Program Logic and Indefinite Loops - Building Java Programs: A Back to Basics Approach - page 304

Table 5.1 Examples of Factors

Number Factors Smallest divisor

10 2 * 5 2

15 3 * 5 3

25 5 * 5 5

31 31 31

77 7 * 11 7

You don't start divisor at 1 because you are looking for the first divisor greater
than 1 . To refine this pseudocode, you must be more explicit about what makes a
divisor work. A divisor of a number has no remainder when the number is divided by
that divisor. You can rewrite this rule as the following pseudocode:

start divisor at 2.
while (the remainder of number/divisor is not 0) {
increase divisor.

}

This exercise is a use for the mod operator, which gives the remainder for integer
division. The following while loop performs the task:

int divisor = 2;
while (number % divisor != 0) {
divisor++;

}

One problem you will undoubtedly encounter when you write while loops is the
infamous infinite loop. Consider the following code:

int number = 1;
while (number > 0) {
number++;

}

Because number begins as a positive value and the loop makes it larger, this loop
will continue indefinitely. You must be careful when you formulate your while
loops to avoid situations in which a piece of code will never finish executing. Every
time you write a while loop, you should consider when and how it will finish
executing.