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🔄 Unit 4: Concepts of Loops

Welcome, coder! 👋
In this unit, we’ll explore loops — the magic that lets your program repeat tasks automatically, without writing the same code 100 times.

🤔 Why Use Loops?​

Imagine printing numbers 1 to 100:

// Without loops (boring and impractical)
printf("1\n");
printf("2\n");
printf("3\n");
// ... 97 more times!

// With loops (much better)
for (int i = 1; i <= 100; i++) {
    printf("%d\n", i);
}

That’s the power of loops! 🚀

🔹 Types of Loops in C​

Loop TypeWhen to UseSpecial Feature
whileIterations unknownCondition checked first
do-whileMust run at least onceCondition checked last
forIterations knownAll parts in header

🔸 The while Loop​

Repeats while the condition is true.

Syntax​

initialization;
while (condition) {
    // repeat code
    update;
}

Example: Print Numbers​

int i = 1;
while (i <= 5) {
    printf("Number: %d\n", i);
    i++;
}

Example: Sum of Natural Numbers​

int n, sum = 0, i = 1;
scanf("%d", &n);
while (i <= n) {
    sum += i;
    i++;
}
printf("Sum = %d\n", sum);

🔹 The do-while Loop​

Executes once before checking the condition.

Syntax​

do {
    // code
    update;
} while (condition);

Example​

int y = 10;
do {
    printf("This will print at least once!\n");
} while (y < 5);

Example: Menu Program​

int choice;
do {
    printf("1. Add\n2. Subtract\n3. Exit\n");
    scanf("%d", &choice);
    // process choice
} while (choice != 3);

🔸 The for Loop​

Best when the number of iterations is known.

Syntax​

for (init; condition; update) {
    // repeat code
}

Example: Multiplication Table​

int num;
scanf("%d", &num);
for (int i = 1; i <= 10; i++) {
    printf("%d × %d = %d\n", num, i, num * i);
}

Example: Factorial​

int n; unsigned long long fact = 1;
scanf("%d", &n);
for (int i = 1; i <= n; i++) {
    fact *= i;
}
printf("%d! = %llu\n", n, fact);

🔹 Nested Loops​

Loops inside loops.

Example: Pattern Printing​

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        printf("* ");
    }
    printf("\n");
}

Output:

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

🎮 Loop Control Statements​

break​

Exits loop immediately.

for (int i = 1; i <= 10; i++) {
    if (i == 5) break;
    printf("%d ", i);
}

continue​

Skips current iteration.

for (int i = 1; i <= 10; i++) {
    if (i % 3 == 0) continue;
    printf("%d ", i);
}

⚡ Loop Variations​

Infinite Loops​

while (1) { /* runs forever */ }
for (;;) { /* also infinite */ }

Flexible for Loop​

for (int i = 0, j = 10; i < 5; i++, j--) {
    printf("i=%d, j=%d\n", i, j);
}

🧪 Practice Problems​

  1. ✅ Prime Checker (using for loop)
  2. ✅ Fibonacci Series
  3. ✅ Armstrong Number Checker
  4. ✅ Pattern Programs (hollow rectangle, diamond, Floyd’s triangle)

🎓 Quick Quiz​

Which loop always executes at least once?

  • while
  • do-while ✅
  • for
  • None

👏 That’s it for Unit 4!
You can now repeat, automate, and generate patterns like a coding wizard 🧙‍♂️.
Next up: Unit 5 — Arrays 📦