🔀 Flow of Control

So far, every program you wrote ran line by line, from top to bottom — every line, every time. But real life isn't like that! 🚦 You decide things ("If it rains, take an umbrella") and you repeat things ("Do 20 push-ups"). This chapter answers the BIG questions: How can a program make decisions? How can it repeat a task many times without writing the same line again and again? What do if, elif, else, for, while, range(), break and continue do? Let's take control! 🎮

5.1 🏁 Introduction — Three Kinds of Flow

The order in which statements run is called the flow of control. Every program uses a mix of three kinds:

graph LR
    subgraph SEQ["➡️ Sequence"]
        direction TB
        S1["Step 1"] --> S2["Step 2"] --> S3["Step 3"]
    end
    subgraph SEL["🔀 Selection"]
        direction TB
        C{"Condition?"} -->|"True"| Y["Do this"]
        C -->|"False"| N["Do that"]
    end
    subgraph ITR["🔁 Iteration"]
        direction TB
        L{"Repeat\nagain?"} -->|"Yes"| B["Do the task"]
        B --> L
    end
    SEQ ~~~ SEL ~~~ ITR

Analogy: Getting ready for school 🎒:

• Sequence: wake up → brush → bathe → eat breakfast.
• Selection: if it's Monday, wear the white uniform; else wear the coloured one.
• Iteration: repeat "chew" until the food is finished! 😄

5.1.1 Indentation — Python's Way of Making Blocks 📦

A block (or suite) is a group of statements that belong together. Python shows a block using indentation (spaces at the start of the line) after a line ending with a colon :.

if marks >= 33:                 ← header line ends with a colon
    print("Pass")               ┐
    print("Well done!")         ┘ block — indented by 4 spaces
print("Result declared")        ← back to the left → outside the block

5.1.2 The Empty Statement — pass 🤐

Sometimes Python needs a statement in a block, but you have nothing to write yet. Use pass — it does nothing.

marks = 45
if marks > 90:
    pass            # will add a prize message later
print("Checked")

Output:

Checked

5.2 🔀 Selection — The if Statements

5.2.1 Simple if ☝️

Runs the block only if the condition is True. If it is False, the block is skipped.

if condition:
    statements

temperature = 38
if temperature > 35:
    print("It's very hot! Drink water. 💧")
print("Have a nice day")

Output:

It's very hot! Drink water. 💧
Have a nice day

5.2.2 if–else ✌️

Chooses between two paths: one for True, the other for False.

graph TD
    C{"🤔 Condition\nTrue?"}
    T["✅ if-block runs"]
    F["❌ else-block runs"]
    E["➡️ Next statement"]

    C -->|"True"| T --> E
    C -->|"False"| F --> E

    style T fill:#4CAF50,color:#fff
    style F fill:#F44336,color:#fff

num = int(input("Enter a number: "))
if num % 2 == 0:
    print(num, "is EVEN")
else:
    print(num, "is ODD")

Sample run (the user types 7):

Enter a number: 7
7 is ODD

Analogy: An if–else is like a railway track switch 🚂 — the train goes on ONE track or the OTHER, never both!

5.2.3 if–elif–else Ladder 🪜

When there are more than two choices, use elif (short for "else if"). Python checks the conditions from top to bottom and runs only the first block whose condition is True.

Code Example: grade.py

marks = int(input("Enter marks: "))
if marks >= 90:
    print("Grade A")
elif marks >= 75:
    print("Grade B")
elif marks >= 50:
    print("Grade C")
elif marks >= 33:
    print("Grade D")
else:
    print("Fail - try again!")

Sample run (the user types 82):

Enter marks: 82
Grade B

How Python checks it for 82:

82 >= 90 ?  False  → skip
82 >= 75 ?  True   → print "Grade B"  → STOP checking the rest

5.2.4 Nested if 🪆

An if inside another if is called a nested if. It is used when one decision depends on another.

age = 20
has_licence = False
if age >= 18:
    if has_licence:
        print("You can drive 🚗")
    else:
        print("Get a licence first 📝")
else:
    print("Too young to drive")

Output:

Get a licence first 📝

Largest of three numbers — a very common exam program:

a = int(input("Enter a: "))
b = int(input("Enter b: "))
c = int(input("Enter c: "))
if a >= b and a >= c:
    print("Largest is", a)
elif b >= c:
    print("Largest is", b)
else:
    print("Largest is", c)

Sample run (the user types 12, 45 and 30):

