🔀 Flow of Control

So far, every program you've written ran straight from top to bottom — line 1, line 2, line 3, done. ⬇️ But real programs need to make decisions ("Is the student pass or fail?") and repeat work ("Print the report card for ALL 40 students"). This chapter gives your programs a BRAIN 🧠 and STAMINA 🏃! We'll answer: How does Python choose between two paths? How do we repeat statements using for and while? What does range() really generate? How do break, continue and the loop else clause change the flow? Let's take control! 🎮🐍

8.1 🏁 Introduction

The flow of control is the order in which the statements of a program are executed. In the Introduction to Problem Solving chapter, you learnt that every algorithm uses three basic control structures. Python supports all three:

graph LR
    FOC["🎮 FLOW OF CONTROL"]
    SEQ["➡️ Sequence\nStatements run one\nafter another"]
    SEL["🔀 Selection\nif, if-else, if-elif-else"]
    ITR["🔁 Iteration\nfor loop, while loop"]

    FOC --> SEQ
    FOC --> SEL
    FOC --> ITR

    style SEQ fill:#2196F3,color:#fff
    style SEL fill:#FF9800,color:#fff
    style ITR fill:#4CAF50,color:#fff

Think of it this way: A program is like your school day 🏫. Most periods happen in a fixed sequence. If it rains, the PT period is replaced by a library period (selection). And the school bell rings again and again at the end of every period (iteration)!

8.2 🧱 Types of Statements in Python

Statements are the instructions given to the computer. Python has three types:

8.2.1 The pass Statement 🫥

marks = 45
if marks >= 33:
    pass            # nothing to do yet — will add code later
else:
    print("Fail")

8.2.2 Compound Statements and Indentation 🧩

A compound statement has a header (ending with a colon :) and a body (a block of statements indented under it).

if marks >= 33:            ←  header (ends with a colon)
    print("Pass")          ←┐
    print("Well done!")    ←┘ body / block (indented by 4 spaces)
print("Result declared")   ←  NOT part of the if — back at the left margin

8.3 🔀 Selection — The if Statements

Selection statements let a program test a condition and decide which statements to execute. A condition is any expression that gives True or False.

graph TD
    IFS["🔀 SELECTION STATEMENTS"]
    A["1️⃣ if\nOne-way decision"]
    B["2️⃣ if-else\nTwo-way decision"]
    C["3️⃣ if-elif-else\nMulti-way decision"]
    D["4️⃣ Nested if\nif inside another if"]

    IFS --> A
    IFS --> B
    IFS --> C
    IFS --> D

    style A fill:#2196F3,color:#fff
    style B fill:#FF9800,color:#fff
    style C fill:#4CAF50,color:#fff
    style D fill:#9C27B0,color:#fff

8.3.1 The Simple if Statement 1️⃣

Executes the block only if the condition is True; otherwise the block is skipped.

Syntax:
if condition:
    statement(s)

age = int(input("Enter your age: "))
if age >= 18:
    print("You can vote! 🗳️")
print("Thank you")

Input 20  →  You can vote! 🗳️
             Thank you
Input 15  →  Thank you

8.3.2 The if-else Statement 2️⃣

Chooses one of two blocks: the if block when the condition is True, the else block when it is False. Exactly one of them always runs.

Syntax:
if condition:
    statement(s)      # runs when True
else:
    statement(s)      # runs when False

graph TD
    A{"condition\nTrue?"}
    B["if block"]
    C["else block"]
    D["next statement"]

    A -->|"True"| B --> D
    A -->|"False"| C --> D

    style A fill:#FF9800,color:#fff
    style B fill:#4CAF50,color:#fff
    style C fill:#F44336,color:#fff

Code Example: leap_year.py

# A year is a leap year if it is divisible by 400,
# OR divisible by 4 but NOT by 100
year = int(input("Enter a year: "))
if (year % 400 == 0) or (year % 4 == 0 and year % 100 != 0):
    print(year, "is a leap year")
else:
    print(year, "is not a leap year")

Input 2024  →  2024 is a leap year
Input 1900  →  1900 is not a leap year
Input 2000  →  2000 is a leap year

8.3.3 The if-elif-else Statement 3️⃣

Used when there are more than two possibilities. Python checks the conditions from top to bottom and runs the block of the FIRST condition that is True — the rest are skipped. The final else (optional) runs if no condition is True.

