C++ Loops — for, while, do-while, range-based | Tutorials Logic

Types of Loops

Loops allow you to execute a block of code repeatedly. C++ provides four looping constructs:

Loop	Use when	Checks condition
`for`	Number of iterations is known	Before each iteration
`while`	Number of iterations is unknown	Before each iteration
`do-while`	Must execute at least once	After each iteration
range-based `for`	Iterating over a collection	Automatic

1. for Loop

The for loop is ideal when you know exactly how many times to iterate. It combines initialization, condition check, and update in a single line.

Syntax: for (initialization; condition; update) { body }

for Loop Examples

#include <iostream>
using namespace std;

int main() {
    // Basic for loop - count 1 to 5
    for (int i = 1; i <= 5; i++) {
        cout << i << " ";
    }
    cout << endl;  // 1 2 3 4 5

    // Count down
    for (int i = 5; i >= 1; i--) {
        cout << i << " ";
    }
    cout << endl;  // 5 4 3 2 1

    // Step by 2
    for (int i = 0; i <= 10; i += 2) {
        cout << i << " ";
    }
    cout << endl;  // 0 2 4 6 8 10

    // Nested for - multiplication table
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            cout << i * j << "\t";
        }
        cout << endl;
    }
    // 1  2  3
    // 2  4  6
    // 3  6  9

    // Sum 1 to 100
    int sum = 0;
    for (int i = 1; i <= 100; i++) sum += i;
    cout << "Sum 1..100 = " << sum << endl;  // 5050

    return 0;
}

2. while Loop

The while loop checks the condition before each iteration. If the condition is false from the start, the body never executes. Use it when the number of iterations depends on a runtime condition.

Syntax: while (condition) { body }

while Loop Examples

#include <iostream>
using namespace std;

int main() {
    // Basic while loop
    int i = 1;
    while (i <= 5) {
        cout << i << " ";
        i++;
    }
    cout << endl;  // 1 2 3 4 5

    // Reverse digits of a number
    int num = 12345, reversed = 0;
    while (num > 0) {
        reversed = reversed * 10 + num % 10;
        num /= 10;
    }
    cout << "Reversed: " << reversed << endl;  // 54321

    // Read until user enters 0
    int n, total = 0;
    cout << "Enter numbers (0 to stop):" << endl;
    while (cin >> n && n != 0) {
        total += n;
    }
    cout << "Total: " << total << endl;

    // Condition false from start - body never runs
    int x = 10;
    while (x < 5) {
        cout << "This never prints" << endl;
        x++;
    }
    cout << "Loop skipped entirely" << endl;

    return 0;
}

3. do-while Loop

The do-while loop executes the body first, then checks the condition. This guarantees the body runs at least once - even if the condition is false from the start. It's perfect for menus and input validation.

Syntax: do { body } while (condition);

do-while Loop Examples

#include <iostream>
using namespace std;

int main() {
    // Basic do-while
    int i = 1;
    do {
        cout << i << " ";
        i++;
    } while (i <= 5);
    cout << endl;  // 1 2 3 4 5

    // Runs at least once even when condition is false
    int x = 10;
    do {
        cout << "Runs once: x = " << x << endl;
        x++;
    } while (x < 5);  // false immediately, but body already ran

    // Menu - always show at least once
    int choice;
    do {
        cout << "\n--- Menu ---" << endl;
        cout << "1. Start" << endl;
        cout << "2. Settings" << endl;
        cout << "3. Exit" << endl;
        cout << "Enter choice (1-3): ";
        cin >> choice;
    } while (choice < 1 || choice > 3);  // repeat until valid input

    cout << "You chose: " << choice << endl;

    // Input validation - keep asking until positive number
    int num;
    do {
        cout << "Enter a positive number: ";
        cin >> num;
    } while (num <= 0);
    cout << "You entered: " << num << endl;

    return 0;
}

4. Range-based for Loop (C++11)

The range-based for loop iterates over every element in a collection - arrays, vectors, strings, maps, and any container with begin()/end(). It's cleaner and less error-prone than index-based loops.

Syntax: for (type element : collection) { body }

Use auto to let the compiler deduce the element type.
Use const auto& to avoid copying (read-only access).
Use auto& to modify elements in place.

Range-based for Loop Examples

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

int main() {
    // Over a C-style array
    int nums[] = {10, 20, 30, 40, 50};
    for (int n : nums) {
        cout << n << " ";
    }
    cout << endl;  // 10 20 30 40 50

    // Over a vector with auto
    vector<string> fruits = {"Apple", "Banana", "Cherry"};
    for (const auto& fruit : fruits) {
        cout << fruit << endl;
    }

    // Modify elements with auto&
    vector<int> scores = {85, 92, 78};
    for (auto& s : scores) s += 5;  // add 5 to each score
    for (auto s : scores) cout << s << " ";  // 90 97 83
    cout << endl;

    // Over a string - character by character
    string word = "Hello";
    for (char c : word) {
        cout << c << "-";
    }
    cout << endl;  // H-e-l-l-o-

    // Over a map - structured binding (C++17)
    map<string, int> ages = {{"Alice", 25}, {"Bob", 30}, {"Carol", 28}};
    for (const auto& [name, age] : ages) {
        cout << name << " is " << age << " years old" << endl;
    }

    return 0;
}

for vs while vs do-while - Quick Comparison

Feature	for	while	do-while
Condition check	Before iteration	Before iteration	After iteration
Minimum executions	0	0	1
Best for	Known count	Unknown count	Must run once
Init + update	In loop header	Manual	Manual
Typical use	Arrays, counting	File reading, events	Menus, validation