A while statement is another type of loop that operates on a CONDITION, similar to a for loop. For loops are better for counting, and while loops are better for conditions.

A while loop takes this form:

cout << "Before loop" << endl;

while ( CONDITION )
{
  // Execute this code
  cout << "Repeats!" << endl;
}

cout << "After loop" << endl;

Any code within a while loop gets executed while the CONDITION is TRUE. Once the condition is no longer true, then the while loop ends and the program continues.

Remember that we can build conditional statements with the greater-than, less-than, greater-than-or-equal-to, less-than-or-equal-to, equal-to, and not-equal-to operators.

Warning! Because a while loop will keep going until the condition is false, it is possible to write a program where the condition never becomes false, resulting in an infinite loop!

int age = 0;

while ( age < 18 )
{
  cout << "Can't vote! Try again next year..." << endl;
  age += 1;
}

cout << "You're old enough to vote now!" << endl;

char quitProgram = 'n';

while ( quitProgram == 'n' )
{
  cout << "Do you want to quit? (y/n): " )
  cin >> quitProgram;
}

cout << "Goodbye!" << endl;

The Relational Operators are operators where you compare two values or variables. Their names are:

1. is a equal to b? a == b
2. is a not equal to b? a != b
3. is a less than b? a < b
4. is a less than or equal to b? a <= b
5. is a greater than b? a > b
6. is a greater than or equal to b? a >= b

All of these are basically "yes/no" questions, and can be used to build conditions for if statements and while loops.

int choice = 0;

cout << "1. Save" << endl;
cout << "2. Load" << endl;
cout << "3. Exit" << endl;

while ( choice != 3 )
{
  cout << "Choice: ";
  cin >> choice;
}

cout << "Goodbye" << endl;

An infinite loop error can occur if you're using a while loop and nothing inside the loop makes the CONDITION evaluate to FALSE.

For example, if you run this code, it will keep printing out numbers forever:

#include <iostream>
using namespace std;

int main()
{
  int iterations = 0;
  bool running = true;

  while ( running )
  {
    iterations += 1;
    cout << iterations << endl;
  } // end of while loop


  // never gets to this point
  cout << "THE END" << endl;
  return 0;
}

There needs to be enough logic inside your while loop to effect the CONDITION. In the example above, we have a running variable, but nothing INSIDE the while loop ever sets it to False. Because it's always true, the loop always keeps running.

If we're asking (x > y)?

If it's true, then yes, x > y. If it's false, however, then x <= y is true (NOT x < y!). Make sure to keep these in mind…

1. The OPPOSITE of x > y is x <= y.
2. The OPPOSITE of x < y is x >= y.
3. The OPPOSITE of x == y is x != y.

A do…while loop is just like a while loop, except the condition goes at the end of the code block, and the code within the code block is always executed at least one time.

do
{
  // Do this at least once
} while ( CONDITION );

For example, you might want to always get user input, but if they enter something invalid you'll repeat that step until they enter something valid.

do
{
  cout << "Enter a choice: ";
  cin >> choice;
} while ( choice > 0 );