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Introducing the While Statement

* Note: a while loop is just like an if statement
  with no "else" branches, with one new feature: repetition.

 * Example 1:

          int i;

     1    i = 1;
     2    while (i <= 5) {
     3        printf("This is line %d\n", i);
     4        i = i + 1;
     5    }
     6    i = 100;

 * Here is the output:

     This is line 1
     This is line 2
     This is line 3
     This is line 4
     This is line 5

* How it works:
   * CPU reaches line 2
   * the Control-Expression is true, so CPU executes 
     the compound statement (lines 3 and 4)
   * then  the CPU jumps back to line 2, and checks the 
     Control-Expression again
   * still true, so the CPU executes lines 3 and 4 again.
   * .... and so on ... until
   * Finally, i is changed to 6 in line 4, so when 
     the processor reaches line 2 again, the
     Control-Expression is finally false. 
   * At that point, the while loop is done, and the
     processor jumps to the next statement 
     past the while (line 6).
* Note: if we accidentally leave out line 4, then i
     always has the value of 1 when the Control-Expression in
     line 2 is evaluated. So the Control-Expression always
     evaluates to 1 (true), and we keep doing line 3
     over and over and over. This is an example of an
     "Infinite Loop."

     If your program gets stuck in an infinite loop,
     type Ctrl-c to stop it. (Hold down the control key,
     and type c .)

 * General Form of the While Loop:

     while ( Control-Expression )
         statement;

   This is how it works. The processor first
   tests the Control-Expression. If it is non-zero, the
   statement is executed, and then the expression
   is evaluated again. And so on.
   The statement can be a compound
   statement (it usually is for while loops); if it is,
   then braces are needed.
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While Loops, Example #2: 
A loop for reading and testing input from the user.

    int input;

    printf("enter an integer from 0 to 10:");
    scanf("%d", &input);
    while (input < 0 || input > 10) { /* while input is bad ... */
        printf("please try again\n");
        printf("enter an integer from 0 to 10:");
        scanf("%d", &input);
    }
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A Variation on Example 2:

    More compact:
 
    1 input = -1;
    2 while ( input < 0 || input > 10 ) { /* while input is bad...*/
    3     printf("enter an integer from 0 to 10:");
    4     scanf("%d", &input);
    5 }
 
    Here, we "artificially" set the input value to -1 initially,
    because we want the program to execute lines 3 and 4 at
    least once. (We don't want the Control-Expression to be 
    false when we first come to the while loop.)                     

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While Loop, Example 3: A loop for your whole program:
 
    #include <stdio.h>
 
    int main(void)
    {
        int proceed;
        char reply;
 
        proceed = 1;
        while (proceed == 1) {
 
            /* here is code to do one iteration of your program */

                    write a prompt to the screen
                    get input
                    perform a calculation
                    write the result to the screen

            /*************************************************/
 
            printf("continue? (y/n)-->");
            scanf (" %c", &reply);
            if (reply == 'n')
                proceed = 0;
        }
    }
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