Loops in C++

The for loop is the most commonly used loop when you know in advance how many times you want to iterate. It combines initialization, condition, and update in a single line.

Basic Syntax

#include <iostream>
using namespace std;

int main() {
    // for (initialization; condition; update) { body }
    
    // Print numbers 1 to 5
    for (int i = 1; i <= 5; i++) {
        cout << i << " ";
    }
    cout << endl;  // Output: 1 2 3 4 5
    
    // Execution flow:
    // 1. Initialize i = 1
    // 2. Check condition: i <= 5 (true)
    // 3. Execute body: print 1
    // 4. Update: i++ (i becomes 2)
    // 5. Repeat steps 2-4 until condition is false
    
    return 0;
}

for Loop Variations

#include <iostream>
using namespace std;

int main() {
    // Counting down
    cout << "Countdown: ";
    for (int i = 5; i >= 1; i--) {
        cout << i << " ";
    }
    cout << endl;  // Output: 5 4 3 2 1
    
    // Step by 2
    cout << "Even numbers: ";
    for (int i = 0; i <= 10; i += 2) {
        cout << i << " ";
    }
    cout << endl;  // Output: 0 2 4 6 8 10
    
    // Multiple variables
    cout << "Two variables: ";
    for (int i = 0, j = 10; i < j; i++, j--) {
        cout << "(" << i << "," << j << ") ";
    }
    cout << endl;  // Output: (0,10) (1,9) (2,8) (3,7) (4,6)
    
    // Omitting parts (all three are optional)
    int k = 0;
    for ( ; k < 3; ) {
        cout << k << " ";
        k++;
    }
    cout << endl;  // Output: 0 1 2
    
    return 0;
}

Practical Example: Sum and Factorial

#include <iostream>
using namespace std;

int main() {
    // Calculate sum of 1 to n
    int n = 10;
    int sum = 0;
    
    for (int i = 1; i <= n; i++) {
        sum += i;
    }
    cout << "Sum of 1 to " << n << ": " << sum << endl;  // 55
    
    // Calculate factorial
    int num = 5;
    long long factorial = 1;
    
    for (int i = 1; i <= num; i++) {
        factorial *= i;
    }
    cout << num << "! = " << factorial << endl;  // 120
    
    // Print multiplication table
    int table = 7;
    cout << "\nMultiplication table of " << table << ":" << endl;
    for (int i = 1; i <= 10; i++) {
        cout << table << " x " << i << " = " << (table * i) << endl;
    }
    
    return 0;
}

The while loop checks the condition before each iteration. It's ideal when you don't know in advance how many times you need to loop.

#include <iostream>
using namespace std;

int main() {
    // Basic while loop
    int count = 1;
    
    while (count <= 5) {
        cout << count << " ";
        count++;
    }
    cout << endl;  // Output: 1 2 3 4 5
    
    // If condition is false initially, body never executes
    int x = 10;
    while (x < 5) {
        cout << "This won't print" << endl;
        x++;
    }
    
    return 0;
}

Practical Examples

#include <iostream>
using namespace std;

int main() {
    // Sum digits of a number
    int number = 12345;
    int digitSum = 0;
    int temp = number;
    
    while (temp > 0) {
        digitSum += temp % 10;  // Get last digit
        temp /= 10;             // Remove last digit
    }
    cout << "Sum of digits of " << number << ": " << digitSum << endl;  // 15
    
    // Reverse a number
    int num = 1234;
    int reversed = 0;
    temp = num;
    
    while (temp > 0) {
        int digit = temp % 10;
        reversed = reversed * 10 + digit;
        temp /= 10;
    }
    cout << "Reverse of " << num << ": " << reversed << endl;  // 4321
    
    // Count digits
    int value = 987654;
    int digitCount = 0;
    temp = value;
    
    while (temp > 0) {
        digitCount++;
        temp /= 10;
    }
    cout << value << " has " << digitCount << " digits" << endl;  // 6
    
    return 0;
}

User Input with while

#include <iostream>
using namespace std;

int main() {
    int input;
    int sum = 0;
    
    cout << "Enter numbers (0 to stop):" << endl;
    
    cin >> input;
    while (input != 0) {
        sum += input;
        cin >> input;
    }
    
    cout << "Total sum: " << sum << endl;
    
