While Loops

• Syntax

     while (condition) {
        statement A
     }
     statement B
     

•    While the condition is trun
        execute statement A
     

• Things we do in a loop
• A loop control variable is the variable that we test in the condition.
  • Initialize the loop control variable.
  • Have a condition that will eventually become false.
  • Inside the loop, change the loop control variable so that it gets closer to the exit condition.
• Connter Controlled loops
  • A counter gets closer to the exit condition

  •       int i=4;
          while (i > 0) {
             cout  << "I is " << i << endl;
             i = i - 1; // this could be i--; too
          }
          

• A event controlled loop will execute an event occurs, or does not occur
  • Sentinel controlled loops
    • Make a decision based on the input
    • Good input from a user in a menu

    • char inval;
      
      inval = ' ';
      while ((inval <  'a') || (inval > 'd') ) {
          print_choices();
          cin >> inval;
      }
            

  • Flag controlled loops
    • Like sentinel loops, but use a flag

    • bool ok_input = false;
      char inchar;
      
      while (!ok_input) {
          print_choices() 
          cin >> inval;
          if ((inval == 'a') || (inval == 'A')) {
              cout << "It was an A" << endl;
      	ok_input = true;
          } else if ( condition for b ) {
              cout << "It was an B" << endl;
      	ok_input = true;
          } else if (condition for c ) {
              cout << "It was an C" << endl;
      	ok_input = true;
          } else {
              cout << "Bad Input " << endl;
          }
      }
      
      	 

• Files
  • The file stream returns true when it is not at the end of file marker.
  • here is some code to illustrate this point.
• Some common tasks
  • Count the number of items
  • Sun a list of items