Enter a: 12
Enter b: 45
Enter c: 30
Largest is 45

year = int(input("Enter year: "))
if year % 400 == 0 or (year % 4 == 0 and year % 100 != 0):
    print("Leap year")
else:
    print("Not a leap year")

5.3 🔢 The range() Function

Before learning loops, let's meet their best friend — range(). It produces a sequence of numbers.

print(list(range(5)))
print(list(range(2, 7)))
print(list(range(1, 10, 2)))
print(list(range(10, 0, -3)))
print(list(range(5, 1)))

Output:

[0, 1, 2, 3, 4]
[2, 3, 4, 5, 6]
[1, 3, 5, 7, 9]
[10, 7, 4, 1]
[]

5.4 🔁 The for Loop

A for loop repeats a block once for each item in a sequence (a range, a string, a list…). Use it when you know how many times to repeat.

for variable in sequence:
    statements

for i in range(1, 6):
    print("Round", i)
print("Loop over!")

Output:

Round 1
Round 2
Round 3
Round 4
Round 5
Loop over!

Analogy: A for loop is like a teacher taking attendance 🧑‍🏫 — she goes through the register one name at a time, from the first to the last, and does the same thing for each ("Present?").

A for loop over a string:

for ch in "PYTHON":
    print(ch, end=" ")

Output:

P Y T H O N 

Code Example: table.py — Multiplication Table

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

Sample run (the user types 7):

Enter a number: 7
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

Code Example: sum_n.py — Sum of 1 to N

n = int(input("Enter N: "))
total = 0
for i in range(1, n + 1):
    total += i
print("Sum =", total)

Sample run (the user types 5):

Enter N: 5
Sum = 15

Dry run — how total changes (N = 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, "=", fact)

Sample run (the user types 5):

Enter a number: 5
Factorial of 5 = 120

5.5 🔄 The while Loop

A while loop repeats a block as long as a condition is True. Use it when you don't know in advance how many times to repeat.

while condition:
    statements

graph TD
    I["1️⃣ Initialise\ni = 1"]
    C{"2️⃣ Condition\ni <= 5 ?"}
    B["3️⃣ Body\nprint(i)"]
    U["4️⃣ Update\ni += 1"]
    X["🏁 Exit loop"]

    I --> C
    C -->|"True"| B --> U --> C
    C -->|"False"| X

    style C fill:#FF9800,color:#fff
    style U fill:#2196F3,color:#fff
    style X fill:#4CAF50,color:#fff

i = 1
while i <= 5:
    print(i, end=" ")
    i += 1
print()
print("After the loop, i =", i)

Output:

1 2 3 4 5 
After the loop, i = 6

Analogy: A while loop is like filling a bucket 🪣 — "while the bucket is not full, keep pouring water". You don't count the mugs; you just check the condition each time!

Code Example: digits.py — Sum of Digits

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

Sample run (the user types 4528):

Enter a number: 4528
Sum of digits = 19

Dry run for 4528:

5.5.1 for vs while ⚖️

5.6 🛑 Jump Statements — break and continue

for i in range(1, 8):
    if i == 5:
        break
    print(i, end=" ")
print("| break done")

for i in range(1, 8):
    if i == 5:
        continue
    print(i, end=" ")
print("| continue done")

Output:

1 2 3 4 | break done
1 2 3 4 6 7 | continue done

Analogy: You're eating a plate of 7 momos 🥟:

• break → at the 5th momo you're full — you stop eating completely.
• continue → the 5th momo fell on the floor — you skip just that one and eat the rest!

Code Example: prime.py — Prime Number Check

n = int(input("Enter a number: "))
is_prime = True
if n < 2:
    is_prime = False
for i in range(2, n):
    if n % i == 0:
        is_prime = False
        break              # a factor is found — no need to check further
if is_prime:
    print(n, "is a prime number")
else:
    print(n, "is not a prime number")

Sample run (the user types 13):

Enter a number: 13
13 is a prime number

5.7 🪆 Nested Loops

A loop inside another loop is a nested loop. For each round of the outer loop, the inner loop runs completely.

Analogy: A clock 🕐 — for every 1 hour (outer loop), the minute hand goes round 60 times (inner loop)!

for i in range(1, 4):
    for j in range(1, 3):
        print("i =", i, "j =", j)

Output:

i = 1 j = 1
i = 1 j = 2
i = 2 j = 1
i = 2 j = 2
i = 3 j = 1
i = 3 j = 2

The outer loop runs 3 times and the inner loop runs 2 times for each → 3 × 2 = 6 lines.