    // Validation loop
    int age;
    cout << "Enter your age (1-120): ";
    cin >> age;
    
    while (age < 1 || age > 120) {
        cout << "Invalid! Enter age (1-120): ";
        cin >> age;
    }
    
    cout << "Valid age: " << age << endl;
    
    return 0;
}

The do-while loop executes the body first, then checks the condition. This guarantees at least one execution, making it perfect for menus and input validation.

#include <iostream>
using namespace std;

int main() {
    // Basic do-while
    int count = 1;
    
    do {
        cout << count << " ";
        count++;
    } while (count <= 5);
    
    cout << endl;  // Output: 1 2 3 4 5
    
    // Executes at least once even if condition is false
    int x = 10;
    do {
        cout << "This prints once!" << endl;
        x++;
    } while (x < 5);  // Condition false, but body ran once
    
    return 0;
}

Menu System Example

#include <iostream>
using namespace std;

int main() {
    int choice;
    
    do {
        cout << "\n===== MENU =====" << endl;
        cout << "1. Say Hello" << endl;
        cout << "2. Say Goodbye" << endl;
        cout << "3. Show Time" << endl;
        cout << "0. Exit" << endl;
        cout << "Choice: ";
        cin >> choice;
        
        switch (choice) {
            case 1:
                cout << "Hello, World!" << endl;
                break;
            case 2:
                cout << "Goodbye, World!" << endl;
                break;
            case 3:
                cout << "It's coding time!" << endl;
                break;
            case 0:
                cout << "Exiting..." << endl;
                break;
            default:
                cout << "Invalid choice!" << endl;
        }
    } while (choice != 0);
    
    return 0;
}

Input Validation with do-while

#include <iostream>
using namespace std;

int main() {
    int password;
    const int CORRECT_PIN = 1234;
    int attempts = 0;
    const int MAX_ATTEMPTS = 3;
    
    do {
        cout << "Enter PIN: ";
        cin >> password;
        attempts++;
        
        if (password != CORRECT_PIN) {
            cout << "Wrong PIN! " << (MAX_ATTEMPTS - attempts) 
                 << " attempts remaining." << endl;
        }
    } while (password != CORRECT_PIN && attempts < MAX_ATTEMPTS);
    
    if (password == CORRECT_PIN) {
        cout << "Access granted!" << endl;
    } else {
        cout << "Account locked!" << endl;
    }
    
    return 0;
}

Loop Comparison

The range-based for loop provides a cleaner syntax for iterating over containers like arrays, vectors, and strings. Introduced in C++11, it's the preferred way to iterate when you don't need the index.

#include <iostream>
#include <vector>
#include <string>
using namespace std;

int main() {
    // Array iteration
    int numbers[] = {10, 20, 30, 40, 50};
    
    cout << "Array elements: ";
    for (int num : numbers) {
        cout << num << " ";
    }
    cout << endl;  // Output: 10 20 30 40 50
    
    // Vector iteration
    vector<string> fruits = {"Apple", "Banana", "Cherry"};
    
    cout << "Fruits: ";
    for (string fruit : fruits) {
        cout << fruit << " ";
    }
    cout << endl;  // Output: Apple Banana Cherry
    
    // String iteration (character by character)
    string text = "Hello";
    
    cout << "Characters: ";
    for (char c : text) {
        cout << c << "-";
    }
    cout << endl;  // Output: H-e-l-l-o-
    
    return 0;
}

By Value vs By Reference

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<int> nums = {1, 2, 3, 4, 5};
    
    // By value (copy) - cannot modify original
    cout << "By value (doubled locally): ";
    for (int n : nums) {
        n *= 2;  // Only modifies the copy
        cout << n << " ";
    }
    cout << endl;  // Output: 2 4 6 8 10
    
