Lesson 58 min read

Loops

The old classics meet modern convenience — range-based for is the star of the show

The Loop Family

Loops are the workhorses of programming — they repeat a block of code until a condition is met. C++ inherits all three classic loop forms from C (for, while, do-while) and adds a modern superstar: the range-based for loop.

If the classic for loop is a manual transmission — powerful but requires you to manage every gear shift — the range-based for is automatic transmission. Same destination, less fiddling.

The Classic for Loop

#include <iostream>
using namespace std;

int main() {
    // Classic for — you control init, condition, and increment
    cout << "Counting up: ";
    for (int i = 1; i <= 5; i++) {
        cout << i << " ";
    }
    cout << endl;

    // Counting down
    cout << "Countdown: ";
    for (int i = 5; i > 0; i--) {
        cout << i << " ";
    }
    cout << "Liftoff!" << endl;

    // Stepping by 2
    cout << "Evens: ";
    for (int i = 0; i <= 10; i += 2) {
        cout << i << " ";
    }
    cout << endl;

    return 0;
}

Output

Counting up: 1 2 3 4 5
Countdown: 5 4 3 2 1 Liftoff!
Evens: 0 2 4 6 8 10

while and do-while

while checks the condition before each iteration — if the condition is false from the start, the body never runs. do-while checks after — the body always runs at least once.

do-while is perfect for menus: you always want to show the menu at least once before asking if the user wants to continue.

while and do-while

#include <iostream>
using namespace std;

int main() {
    // while — might not execute at all
    int fuel = 3;
    cout << "Driving..." << endl;
    while (fuel > 0) {
        cout << "  Fuel left: " << fuel << endl;
        fuel--;
    }
    cout << "Out of gas!" << endl;

    cout << endl;

    // do-while — always executes at least once (great for menus)
    int choice;
    do {
        cout << "--- Menu ---" << endl;
        cout << "1. Play" << endl;
        cout << "2. Settings" << endl;
        cout << "3. Quit" << endl;
        choice = 3;  // simulate user picking Quit
        cout << "You chose: " << choice << endl;
    } while (choice != 3);

    cout << "Goodbye!" << endl;

    return 0;
}

Output

Driving...
  Fuel left: 3
  Fuel left: 2
  Fuel left: 1
Out of gas!

--- Menu ---
1. Play
2. Settings
3. Quit
You chose: 3
Goodbye!

Range-Based for — The Star of the Show

Introduced in C++11, the range-based for loop is the modern way to iterate over collections. No index management, no off-by-one errors, no iterator boilerplate. Just tell C++ what to loop over, and it handles the rest.

The syntax is: for (auto& element : collection)

Think of it as saying: "for each element in this collection, do something."

Range-Based for with Vectors

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

int main() {
    vector<int> scores = {95, 87, 92, 78, 100};

    // Range-based for — clean and readable
    cout << "Scores: ";
    for (const auto& score : scores) {
        cout << score << " ";
    }
    cout << endl;

    // Modifying elements — use auto& (without const)
    for (auto& score : scores) {
        score += 5;  // curve everyone up by 5
    }

    cout << "Curved: ";
    for (const auto& score : scores) {
        cout << score << " ";
    }
    cout << endl;

    // Works with strings too
    vector<string> names = {"Alice", "Bob", "Carol"};
    for (const auto& name : names) {
        cout << "Hello, " << name << "!" << endl;
    }

    return 0;
}

Output

Scores: 95 87 92 78 100
Curved: 100 92 97 83 105
Hello, Alice!
Hello, Bob!
Hello, Carol!

Note: Always use & in range-based for: for (auto& x : vec). Without &, you get a copy of each element — modifications won't affect the original, and you pay a performance cost copying every element. Use const auto& when you just need to read, and auto& when you need to modify.

C++17: Structured Bindings in Loops

When iterating over a map, each element is a pair with .first (key) and .second (value). Before C++17, this was clunky. With structured bindings, you can unpack the pair right in the loop header — naming the key and value whatever you want.

Structured Bindings with Maps (C++17)

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

int main() {
    map<string, int> ages = {
        {"Alice", 30},
        {"Bob", 25},
        {"Carol", 28}
    };

    // Before C++17 — clunky .first and .second
    cout << "Old style:" << endl;
    for (const auto& pair : ages) {
        cout << "  " << pair.first << " is " << pair.second << endl;
    }

    // C++17 structured bindings — clean and readable!
    cout << "\nC++17 style:" << endl;
    for (const auto& [name, age] : ages) {
        cout << "  " << name << " is " << age << endl;
    }

    // Modifying values with structured bindings
    for (auto& [name, age] : ages) {
        age++;  // happy birthday everyone!
    }

    cout << "\nAfter birthdays:" << endl;
    for (const auto& [name, age] : ages) {
        cout << "  " << name << " is now " << age << endl;
    }

    return 0;
}

Output

Old style:
  Alice is 30
  Bob is 25
  Carol is 28

C++17 style:
  Alice is 30
  Bob is 25
  Carol is 28

After birthdays:
  Alice is now 31
  Bob is now 26
  Carol is now 29

break and continue — Loop Control

break exits the loop entirely — like pulling the emergency stop on a conveyor belt. continue skips the rest of the current iteration and jumps to the next one — like tossing a defective item off the belt and keeping the line moving.

break and continue

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

int main() {
    vector<int> numbers = {1, 2, -1, 4, 5, -1, 7, 8};

    // continue — skip negative numbers
    cout << "Positives only: ";
    for (const auto& n : numbers) {
        if (n < 0) continue;  // skip this iteration
        cout << n << " ";
    }
    cout << endl;

    // break — stop at first negative
    cout << "Until negative: ";
    for (const auto& n : numbers) {
        if (n < 0) break;  // exit the entire loop
        cout << n << " ";
    }
    cout << endl;

    // Searching with break
    vector<string> names = {"Alice", "Bob", "Carol", "Dave"};
    string target = "Carol";
    for (const auto& name : names) {
        if (name == target) {
            cout << "Found " << target << "!" << endl;
            break;
        }
    }

    return 0;
}

Output

Positives only: 1 2 4 5 7 8
Until negative: 1 2
Found Carol!

Challenge

Quick check

What is the difference between for (auto x : vec) and for (auto& x : vec)?

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