1 - num2}")
    elif choice == 3:
        print(f"Result: {num1 * num2}")
    elif choice == 4:
        if num2 != 0:
            print(f"Result: {num1 / num2}")
        else:
            print("Error: Division by zero!")

Output:

=== Calculator ===
1. Addition
2. Subtraction
3. Multiplication
4. Division
5. Exit
Enter choice (1-5): 1
Enter first number: 10
Enter second number: 5
Result: 15.0

=== Calculator ===
...

Example 8: Nested Loop - Multiplication Table

# Multiplication table from 1 to 5
for i in range(1, 6):
    print(f"\nTable of {i}:")
    for j in range(1, 11):
        print(f"{i} x {j} = {i*j}")

Output:

Table of 1:
1 x 1 = 1
1 x 2 = 2
...
1 x 10 = 10

Table of 2:
2 x 1 = 2
2 x 2 = 4
...

Example 9: Finding Maximum in List

numbers = [45, 23, 67, 12, 89, 34]
max_num = numbers[0]

for num in numbers:
    if num > max_num:
        max_num = num

print(f"Maximum number: {max_num}")

# Alternative using enumerate
numbers = [45, 23, 67, 12, 89, 34]
max_num = numbers[0]
max_index = 0

for index, num in enumerate(numbers):
    if num > max_num:
        max_num = num
        max_index = index

print(f"Maximum: {max_num} at index {max_index}")

Output:

Maximum number: 89
Maximum: 89 at index 4

Example 10: Count Vowels and Consonants

text = "Hello World"
vowels = "aeiouAEIOU"
vowel_count = 0
consonant_count = 0

for char in text:
    if char.isalpha():
        if char in vowels:
            vowel_count += 1
        else:
            consonant_count += 1

print(f"Vowels: {vowel_count}")
print(f"Consonants: {consonant_count}")

Output:

Vowels: 3
Consonants: 7

🚀 Quick Reference Summary

Conditional Statements

Statement	Description	Example
if	Execute if condition is True	`if x > 0: print(x)`
if-else	Execute different blocks	`if x > 0: ... else: ...`
if-elif-else	Multiple conditions	`if x > 0: ... elif x == 0: ... else: ...`
Ternary	Conditional expression	`result = a if condition else b`

Loops

Loop	Description	Use Case
for	Iterate over sequence	Known iterations
while	Loop while condition is True	Unknown iterations

Loop Control

Statement	Description	Effect
break	Exit loop	Terminates loop immediately
continue	Skip iteration	Jump to next iteration
pass	Do nothing	Placeholder
else	Execute after loop	Runs if loop completes normally

range() Function

range(stop)              # 0 to stop-1
range(start, stop)       # start to stop-1
range(start, stop, step) # with step increment

📝 Exam Important Questions

Write a program to check if a number is positive, negative, or zero using if-elif-else.
Explain the difference between break and continue with examples.
Write a program to print all prime numbers between 1 and 50 using for loop.
Demonstrate the use of else clause with loops.
Write a program to find factorial of a number using:
- for loop
- while loop
Create a pattern using nested loops:
```
*
**
***
****
*****
```
Write a program to reverse a string using loop.
Explain range() function with examples of all three forms.
Write a menu-driven program for:
- Area of circle
- Area of rectangle
- Area of triangle
- Exit
Write a program to:
- Take a number as input
- Print multiplication table of that number
- Use while loop

End of Unit-2

Uh oh!

FilesExpand file tree

Unit-2.md

Latest commit

History

Unit-2.md

File metadata and controls

Unit-2: Python Program Flow Control

1. Conditional Blocks

if Statement

if-else Statement

if-elif-else Statement

Nested if Statements

Practical Example: Calculator

Ternary Operator (Conditional Expression)

2. for Loops in Python

Simple for Loop

for Loop with range()

range(stop)

range(start, stop)

range(start, stop, step)

for Loop with Strings

for Loop with Lists

for Loop with Dictionaries

Nested for Loops

3. while Loops in Python

Basic while Loop

while Loop with Input

Infinite while Loop

while vs for Loop

4. Loop Manipulation

pass Statement

continue Statement

break Statement

else with Loops

Combining break, continue, and pass

5. Programming Examples

Example 1: Prime Number Check

Example 2: Factorial Calculation

Example 3: Fibonacci Series

Example 4: Sum of Digits

Example 5: Reverse a Number

Example 6: Pattern Printing

Example 7: Menu-Driven Calculator

Example 8: Nested Loop - Multiplication Table

Example 9: Finding Maximum in List

Example 10: Count Vowels and Consonants

🚀 Quick Reference Summary

Conditional Statements

Loops

Loop Control

range() Function

📝 Exam Important Questions