    // Original unchanged
    cout << "Original: ";
    for (int n : nums) {
        cout << n << " ";
    }
    cout << endl;  // Output: 1 2 3 4 5
    
    // By reference - modifies original
    for (int& n : nums) {
        n *= 2;  // Modifies original
    }
    
    cout << "After reference modification: ";
    for (int n : nums) {
        cout << n << " ";
    }
    cout << endl;  // Output: 2 4 6 8 10
    
    // By const reference - efficient for large objects, read-only
    vector<string> names = {"Alice", "Bob", "Charlie"};
    
    for (const string& name : names) {
        cout << name << " ";
        // name = "Changed";  // Error! Can't modify const reference
    }
    cout << endl;
    
    return 0;
}

Using auto Keyword

#include <iostream>
#include <vector>
#include <map>
using namespace std;

int main() {
    vector<int> numbers = {1, 2, 3, 4, 5};
    
    // auto deduces the type automatically
    cout << "With auto: ";
    for (auto num : numbers) {
        cout << num << " ";
    }
    cout << endl;
    
    // auto with reference
    for (auto& num : numbers) {
        num += 10;
    }
    
    // auto with const reference (recommended for read-only)
    cout << "Modified: ";
    for (const auto& num : numbers) {
        cout << num << " ";
    }
    cout << endl;  // Output: 11 12 13 14 15
    
    // Useful with complex types like maps
    map<string, int> ages = {{"Alice", 25}, {"Bob", 30}};
    
    for (const auto& pair : ages) {
        cout << pair.first << ": " << pair.second << endl;
    }
    
    return 0;
}

Loop control statements (break, continue, goto) allow you to alter the normal flow of loop execution.

break Statement

#include <iostream>
using namespace std;

int main() {
    // break exits the loop immediately
    cout << "Finding first multiple of 7: ";
    for (int i = 1; i <= 100; i++) {
        if (i % 7 == 0) {
            cout << i << endl;  // 7
            break;  // Exit loop
        }
    }
    
    // Search in array
    int arr[] = {10, 25, 30, 45, 50};
    int target = 30;
    bool found = false;
    
    for (int i = 0; i < 5; i++) {
        if (arr[i] == target) {
            cout << "Found " << target << " at index " << i << endl;
            found = true;
            break;
        }
    }
    
    if (!found) {
        cout << target << " not found" << endl;
    }
    
    return 0;
}

continue Statement

#include <iostream>
using namespace std;

int main() {
    // continue skips the rest of current iteration
    cout << "Odd numbers 1-10: ";
    for (int i = 1; i <= 10; i++) {
        if (i % 2 == 0) {
            continue;  // Skip even numbers
        }
        cout << i << " ";
    }
    cout << endl;  // Output: 1 3 5 7 9
    
    // Skip specific values
    cout << "Numbers except 5: ";
    for (int i = 1; i <= 10; i++) {
        if (i == 5) {
            continue;
        }
        cout << i << " ";
    }
    cout << endl;  // Output: 1 2 3 4 6 7 8 9 10
    
    // Process only positive numbers
    int nums[] = {3, -1, 4, -2, 5, -3};
    int sum = 0;
    
    for (int num : nums) {
        if (num < 0) {
            continue;  // Skip negative numbers
        }
        sum += num;
    }
    cout << "Sum of positives: " << sum << endl;  // 12
    
    return 0;
}

break vs continue

#include <iostream>
using namespace std;

int main() {
    cout << "Using break at 5: ";
    for (int i = 1; i <= 10; i++) {
        if (i == 5) break;
        cout << i << " ";
    }
    cout << endl;  // Output: 1 2 3 4
    
    cout << "Using continue at 5: ";
    for (int i = 1; i <= 10; i++) {
        if (i == 5) continue;
        cout << i << " ";
    }
    cout << endl;  // Output: 1 2 3 4 6 7 8 9 10
    
    return 0;
}

Nested loops are loops inside other loops. The inner loop completes all its iterations for each iteration of the outer loop. They're essential for working with multi-dimensional data and patterns.

