C++ While Loop

The while loop

The while loop checks a condition before every pass through the loop body. As long as that condition evaluates to true, the code inside the braces runs. The moment the condition becomes false, the loop stops and your program continues with whatever comes after it.

#include <iostream>
using namespace std;

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

Notice the three moving parts: we start with i = 0, we test i < 5 before each pass, and we increase i with i++ inside the loop. If you forget to change i, the condition would stay true forever.

How the condition is checked

• The condition is tested first, before the body ever runs.
• If it is false right away, the loop body runs zero times.
• After each pass, control jumps back to the top and the condition is tested again.
• The loop ends only when the condition becomes false.

#include <iostream>
using namespace std;

int main() {
  int count = 10;
  while (count < 5) {
    cout << "This never prints\n";
    count++;
  }
  cout << "Done\n";
  return 0;
}

Use a while loop when you do not know in advance exactly how many times you need to repeat, but you do know the condition that should keep it going, such as reading numbers until the user types zero.