Loops

Entry-controlled vs exit-controlled (deep)

• for and while test **before** entry — zero iterations are possible.
• do-while tests **after** the body — exactly the right model for “act, then decide”.
• You can rewrite do-while as while by duplicating the body once before the loop — that duplication is why do-while exists.
• Infinite loops: while (true) or for (;;) — always plan a break / return exit.

for details and range-for pitfalls

• Any of the three for clauses may be empty: for (;;) is an intentional infinite loop.
• Comma operator packs multiple inits/updates: for (int i = 0, j = n-1; i < j; i++, j--).
• Range-for copies by default: for (auto x : v) copies; use auto& / const auto& when needed.
• Do not add/remove elements of a vector while ranging over it — iterators can invalidate.

Real-life loop models (combined)

• Factory belt: while items remain on the belt, pack them.
• TV remote menu: do-while until user selects Exit.
• School timetable: nested for over days and periods.
• Cinema booking screen: nested for over rows and seats — mark booked seats.
• Playlist: range-based for over every song title.
• OTP retry: do-while until code matches or attempts exhausted (combine with a counter).

for — print 1 to N

#include <iostream>
using namespace std;

int main() {
    int n = 5;
    for (int i = 1; i <= n; i++) cout << i << " ";
    return 0;
}

while — sum of digits

#include <iostream>
using namespace std;

int main() {
    int n = 1234, sum = 0;
    while (n > 0) {
        sum += n % 10;
        n /= 10;
    }
    cout << "Sum = " << sum << "\n";
    return 0;
}

do-while — cafe menu (required pattern)

#include <iostream>
using namespace std;

int main() {
    int choice;
    do {
        cout << "1. Tea  2. Coffee  3. Exit\nChoice: ";
        cin >> choice;
        if (choice == 1) cout << "Tea\n";
        else if (choice == 2) cout << "Coffee\n";
    } while (choice != 3);
    return 0;
}

do-while — validate marks 0–100

#include <iostream>
using namespace std;

int main() {
    int marks;
    do {
        cout << "Marks (0-100): ";
        cin >> marks;
    } while (marks < 0 || marks > 100);
    cout << "OK: " << marks << "\n";
    return 0;
}

while vs do-while — false start condition

#include <iostream>
using namespace std;

int main() {
    int x = 0;
    cout << "while: ";
    while (x > 0) { cout << "W"; x--; }
    cout << "(done)\n";

    x = 0;
    cout << "do-while: ";
    do { cout << "D"; x--; } while (x > 0);
    cout << "(done)\n";
    return 0;
}

Range-based for — cart total

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

int main() {
    vector<int> prices = {50, 120, 30};
    int total = 0;
    for (int p : prices) total += p;
    cout << "Total = " << total << "\n";
    return 0;
}

Nested for — seat grid

#include <iostream>
using namespace std;

int main() {
    for (int row = 1; row <= 3; row++) {
        for (int col = 1; col <= 4; col++)
            cout << "R" << row << "C" << col << " ";
        cout << "\n";
    }
    return 0;
}

Nested for — right triangle stars

#include <iostream>
using namespace std;

int main() {
    int n = 5;
    for (int row = 1; row <= n; row++) {
        for (int s = 1; s <= row; s++) cout << "* ";
        cout << "\n";
    }
    return 0;
}

Nested for — multiplication table

#include <iostream>
using namespace std;

int main() {
    for (int i = 1; i <= 5; i++) {
        for (int j = 1; j <= 10; j++)
            cout << i * j << "\t";
        cout << "\n";
    }
    return 0;
}

Nested for — timetable (days × periods)

#include <iostream>
using namespace std;

int main() {
    for (int day = 1; day <= 3; day++) {
        cout << "Day " << day << ": ";
        for (int period = 1; period <= 4; period++)
            cout << "P" << period << " ";
        cout << "\n";
    }
    return 0;
}