Syntax:
if condition1:
    block1
elif condition2:
    block2
elif condition3:
    block3
else:
    block4

Code Example: grade.py

marks = float(input("Enter percentage: "))
if marks >= 90:
    grade = "A"
elif marks >= 75:
    grade = "B"
elif marks >= 60:
    grade = "C"
elif marks >= 33:
    grade = "D"
else:
    grade = "E (Fail)"
print("Grade:", grade)

Input 82  →  Grade: B
Input 30  →  Grade: E (Fail)

8.3.4 Nested if Statements 4️⃣

An if statement inside another if (or else) block is called a nested if.

Code Example: positive_even.py

num = int(input("Enter a number: "))
if num > 0:
    if num % 2 == 0:
        print("Positive even number")
    else:
        print("Positive odd number")
elif num < 0:
    print("Negative number")
else:
    print("Zero")

Input 8   →  Positive even number
Input -3  →  Negative number
Input 0   →  Zero

8.4 🔢 The range() Function

Before learning loops, you must master range(). It generates a sequence of integers, and is most often used with the for loop.

Syntax:  range(start, stop, step)

print(list(range(1, 10, 3)))    # [1, 4, 7]
print(list(range(0, -5, -1)))   # [0, -1, -2, -3, -4]
print(len(range(3, 15)))        # 12

8.5 🔁 Iteration — Loops

Iteration (looping) means repeating a set of statements multiple times. Python has two loops:

graph TD
    LOOPS["🔁 PYTHON LOOPS"]
    FOR["🔢 for loop\nCounting loop\nRepeats for each item\nin a sequence"]
    WHILE["⏳ while loop\nConditional loop\nRepeats as long as a\ncondition is True"]

    LOOPS --> FOR
    LOOPS --> WHILE

    style FOR fill:#2196F3,color:#fff
    style WHILE fill:#FF9800,color:#fff

8.5.1 The for Loop 🔢

The for loop repeats a block once for each item in a sequence (a range, string, list, tuple, etc.). The loop variable takes the value of each item, one by one.

Syntax:
for variable in sequence:
    statement(s)

for i in range(1, 6):
    print(i, end=" ")
# Output: 1 2 3 4 5

for ch in "PYTHON":
    print(ch, end="-")
# Output: P-Y-T-H-O-N-

How a for loop works:

graph TD
    A(["Start"])
    B{"Any items left\nin the sequence?"}
    C["Assign next item\nto loop variable"]
    D["Execute loop body"]
    E(["Exit loop"])

    A --> B
    B -->|"Yes"| C --> D --> B
    B -->|"No"| E

    style B fill:#FF9800,color:#fff
    style D fill:#2196F3,color:#fff
    style E fill:#4CAF50,color:#fff

Code Example: multiplication_table.py

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

Input 7  →  7 x 1 = 7
            7 x 2 = 14
            ...
            7 x 10 = 70

Code Example: sum_and_average.py

total = 0
for i in range(1, 6):
    marks = float(input("Enter marks of subject " + str(i) + ": "))
    total += marks
print("Total =", total)
print("Average =", total / 5)

Code Example: factorial.py

n = int(input("Enter a number: "))
fact = 1
for i in range(1, n + 1):
    fact *= i
print("Factorial of", n, "is", fact)
# Input 6 → Factorial of 6 is 720

8.5.2 The while Loop ⏳

The while loop repeats a block as long as a condition is True. It is used when we don't know in advance how many times the loop must run.

Syntax:
while condition:
    statement(s)

A while loop has four parts:

count = 1                   ① Initialisation — give the loop variable a starting value
while count <= 5:           ② Test condition — checked BEFORE every iteration
    print(count)            ③ Body — the statements to repeat
    count += 1              ④ Update — change the loop variable

graph TD
    A["① count = 1"]
    B{"② count <= 5 ?"}
    C["③ print(count)"]
    D["④ count += 1"]
    E(["Loop ends"])

    A --> B
    B -->|"True"| C --> D --> B
    B -->|"False"| E

    style B fill:#FF9800,color:#fff
    style C fill:#2196F3,color:#fff
    style D fill:#9C27B0,color:#fff
    style E fill:#4CAF50,color:#fff

