1st PUC Computer Science Chapter 6 Flow of Control Notes

Chapter Overview

Flow of control means the order in which statements in a program are executed. This chapter explains selection, indentation, repetition, for and while loops, range(), break, continue and nested loops.

Table of Contents

• 1. Introduction
• 2. Selection
• 3. Indentation
• 4. Repetition
• 4.1 for Loop
• 4.1.1 range()
• 4.2 while Loop
• 5. Break & Continue
• 6. Nested Loops
• Important Questions
• Quick Revision

1. Introduction to Flow of Control

In a sequential program, Python executes statements one after another from the beginning to the end. Control structures are used when a program needs decision making or repetition.

Main Control Structures

• Selection
• Repetition

Sequential Program Example

num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))
diff = num1 - num2
print("The difference of", num1, "and", num2, "is", diff)

2. Selection

2.1 if Statement

if condition:
    statement(s)

If the condition is true, the indented statement block is executed.

age = int(input("Enter your age "))

if age >= 18:
    print("Eligible to vote")

2.2 if..else Statement

if condition:
    statement(s)
else:
    statement(s)

age = int(input("Enter your age: "))

if age >= 18:
    print("Eligible to vote")
else:
    print("Not eligible to vote")

Positive Difference

if num1 > num2:
    diff = num1 - num2
else:
    diff = num2 - num1

Decision-Making Flowchart

2.3 if..elif..else

if condition:
    statement(s)
elif condition:
    statement(s)
elif condition:
    statement(s)
else:
    statement(s)

Positive, Negative or Zero

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

if number > 0:
    print("Number is positive")
elif number < 0:
    print("Number is negative")
else:
    print("Number is zero")

Nested if

A selection structure placed inside another selection structure is called a nested if. The calculator example in the chapter uses nested conditions.

Simple Calculator Logic

if op == "+":
    result = val1 + val2
elif op == "-":
    if val1 > val2:
        result = val1 - val2
    else:
        result = val2 - val1
elif op == "*":
    result = val1 * val2
elif op == "/":
    if val2 == 0:
        print("Error! Division by zero is not allowed.")
    else:
        result = val1 / val2
else:
    print("Wrong input, program terminated")

3. Indentation

• Statements at the same indentation level belong to the same block.
• Python checks indentation strictly.
• Incorrect indentation may produce syntax errors.
• The body of an if or loop must be indented.
• Nested blocks require further indentation.

if num1 > num2:
    print("first number is larger")
    print("Bye")
else:
    print("second number is larger")
    print("Bye Bye")

4. Repetition

Loops avoid writing a large number of repeated statements. The chapter describes two looping constructs: for and while.

4.1 The for Loop

The for loop iterates over a range of values or a sequence. It executes its body for each item and then transfers control to the statement following the loop.

for <control-variable> in <sequence/items in range>:
    <statements inside body of the loop>

Example

for letter in 'PYTHON':
    print(letter)

Even Number Example

numbers = [1,2,3,4,5,6,7,8,9,10]

for num in numbers:
    if (num % 2) == 0:
        print(num, 'is an even Number')

4.1.1 range() Function

• Generates a sequence of integers.
• The stop value is excluded.
• start is optional.
• step is optional.
• Default start is 0.
• Default step is 1.
• step may be positive or negative, but not zero.

list(range(10))
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

list(range(2, 10))
# [2, 3, 4, 5, 6, 7, 8, 9]

list(range(0, 30, 5))
# [0, 5, 10, 15, 20, 25]

list(range(0, -9, -1))
# [0, -1, -2, -3, -4, -5, -6, -7, -8]

4.2 The while Loop

while test_condition:
    body of while

• The condition is checked before the body.
• If initially false, the body is not executed.
• The condition is checked again after every iteration.
• The condition should eventually become false.
• Otherwise an infinite loop may occur.

Flowchart of while Loop

Example

count = 1

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

Infinite Loop

If the loop condition never becomes false, the loop may continue indefinitely. The chapter identifies this as a logical error.

5. Break and Continue Statements

5.1 break Statement

for num in range(10):
    num = num + 1

    if num == 8:
        break

    print('Num has value ' + str(num))

5.2 continue Statement

for num in range(6):
    num = num + 1

    if num == 3:
        continue

    print('Num has value ' + str(num))

6. Nested Loops

The chapter states that any type of loop can be nested within another loop. Nested loops are used for patterns and other repeated calculations.

Nested Loop Example

for i in range(1, num + 1):
    for j in range(1, i + 1):
        print(j, end=" ")
    print()

Pattern for num = 5

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

Important Uses in the Chapter

• Pattern generation.
• Finding prime numbers.
• Repeated calculations.
• Using different loop types together.

📝 Important Questions & Answers

⚡ Quick Revision

if condition:
    statement

if condition:
    statement
else:
    statement

if condition:
    statement
elif condition:
    statement
else:
    statement

for variable in sequence:
    statement

while condition:
    statement

range([start], stop[, step])

Programming Practice – Chapter Focus

1. Check eligibility using age.
2. Print the table of a number.
3. Find minimum and maximum of five numbers.
4. Check leap year.
5. Generate the sequence −5, 10, −15, 20, −25….
6. Find the sum 1 + 1/8 + 1/27 + … + 1/n³.
7. Find sum of digits.
8. Check palindrome.
9. Generate star and number patterns.
10. Find student grade according to the given percentage table.
11. Create the menu-driven student-information case study described in the chapter.

Final Revision