6.1 Introduction

When we write a program, Python normally executes statements one after another, in the order in which they appear. This is called sequential execution.

Example:

print("Start")
print("Processing")
print("End")

In many real-life situations, however, decisions must be made:

• If it rains, take an umbrella
• If marks are greater than 40, declare pass
• Repeat an action until a condition is met

Programs must therefore be able to:

1. Choose between alternative paths
2. Repeat certain instructions multiple times
3. Stop or skip parts of execution when required

The ability to control the order of execution of statements is called flow of control.

Python provides control structures to manage this flow:

• Selection (decision making)
• Repetition (looping)

6.2 Selection (Decision Making)

Why Selection is Needed

A computer cannot “decide” on its own. It can only evaluate conditions and follow instructions based on results.

Selection allows a program to:

• execute certain statements only if a condition is true
• choose between multiple alternatives

In Python, selection is implemented using:

• if
• if–else
• if–elif–else

All selection statements depend on Boolean expressions (True or False).

6.2.1 The if Statement

The simplest form of selection.

Logical Meaning

If a condition is true, perform an action. If it is false, do nothing.

Syntax

if condition:
    statement(s)

Important Rules

• The condition must evaluate to True or False
• Statements under if must be indented
• No parentheses are required around the condition

Example

marks = 75

if marks >= 40:
    print("Pass")

Explanation:

• Python checks marks >= 40
• If true, it executes the indented statement
• If false, it skips it completely

6.2.2 The if–else Statement

Used when two alternative actions are possible.

Logical Meaning

If the condition is true, do one thing. Otherwise, do something else.

Syntax

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

Example

marks = 35

if marks >= 40:
    print("Pass")
else:
    print("Fail")

Explanation:

• Exactly one block will execute
• else has no condition; it handles all remaining cases

6.2.3 The if–elif–else Statement

Used when multiple conditions must be checked.

Logical Meaning

Check conditions one by one. Execute the first block whose condition is true.

Syntax

if condition1:
    statement(s)
elif condition2:
    statement(s)
else:
    statement(s)

Example

marks = 82

if marks >= 90:
    print("Grade A")
elif marks >= 75:
    print("Grade B")
elif marks >= 40:
    print("Grade C")
else:
    print("Fail")

Explanation:

• Conditions are checked top to bottom
• Once a condition is satisfied, remaining checks are skipped
• else acts as a default case

6.3 Indentation

Why Indentation is Critical in Python

Most programming languages use braces { } to define blocks. Python does not.

Instead, Python uses indentation (spaces) to define:

• blocks of code
• scope of conditions and loops

This is not a style choice — it is a syntax rule.

Correct Indentation Example

if marks >= 40:
    print("Pass")
    print("Congratulations")

Both print statements belong to the if block.

Incorrect Indentation (Syntax Error)

if marks >= 40:
print("Pass")

Python raises an IndentationError because it cannot determine the block.

Key Rules

• All statements in a block must be indented equally
• Common practice: 4 spaces
• Mixing tabs and spaces causes errors

Indentation makes Python programs:

• cleaner
• more readable
• less ambiguous

6.4 Repetition (Loops)

Why Repetition is Needed

Many tasks involve repeating actions:

• printing numbers from 1 to 10
• calculating total marks for multiple subjects
• reading multiple inputs

Writing the same statement repeatedly is inefficient.

Loops allow us to repeat instructions automatically.

Python provides two main loops:

• while
• for

6.4.1 The while Loop

Logical Meaning

*Repeat a block of code as long as a condition remains true.*

Syntax

while condition:
    statement(s)

Example: Printing numbers 1 to 5

count = 1

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

Explanation:

• Condition is checked before every iteration
• Loop stops when condition becomes false
• Variable count is called a loop control variable

Infinite Loop (Logical Error)

count = 1

while count <= 5:
    print(count)

Why is this dangerous?

• count never changes
• Condition always remains true
• Loop never ends

6.4.2 The for Loop

Used when the number of repetitions is known in advance.

Syntax

for variable in range(start, stop, step):
    statement(s)

Example

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

Explanation:

• range(1,6) generates numbers 1 to 5
• i takes one value at a time
• Loop ends automatically

Comparison: while vs for

| Aspect | while | for | | - | - | – | | Condition based | Yes | No | | Count controlled manually | Yes | No | | Preferred when | repetitions unknown | repetitions known |

6.5 Break and Continue Statement

Sometimes we need extra control inside loops.

6.5.1 break Statement

Purpose

Immediately terminates the loop, regardless of condition.

Example

for i in range(1, 10):
    if i == 5:
        break
    print(i)

Output:

1
2
3
4

Explanation:

• Loop stops when i == 5
• Control moves outside the loop

6.5.2 continue Statement

Purpose

Skips the current iteration and moves to the next one.

Example

for i in range(1, 6):
    if i == 3:
        continue
    print(i)

Output:

1
2
4
5

Explanation:

• When i == 3, print is skipped
• Loop continues normally

Difference Between break and continue

| break | continue | | | – | | Ends the loop | Skips current iteration | | Control exits loop | Control stays in loop |

6.6 Nested Loops

What Are Nested Loops?

A loop inside another loop is called a nested loop.

Used when a task requires repetition within repetition.

Example: Pattern Printing

for i in range(1, 4):
    for j in range(1, 4):
        print("*", end=" ")
    print()

Output:

* * *
* * *
* * *

Explanation:

• Outer loop controls rows
• Inner loop controls columns
• Inner loop runs completely for each outer loop iteration

Example: Multiplication Table

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

Key Conceptual Takeaways

1. The if statement is used for selection or decision making.
2. The looping constructs while and for allow sections of code to be executed repeatedly under some condition.
3. for statement iterates over a range of values or a sequence.
4. The statements within the body of for loop are executed till the range of values is exhausted.
5. The statements within the body of a while are executed over and over until the condition of the while is false.
6. If the condition of the while loop is initially false, the body is not executed even once.
7. The statements within the body of the while loop must ensure that the condition eventually becomes false; otherwise, the loop will become an infinite loop, leading to a logical error in the program.
8. TThe break statement immediately exits a loop, skipping the rest of the loop’s body. Execution continues with the statement immediately following the body of the loop.
9. When a continue statement is encountered, the control jumps to the beginning of the loop for the next iteration.
10. A loop contained within another loop is called a nested loop.