Lesson 9 Part 2: Loops A loop is a control structure that causes a statement or group of statements to repeat. We will discuss three (possibly four) looping control structures. They differ in how they control the repetition. The while Loop General Form: while (BooleanExpression) Statement or Block First, the BooleanExpression is tested If it is true, the Statement or Block is executed After the Statement or Block is done executing, the BooleanExpression is tested again If it is still true, the Statement or Block is executed again o This continues until the test of the BooleanExpression results in false. Note: the programming style rules that apply to decision statements also apply. while Loop Example public class WhileLoop { public static void main(String[] args) { int number = 1; //Must have a starting value for While Loop while (number <= 5) { System.out.println("Hello!"); number++; } } } Flowchart for the above loop Here, number is called a loop control variable. A loop control variable determines how many times a loop repeats. Each repetition of a loop is called an iteration. The while loop is known as a pretest loop, because it tests the boolean expression before it executes the statements in its body. Note: This implies that if the boolean expression is not initially true, the body is never executed. Infinite Loops In all but rare cases, loops must contain a way to terminate within themselves. In the previous example number was incremented so that eventually number <= 5 would be false. If a loop does not have a way of terminating it’s iteration, it is said to be an infinite loop, because it will iterate indefinitely. This is a bad logic error! If we removed number++ from the previous example, it would be an infinite loop. Can also be created by putting a semicolon after the loop header or not using brackets properly. Infinite Loop Example: public class WhileLoop { public static void main(String[] args) { int number = 1; while (number <= 5);// semicolon causes infinite loop { System.out.println("Hello!"); number++; } } } Example 2 public class WhileLoop { public static void main(String[] args) { int number = 1; while (number <= 5) { System.out.println("Hello!"); // Number never reaches 5 //causing infinite loop } } } The do-while Loop while loops are considered pretest, but Java also provides a posttest loop called the do-while loop: do Statement or Block while (BooleanExpression); Here, the Statement or Block is executed first Next, the BooleanExpression is tested If true, the Statement or Block is executed Then the BooleanExpression is tested This continues until the BooleanExpression is false. Again, this is a posttest loop, meaning the BooleanExpression is tested at the end. Note that this means the Statement or Block will ALWAYS be executed at least once. Also not the semicolon at the end of the last line. In a Do While Loop, the statements are excuted 1 time before the Boolean expression has been checked. Example Do-While Loop public class DoWhileLoop { public static void main(String[] args) { int number = 1; do { System.out.println("Hello!"); number++; } while(number <= 5); } } /*Always make sure that your Boolean Expression will become False sometime during the execution of your program.*/ .