Code Example: sum_of_digits.py

num = int(input("Enter a number: "))
total = 0
while num > 0:
    digit = num % 10        # get the last digit
    total += digit
    num = num // 10         # remove the last digit
print("Sum of digits =", total)
# Input 4721 → Sum of digits = 14

Dry run for num = 4721:

Code Example: palindrome_number.py

num = int(input("Enter a number: "))
original = num
reverse = 0
while num > 0:
    reverse = reverse * 10 + num % 10
    num = num // 10
if original == reverse:
    print(original, "is a palindrome")
else:
    print(original, "is not a palindrome")
# Input 1221 → 1221 is a palindrome
# Input 1234 → 1234 is not a palindrome

count = 1
while count <= 5:
    print(count)     # ❌ count never changes → prints 1 forever!

for vs while — The Critical Comparison:

8.6 🦘 Jump Statements — break and continue

Jump statements change the normal flow of a loop.

for i in range(1, 8):
    if i == 4:
        break
    print(i, end=" ")
# Output: 1 2 3

for i in range(1, 8):
    if i == 4:
        continue
    print(i, end=" ")
# Output: 1 2 3 5 6 7

graph LR
    subgraph BR["🛑 break"]
        B1["1"] --> B2["2"] --> B3["3"] --> B4["4 ✋ STOP"]
    end
    subgraph CO["⏭️ continue"]
        C1["1"] --> C2["2"] --> C3["3"] --> C4["4 ⏭️ skip"] --> C5["5"] --> C6["6"] --> C7["7"]
    end

    style B4 fill:#F44336,color:#fff
    style C4 fill:#FF9800,color:#fff

Analogy: You're eating a plate of 7 pani-puris 🥟. break: at puri 4 you find a chilli 🌶️ and STOP eating altogether. continue: you find puri 4 is broken, so you SKIP just that one and carry on with 5, 6 and 7!

Code Example: prime_check.py

num = int(input("Enter a number: "))
is_prime = True
if num < 2:
    is_prime = False
for d in range(2, num):
    if num % d == 0:
        is_prime = False
        break               # a factor found — no need to check further
if is_prime:
    print(num, "is a prime number")
else:
    print(num, "is not a prime number")
# Input 29 → 29 is a prime number
# Input 91 → 91 is not a prime number   (91 = 7 x 13)

8.7 🎁 The Loop else Clause

Both for and while loops can have an optional else block. It runs only when the loop ends normally — that is, NOT because of a break.

graph TD
    L["🔁 Loop runs"]
    Q{"How did the\nloop end?"}
    N["✅ Normally\n(sequence over / condition False)"]
    B["🛑 By break"]
    E["else block RUNS"]
    S["else block SKIPPED"]

    L --> Q
    Q --> N --> E
    Q --> B --> S

    style E fill:#4CAF50,color:#fff
    style S fill:#F44336,color:#fff

Code Example: prime_with_else.py

num = int(input("Enter a number (2 or more): "))
for d in range(2, num):
    if num % d == 0:
        print(num, "is not prime; it is divisible by", d)
        break
else:
    print(num, "is prime")      # runs only if break never happened
# Input 17 → 17 is prime
# Input 15 → 15 is not prime; it is divisible by 3

i = 1
while i <= 3:
    print(i, end=" ")
    i += 1
else:
    print("-> loop over")
# Output: 1 2 3 -> loop over

8.8 🪆 Nested Loops

A loop inside another loop is a nested loop. For every single iteration of the outer loop, the inner loop runs completely.

for i in range(1, 4):           # outer loop — 3 times
    for j in range(1, 3):       # inner loop — 2 times for EACH i
        print(i, j)
# Total prints = 3 x 2 = 6

1 1
1 2
2 1
2 2
3 1
3 2

Analogy: A clock ⏰! The hour hand is the outer loop and the minute hand is the inner loop. For every ONE move of the hour hand, the minute hand completes a FULL round of 60 minutes.

