010605
J. W. Rider

Iteration (Loops)

The Java programming language provides for the repetitious execution of a statement or sequence of statements through three flavors of iteration control structures: the while statement, the do statement, and the for statement. In addition, Java provides for early termination of a single iteration, or of an entire loop, through the continue and break statements.

All loops may be labelled and the target of embedded break and continue statements.

While Statement

  while (test) statement;           // "execute statement while test is true".
      				    // The statement is never executed if test
				    // is false the first time.


Common Programming Error 
  while (test);                     // Putting a semicolon after the test
     statement;                     // usually results in an infinite loop
      				    // situatin is test is true, or
				    // unintended execution of the
				    // loop body statement if test is false.


Do Statement

  do                                // execute a sequence of statements.
  {
    statement1;
    statement2;
    statement3;
  } while (test);                   // continue executing the sequence while
                                    // test is true.
				    // Even if the test is false the first
				    // time, the sequence is always executed at
				    // least once.


In the process of executing while and do loops, something should be happening that will eventually cause test to be false. Otherwise, you get into an "infinite loop" situation, where your program appears to be doing nothing, or writing the same message to the screen, again and again.

For Statement

//for (initpart; testpart; incrpart)
  for (int x=0;  x < 10;     x++     )
      statement;


    When a for loop is entered,
  1. The initpart is evaluated. This entry is the only time the initpart is evaluated.
  2. The testpart is evaluated.
  3. If the testpart is true, the loop body statement is executed.
  4. When the statement finishes, the incrpart is evaluated.
  5. When the incrpart is finished, the testpart is evaluated again.
  6. If the testpart is true, the loop body statement is executed again. And so on.

If the testpart evaluates to false, the loop terminates. The statement is not executed. The incrpart is not evaluated.

while loop: 
   while (test) statement;
equivalent for loop: 
   for ( ;test; ) statement;
do loop: 
   do{ statement; } while (test);
near-equivalent1 for loop: 
   for (boolean dummy=true; dummy || test; dummy=false) statement;
near-equivalent2 for loop: 
   statement;
   for ( ;test; ) statement;

Break Statement

A break statement may appear in the body of switch statements or loops.

In all cases, statements following the break are not executed, and the control structure terminates.

In the case of the for loop, the incrpart is NOT evaluated.

In the case of any loop, the test is NOT evaluated.

If the loop is in a sequence of statements, execution continues with the statement following the loop.

Break statements appear in two forms: 
   break;
 break the innermost switch or loop 
   break label;
 break all switches or loops nested inside of the switch or loop labeled "label"

Break statements rarely appear by themselves (in an absolute context). The statements are usually conditional. 

   if (isInfiniteLoop) break;

Continue Statement

A continue statement may appear in the body of a loop.

Like a break statement, the continue statement causes statements following the break to be ignored, but for the current iteration only. The loop does not necessarily terminate.

In the case of the for loop, the increment part IS evaluated next.

Then the loop test expression is evaluated. If the expression is true, the statements in the loop body are executed from the beginning. Otherwise, the loop terminates normally.

Continue statements appear in two forms: 
    continue;
 only the current iteration of the loop is ended. the loop is not broken. 
    continue label;
 breaks all control structures nested within the loop with the named label. The loop with the named label is continued.
1