Examples of while, for, and do-while loops in c++ for CS 142

loops-example.cpp

#include <iostream>
#include <string>
#include <limits> // For numeric_limits<streamsize>::max()
using namespace std;

int main() {
    int number;
    string name;

    // This first example checks cin.fail() to see if the input was not
    // able to be stored in an integer variable.
    cout << "Enter a number: ";
    cin >> number;
    while (cin.fail()) {
        cout << "Invalid input!" << endl;
        cin.clear();
        // Ignore all charracters until the next line ('\n')
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
        // Ask again!
        cin >> number;
    }

    // This loop could validate ranges in inputs (make sure the number is within a range)
    cin >> number;
    while (number < 0) {
        cout << "Invalid input!" << endl;
        cin >> number;
    }

    // This can check for specific allowed inputs
    char option;
    cin >> option;
    while (option != 'n' && option != 'y') {
        cout << "Invalid input!" << endl;
        cin >> option;
    }

    cout << "Enter your name: ";
    cin >> name;

    // All 4 following loops are equivalent
    int numberOfLoops = 0;
    while (numberOfLoops < number) {
        cout << "Hello " << name << "!" << endl;
        numberOfLoops += 1;
    }

    numberOfLoops = 0;
    do {
        cout << "Hello " << name << "!" << endl;
        numberOfLoops += 1;
    } while (numberOfLoops < number);

    for (int i = 0; i < number; i += 1) {
        cout << "Hello " << name << "!" << endl;
    }

    numberOfLoops = 0;
    for (; numberOfLoops < number; ) {
        cout << "Hello " << name << "!" << endl;
        numberOfLoops += 1;
    }

    // Note that the for loop's counter variable (i) is only available
    // within the loop. Trying to output i outside the loop would fail
    // cout << i;     <- THIS FAILS!
    // Sometimes it is better to keep the variable outside the loop
    // (like in the case of numberOfLoops) but most often it is fine to
    // leave it in the initializer part of the for loop.
    
    // One last example of an equivalent loop
    while (number > 0) {
        cout << "Hello " << name << "!" << endl;
        number--;
    }
    // This example actually modifies "number" which might
    // not be what you expect! If you need that variable later then
    // you no longer have the original value after the loop.
    // In some programs though this is perfectly fine.
    
    return 0;
}