While Loops, More Details

Objectives

1. Better understand the while loop, and loops in general.

Notes

• Infinite loops

  • while (true) {
        System.out.println("In the loop");
    } 

  • You will write these.
  • You need to know how to stop them.
  • In jGRASP
    • Click on the compile button
    • OR right click in the Run I/O area and select End Process or <ctrl>-e
• Let's take a look at our first loop.

  • i = 0;
    while (i < 4) {
        System.out.printf("i = %d\n",i);
        i = i + 1;
    } 

  • the variable i is the Loop Control Variable (LCV)
    • Because it controls the loop.
  • To have a successful loop we need three things.
    1. Before the loop, initialize the LCV
       • This is done in line 1.
    2. Have an exit condition or test on the LCV that can become false
       • This is done in line 2.
       • Any time i is 4 or more the loop will exit.
    3. Change the LCV to get closer to the exit condition.
       • This is done in line 5.
       • Don't say increment the LCV
• By the way {} are called blocks or conde blocks.
  • They turn multiple lines of code into a single line.
  • Well, not completely true, but to the compiler they do.
  • Remember the syntax for a while loop is

    while (boolean-expression)
       statement; 

  • This will work, but will probably cause problems.
  • So we change the style to be

    while (boolean-expression) {
       statement; 
    } 

• Consider this loop

  • double x = 900;
    while (x != .01) 
       x = x /10; 

  • As we saw before, this is an infinite loop.
  • To debug it we might throw in a print statement.
  • Case I

  • double x = 900;
    while (x != .01) 
       System.out.printf("x = %f\n",x);
       x = x /10; 

  • Case II

  • double x = 900;
    while (x != .01) 
       x = x /10;
       System.out.printf("x = %f\n",x);

  • In either case, we don't get what we want.
  • We have to throw in a code block.
  • So just do it by default.

  • double x = 900;
    while (x != .01) {
       x = x /10;
       System.out.printf("x = %f\n",x);
    } 

• A note on code blocks.
  • There are many acceptable indentation/code block styles.
  • You might look at the related Wikipedia article
  • I mostly use K&R style.
  • But you can use anything as long as
    1. You are consistent.
    2. You indent after a starting {
    3. You stop indenting after the ending }