ITSC 1213 - Introduction to CS 2 How to Design Programs with Object Oriented Programming

The simplest loops are counter-controlled loops like below, where the loop control variable is a counter that controls how many times to repeat the loop. There are 3 steps to writing a loop using this loop control variable as seen below in a loop that counts from 1 to 10.

Java doesn’t require your code to be correctly indented (code moved to the right a few spaces) to make it clear what statements are part of the body of the loop, but it is standard practice to do so.

Practice

int count = 0;
/* missing loop header */
{
    System.out.print(count + " ");
    count += 2;
}

A really important skill to develop is the ability to trace the values of variables and how they change during each iteration of a loop.

You can create a tracing table that keeps track of the variable values each time through the loop as shown below. This is very helpful on the exam. Studies have shown that students who create tables like this do much better on code tracing problems on multiple choice exams.

Watch the following video for a tracing demo. When you are tracing through code, pretend to be the computer running the code line by line, repeating the code in the loop, and keeping track of the variable values and output.

Practice

int count = 1;
while (count <= 10)
{
    count *= 2;
}
count = count - 10;

Step through the code above with the visualizer.

int x = -5;
while (x < 0)
{
   x++;
   System.out.print(x + " ");
}

One common error with loops is infinite loops. An infinite loop is one that never stops (the condition is always true).

// an infinite loop
while (true)
{
   System.out.println("This is a loop that never ends");
}

The infinite loop above is pretty obvious. But, most infinite loops are accidental. They usually occur because you forget to change the loop variable in the loop (step 3 of a loop).

Another common error with loops is an off-by-one error where the loop runs one too many or one too few times. This is usually a problem with step 2 the test condition and using the incorrect relational operator < or <=.

You can use a while loop to repeat the body of the loop a certain number of times as shown above. However, a while loop is typically used when you don’t know how many times the loop will execute. It is often used for a input-controlled loop where the user’s input indicates when to stop. For example, in the Magpie chatbot lab on repl.it below, the while loop stops when you type in “Bye”. The stopping value is often called the sentinel value for the loop. Notice that if you type in “Bye” right away, the loop will never run. If the loop condition evaluates to false initially, the loop body is not executed at all. Another way to stop the loop prematurely is to put in a return statement that makes it immediately return from the method.