🔄 Session 10: Loops

The Power of Repetition

Make the computer do the boring work—again and again!

Print 1 to 1000? Sum a list? Draw patterns? One loop does it all.

✨ Write once → Run many times ✨

Today's Loop Mission

🔄

for Loop
Count-based repetition

⏳

while Loop
Condition-based repetition

🔁

do-while Loop
Execute at least once

🛑

break & continue
Control loop flow

📐

Nested Loops
Loops inside loops

🎯

Real Programs
Patterns, sums, tables

🤔 Why Loops? Without vs With

❌ Without loop (print 1 to 100)

printf("%d\n",1);
printf("%d\n",2);
printf("%d\n",3);
// ... 97 more lines! 😫
                        

100 lines of the same thing!

✅ With loop

for(i=1; i<=100; i++)
    printf("%d\n", i);
                        

3 lines. Done. 🎉

Less code • Easy to change • No copy-paste mistakes

🔄 How a Loop Runs

Follow the cycle—it keeps going until the condition is false!

1. Init (i=1) → 2. Condition? → 3. Run body → 4. Update (i++) ↻ (back to 2)

When condition is false → exit loop

🔄 The for Loop

Best when you know how many times to repeat!

// Syntax: for(init; condition; update)
for (int i = 1; i <= 5; i++) {
    printf("%d\n", i);
}
// Output: 1 2 3 4 5
            

1️⃣ Init

Start: i = 1

2️⃣ Condition

Continue while i <= 5

3️⃣ Update

After each run: i++

📝 for Loop: Print 1 to 10

#include <stdio.h>

int main() {
    int i;
    
    for (i = 1; i <= 10; i++) {
        printf("%d ", i);
    }
    printf("\n");
    
    return 0;
}
// Output: 1 2 3 4 5 6 7 8 9 10
            

➕ for Loop: Sum of 1 to N

int n, sum = 0;
printf("Enter n: ");
scanf("%d", &n);

for (int i = 1; i <= n; i++) {
    sum = sum + i;
}
printf("Sum = %d\n", sum);
            

Example: n = 5 → Sum = 1+2+3+4+5 = 15

⏳ The while Loop

Repeat while a condition is true. Check condition first!

// Syntax: while(condition) { ... }
int count = 1;
while (count <= 5) {
    printf("%d ", count);
    count++;
}
// Output: 1 2 3 4 5
            

⚠️ Update Variable

Must change condition or infinite loop!

🔁 The do-while Loop

Execute at least once, then check condition!

// Syntax: do { ... } while(condition);
int x = 1;
do {
    printf("%d ", x);
    x++;
} while (x <= 5);
// Output: 1 2 3 4 5
            

Difference: do-while runs body first, then checks. while checks first.

🔄 When to Use Which?

🔄 for loop

Use when: You know how many times!

• Counting 1 to N, tables

• Most common for repetition

⏳ while loop

Use when: "Repeat until..."

• User quits, condition becomes false

• Condition checked first

🔁 do-while loop

Use when: Run at least once!

• Menus: show then ask again

• Condition checked last

🛑 break and continue

// break: exit the loop immediately
for (i = 1; i <= 10; i++) {
    if (i == 5) break;
    printf("%d ", i);
}
// Output: 1 2 3 4

// continue: skip to next iteration
for (i = 1; i <= 5; i++) {
    if (i == 3) continue;
    printf("%d ", i);
}
// Output: 1 2 4 5 (skips 3)
            

📐 Nested Loops

A loop inside another loop! Great for patterns and tables.

// Print 3 rows, 4 stars each
for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 4; col++) {
        printf("*");
    }
    printf("\n");
}
            

Output:

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

📊 Multiplication Table Example

int num = 5;
for (int i = 1; i <= 10; i++) {
    printf("%d x %d = %d\n", num, i, num * i);
}
// 5 x 1 = 5, 5 x 2 = 10, ... 5 x 10 = 50
            

🚨 Loop Traps

❌ Infinite Loop

Forgetting to update loop variable in while

❌ Off-by-One

i <= 10 vs i < 10 — count carefully!

❌ Semicolon

for(i=1; i<=5; i++); ← wrong! Loop does nothing

🧠 Loop Quiz!

for loop is best when we know the number of repetitions

do-while checks condition before running the body

break exits the loop immediately

continue restarts the program from the beginning

🏆 Loop Mastery Unlocked!

You've earned these badges:

🔄 for Loop Pro

⏳ while Wizard

🔁 do-while Expert

📐 Pattern Builder

🛑 break & continue

You can make programs repeat anything—like a pro! 🚀

🏠 Your Coding Mission!

🔢 Print 1 to N

Use for loop to print numbers

➕ Sum & Product

Sum and product of 1 to N

📊 Table

Multiplication table for any number

⭐ Pattern

Print triangle or rectangle with *

🚀 What's Coming Next!

Arrays: Storing Many Values

Store 100 numbers in one variable? Yes, with arrays!

🤔 Think About This Week:

How would you store marks of 50 students?