Loops & Iteration in C++ (`for`, `while`, `do-while`, Range-based `for`)

🟢 Beginner

📖 Definition

Loops allow a block of C++ statements to execute repeatedly as long as a specified condition remains true. C++ supports four primary loop structures:

for Loop: Best when the number of iterations is known in advance.
while Loop: Best when iterating based on a condition until it becomes false.
do-while Loop: Guarantees execution at least once before checking the condition.
Range-based for Loop (C++11): Cleanly iterates over containers like std::vector and arrays.

🇮🇳 Hindi Explanation

Loop ka use ek hi code block ko baar-baar repeat karne ke liye hota hai.

for loop tab use hota hai jab pata ho ki loop kitni baar chalana hai (e.g. 1 se 10 tak).
while loop pehle condition check karta hai, fir andar ka code chalata hai.
do-while loop kam se kam ek baar zaroor chalta hai, chahe condition shuru mein hi false ho.
Range-based for loop C++11 ka modern feature hai jo array ya vector ke har element par aasani se iterate karta hai.

🚩 Marathi Explanation

Loop mule aapan ekapeksha jast vela ekach code block punha punha run karu shakto.

for loop: Iteration chi sankhya mahit aslyas.
while loop: Condition true ahe tovar.
do-while loop: Shart (condition) magun tapasli jate, mhanun ha loop kamit kami eka vela nakkich chaltos.
Range-based for (C++11): Vector kiwa array madhil sarva elements varun iterate karnyasathi.

📝 Syntax Comparison

// 1. Standard for Loop
for (int i = 0; i < 5; i++) { /* Statements */ }

// 2. while Loop
int j = 0;
while (j < 5) { /* Statements */ j++; }

// 3. do-while Loop
int k = 0;
do { /* Statements */ k++; } while (k < 5);

// 4. Range-based for Loop (C++11)
int numbers[] = {10, 20, 30};
for (int val : numbers) { /* Statements */ }

💡 Practical Example

#include <iostream>
#include <vector>

int main() {
    // 1. Standard for Loop
    std::cout << "--- 1. STANDARD FOR LOOP (1 to 5) ---" << std::endl;
    for (int i = 1; i <= 5; i++) {
        std::cout << "Iteration Count: " << i << std::endl;
    }

    // 2. while Loop (Countdown)
    std::cout << "\n--- 2. WHILE LOOP (Countdown) ---" << std::endl;
    int count = 3;
    while (count > 0) {
        std::cout << "Countdown: " << count << "..." << std::endl;
        count--;
    }
    std::cout << "Blastoff! 🚀" << std::endl;

    // 3. do-while Loop (Guaranteed At Least 1 Run)
    std::cout << "\n--- 3. DO-WHILE LOOP ---" << std::endl;
    int choice = 0;
    do {
        std::cout << "Executed choice = " << choice << " (Condition checked after execution)" << std::endl;
        choice++;
    } while (choice < 1);

    // 4. Range-based for Loop (C++11)
    std::cout << "\n--- 4. RANGE-BASED FOR LOOP ---" << std::endl;
    std::vector<std::string> fruits = {"Apple", "Banana", "Cherry", "Mango"};
    for (const std::string& fruit : fruits) {
        std::cout << "Fruit Item: " << fruit << std::endl;
    }

    return 0;
}

🔍 Code Breakdown

for (int i = 1; i <= 5; i++): Initializes i = 1, checks i <= 5, runs body, then increments i++.
const std::string& fruit: Range-based for loop uses a const reference (&) to avoid copying strings in memory during iteration.

👀 Output

--- 1. STANDARD FOR LOOP (1 to 5) ---
Iteration Count: 1
Iteration Count: 2
Iteration Count: 3
Iteration Count: 4
Iteration Count: 5

--- 2. WHILE LOOP (Countdown) ---
Countdown: 3...
Countdown: 2...
Countdown: 1...
Blastoff! 🚀

--- 3. DO-WHILE LOOP ---
Executed choice = 0 (Condition checked after execution)

--- 4. RANGE-BASED FOR LOOP ---
Fruit Item: Apple
Fruit Item: Banana
Fruit Item: Cherry
Fruit Item: Mango

⚠️ Common Mistakes & Infinite Loops

Infinite Loop Bug: Forgetting to update loop variables (count-- or i++) causes the condition to remain true forever, freezing terminal execution!
Off-By-One Errors: Writing i <= 5 when i < 5 was intended, leading to 1 extra iteration.
Modifying Range-based for Targets Without Reference: Writing for (int x : vec) x *= 2; modifies the local copy x, NOT the vector elements! Write for (int& x : vec) x *= 2; (with reference &) to modify vector elements directly.

🛡️ Best Practices

Prefer Range-based for loops when reading or modifying C++ containers like std::vector.
Use break to exit loops early and continue to skip the remainder of the current iteration.

🧪 Try It Yourself

Print all Even numbers between 1 and 20 using a for loop.
Calculate the factorial of 5 ($5! = 5 \times 4 \times 3 \times 2 \times 1 = 120$) using a while loop.

🎯 Mini Challenge

Write a program that uses a loop to prompt the user to guess a secret passcode number (e.g. 1234). Keep looping until the user enters the correct passcode!

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Loops & Iteration in C++ (for, while, do-while, Range-based for)