Code Example: pattern.py — Star Pattern

for row in range(1, 5):
    for col in range(row):
        print("*", end=" ")
    print()

Output:

* 
* * 
* * * 
* * * * 

Code Example: number_pattern.py

for row in range(1, 5):
    for col in range(1, row + 1):
        print(col, end=" ")
    print()

Output:

1 
1 2 
1 2 3 
1 2 3 4 

⚠️ Common Errors and Misconceptions

🔑 Quick Revision — Exam Ready!

• Flow of control → Sequence, Selection (if), Iteration (loops)
• Block → statements indented equally after a line ending with :
• pass → empty statement; does nothing
• if → one choice; if–else → two choices; if–elif–else → many choices, only the first True block runs
• Nested if → an if inside another if
• range(stop) → 0 to stop−1; range(start, stop); range(start, stop, step); stop is never included
• for → repeats for each item; number of rounds known
• while → repeats while a condition is True; initialise, test, update
• Infinite loop → condition never becomes False
• break → exits the loop; continue → skips to the next round
• Nested loop → inner loop runs completely for each round of the outer loop (total = outer × inner)
• Accumulator → total = 0 / fact = 1 before the loop, update inside

🎯 Sample Exam Questions

Q1: Very Short Answer [1 mark each]

a) Which statement does nothing and is used as a placeholder? Answer: pass

b) What numbers does range(3, 12, 3) produce? Answer: 3 6 9

c) Which statement ends a loop immediately? Answer: break

d) How many times will for i in range(5, 10): run? Answer: 5 times (5, 6, 7, 8, 9)

e) What is a loop that never ends called? Answer: An infinite loop

Q2: Output Based [2 marks]

x = 15
if x > 10:
    print("A")
elif x > 5:
    print("B")
else:
    print("C")
print("D")

Answer:

A
D

Only the first True block runs, so B is not printed even though x > 5 is also True.

Q3: Output Based [2 marks]

total = 0
for i in range(2, 11, 2):
    total += i
print(total)

Answer:

30

i takes 2, 4, 6, 8, 10 → 2 + 4 + 6 + 8 + 10 = 30.

Q4: Output Based [2 marks]

n = 1
while n < 20:
    print(n, end=" ")
    n = n * 3
print("End")

Answer:

1 3 9 End

n becomes 1 → 3 → 9 → 27; 27 < 20 is False, so the loop stops.

Q5: Rewrite [2 marks]

Rewrite the following for loop using a while loop:

for i in range(10, 0, -2):
    print(i)

Answer:

i = 10
while i > 0:
    print(i)
    i -= 2

Both print 10, 8, 6, 4, 2 on separate lines.

Q6: Error Finding [2 marks]

Find and correct the errors:

n = int(input("Enter n: "))
if n % 2 = 0
    print("Even")
else if:
    print("Odd")

Answer:

n = int(input("Enter n: "))
if n % 2 == 0:
    print("Even")
else:
    print("Odd")

Errors: = should be ==, the colon after the condition was missing, and else if: should be else:.

Q7: Program [3 marks]

Write a program to print all the even numbers from 1 to 20 and their sum.

Answer:

total = 0
for i in range(2, 21, 2):
    print(i, end=" ")
    total += i
print()
print("Sum =", total)

Output:

2 4 6 8 10 12 14 16 18 20 
Sum = 110

✏️ Practice Problems

1. Write a program to input a number and print whether it is positive, negative or zero.
2. Write a program to input the cost price and selling price of an item and print "Profit", "Loss" or "No profit no loss" with the amount.
3. Write the numbers produced by: (a) range(4) (b) range(3, 9) (c) range(20, 5, -5) (d) range(1, 2, 3)
4. Write a program to print the squares of numbers from 1 to 10.
5. Write a program to input N and print the sum of all odd numbers from 1 to N.
6. Write a program to input a number and print its reverse (e.g. 1234 → 4321) using a while loop.
7. Write the output: for i in range(1, 10): if i % 3 == 0: continue then print(i, end=" ") inside the loop.
8. Write a program that keeps asking for a password until the user types python123, then prints "Access granted".
9. Write a program to print this pattern:

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

10. Write a program to input a number and print whether it is an Armstrong number (e.g. 153 = 1³ + 5³ + 3³).
11. How many times will print("Hi") run in for i in range(3): for j in range(4): print("Hi")?
12. Write a program to input the marks of 5 students using a loop and print the highest marks.