#include <iostream>
using namespace std;

int main() {
    // Basic nested loop
    cout << "Nested loop output:" << endl;
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            cout << "(" << i << "," << j << ") ";
        }
        cout << endl;
    }
    // Output:
    // (1,1) (1,2) (1,3)
    // (2,1) (2,2) (2,3)
    // (3,1) (3,2) (3,3)
    
    return 0;
}

Pattern Printing

#include <iostream>
using namespace std;

int main() {
    int n = 5;
    
    // Right triangle
    cout << "Right Triangle:" << endl;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= i; j++) {
            cout << "* ";
        }
        cout << endl;
    }
    /*
    *
    * *
    * * *
    * * * *
    * * * * *
    */
    
    // Number pyramid
    cout << "\nNumber Pyramid:" << endl;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= i; j++) {
            cout << j << " ";
        }
        cout << endl;
    }
    /*
    1
    1 2
    1 2 3
    1 2 3 4
    1 2 3 4 5
    */
    
    // Inverted triangle
    cout << "\nInverted Triangle:" << endl;
    for (int i = n; i >= 1; i--) {
        for (int j = 1; j <= i; j++) {
            cout << "* ";
        }
        cout << endl;
    }
    
    return 0;
}

2D Array Traversal

#include <iostream>
using namespace std;

int main() {
    int matrix[3][4] = {
        {1, 2, 3, 4},
        {5, 6, 7, 8},
        {9, 10, 11, 12}
    };
    
    // Print matrix
    cout << "Matrix:" << endl;
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++) {
            cout << matrix[i][j] << "\t";
        }
        cout << endl;
    }
    
    // Find sum of all elements
    int sum = 0;
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++) {
            sum += matrix[i][j];
        }
    }
    cout << "Sum of all elements: " << sum << endl;  // 78
    
    // Find maximum element
    int maxVal = matrix[0][0];
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++) {
            if (matrix[i][j] > maxVal) {
                maxVal = matrix[i][j];
            }
        }
    }
    cout << "Maximum element: " << maxVal << endl;  // 12
    
    return 0;
}

Breaking from Nested Loops

#include <iostream>
using namespace std;

int main() {
    // Using a flag to break from nested loops
    bool found = false;
    int target = 7;
    
    for (int i = 1; i <= 5 && !found; i++) {
        for (int j = 1; j <= 5 && !found; j++) {
            cout << "Checking (" << i << "," << j << ")" << endl;
            if (i * j == target) {
                cout << "Found! " << i << " * " << j << " = " << target << endl;
                found = true;
            }
        }
    }
    
    return 0;
}

An infinite loop runs forever unless explicitly broken. While usually a bug, infinite loops are sometimes intentional for continuous processes like servers or game loops.

#include <iostream>
using namespace std;

int main() {
    // Intentional infinite loops
    
    // Method 1: while(true)
    // while (true) {
    //     // Game loop, server, etc.
    // }
    
    // Method 2: for(;;)
    // for (;;) {
    //     // Runs forever
    // }
    
    // Method 3: while(1)
    // while (1) {
    //     // Runs forever
    // }
    
    // Practical example: Event loop with exit condition
    int input;
    while (true) {
        cout << "Enter a number (negative to quit): ";
        cin >> input;
        
        if (input < 0) {
            cout << "Goodbye!" << endl;
            break;  // Exit the infinite loop
        }
        
        cout << "Square: " << (input * input) << endl;
    }
    
    return 0;
}

Common Infinite Loop Bugs

// Bug 1: Forgetting to update counter
int i = 0;
while (i < 5) {
    cout << i;
    // i++ missing! Loop never ends
}

// Bug 2: Wrong comparison
for (int i = 0; i != 10; i += 3) {
    cout << i;
    // i goes 0, 3, 6, 9, 12, 15... never equals 10!
}

// Bug 3: Modifying loop variable incorrectly
for (int i = 10; i >= 0; i++) {  // Should be i--
    cout << i;
}