Pattern programs — a favourite of board exams:

Code Example: star_triangle.py

n = 4
for i in range(1, n + 1):       # row number
    for j in range(i):          # print i stars in row i
        print("*", end=" ")
    print()                     # move to the next line

*
* *
* * *
* * * *

Code Example: number_pattern.py

for i in range(1, 5):
    for j in range(1, i + 1):
        print(j, end=" ")
    print()

1
1 2
1 2 3
1 2 3 4

Code Example: inverted_pattern.py

for i in range(4, 0, -1):
    print("*" * i)

****
***
**
*

⚠️ Common Errors and Misconceptions

🔑 Quick Revision — Exam Ready!

Statement Types:

• Empty → pass (placeholder, does nothing)
• Simple → single line, e.g. x = 5
• Compound → header with : + indented body, e.g. if, for, while

Selection:

range(start, stop, step):

• start included (default 0) · stop excluded · step default 1, can be negative, never 0
• range(5) → 0 1 2 3 4 · range(2, 10, 3) → 2 5 8 · range(5, 0, -1) → 5 4 3 2 1

Loops:

• for → counting loop; repeats for each item in a sequence
• while → conditional loop; needs initialisation, condition, body, update

Jump Statements:

• break → exits the loop immediately
• continue → skips to the next iteration

Loop else → runs only when the loop ends normally (no break)

Nested loops → inner loop runs fully for each outer iteration; total = outer × inner

🎯 Sample Board Exam Questions

Q1: Very Short Answer [1 mark each]

a) What is the output of print(list(range(2, 11, 3)))? → [2, 5, 8]

b) Which statement is used as a placeholder that does nothing? → pass

c) How many times will for i in range(10, 0, -2): run? → 5 times (10, 8, 6, 4, 2)

d) Which keyword is used in Python instead of "else if"? → elif

e) Which jump statement skips the rest of the current iteration? → continue

Q2: Output Based [2 marks]

Q: Write the output of the following code:

for i in range(1, 10):
    if i % 3 == 0:
        continue
    if i == 8:
        break
    print(i, end=" ")
else:
    print("Done")
print("End")

Answer:

1 2 4 5 7 End

(3 and 6 are skipped by continue; at i = 8 the loop breaks, so the loop's else does NOT run.)

Q3: Find the Errors [2 marks]

Q: Rewrite the code after removing all errors. Underline each correction.

n = int(input("Enter n"))
if n > 0
    for i in range(1, n+1):
    print(i)
else if n == 0:
    print("Zero")

Answer:

n = int(input("Enter n"))
if n > 0:                       # colon added
    for i in range(1, n+1):
        print(i)                # indentation fixed
elif n == 0:                    # 'else if' changed to 'elif'
    print("Zero")

Q4: Output Based [3 marks]

Q: Write the output of the following code:

x = 10
count = 0
while x > 0:
    x = x - 3
    count += 1
print(x, count)

for a in range(3):
    for b in range(a, 3):
        print(a + b, end=" ")
    print()

Answer:

-2 4
0 1 2
2 3
4

(x goes 10 → 7 → 4 → 1 → −2, so the loop runs 4 times. In the nested loop, the inner range starts at a.)

Q5: Program [3 marks]

Q: Write a program to print the Fibonacci series up to n terms (0 1 1 2 3 5 …).

n = int(input("How many terms? "))
a, b = 0, 1
for i in range(n):
    print(a, end=" ")
    a, b = b, a + b
# Input 8 → 0 1 1 2 3 5 8 13

✏️ Practice Problems

1. Write a program to input three numbers and print the largest using if-elif-else.

2. Write a program to input the cost price and selling price of an item and print whether there is a profit, a loss, or neither — and by how much.

3. Write the output of: (a) list(range(7)) (b) list(range(3, 20, 4)) (c) list(range(20, 3, -5)) (d) list(range(4, 4))

4. Write a program to print all even numbers from 1 to 50 using a for loop, and then again using a while loop.

5. Write a program to count the number of digits in an integer using a while loop.

6. Write a program to check whether a number is an Armstrong number (e.g., 153 = 1³ + 5³ + 3³).

7. Write a program to print all prime numbers between 1 and 100 (use a nested loop with break).

8. Write a program to print the following pattern for n = 5:

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

9. Explain the loop else clause with a program that searches for a number in a range and prints "Not found" using the else clause.

10. Convert the following while loop into an equivalent for loop:

    i = 5
    while i <= 25:
        print(i)
        i += 5