LOOPS

Tutorial

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Introduction: The purpose of a loop is to execute some method repeatedly in Java. The loops are really useful in programming because sometimes some program requires some particular method to be executed repeatedly. There are four types of loops in Java while, for, do-while and for-each loop.

· while loop

· do-while loop

· for loop

· for-each loop

while loop

The while-loop has the following syntax.

while ("expression") <"statement(s)">

while-loop executes the statement(s) continually while some "expression" is true.

<"expression"> →true <"statement">

↓ false

Example:

int x = 0;

while (x < 2){

System.out.println("Hello!");

x++; //increments x, same as x = x +1.

}

Output:

Hello!

Hello!

Solution: First time when it goes through the while loop x = 0 which is true, so it prints “Hello!” to the screen. Second time when it goes to the while loop x = 1, the condition is still true so it prints “Hello!” again to the screen. Third time, when it checks for the condition of while, it return false because x = 2.

do-while loop

The do-while-loop has the following syntax.

do{ <"statement(s)"> }while ("expression")

The do-while loop is similar to the while-loop. The only difference is that the statement(s) is executed before the expression is checked to be true or false. If it is false the next time around the loop exits, and if it returns true again the statement(s) is executed.

int x = 0;

do{

System.out.println("Hello!");

x++; //increments x, same as x = x +1.

}while(x < 2);

Output:

Hello!

Hello!

Solution: The do loop executes the statement the first time around and prints “Hello!” to the screen, and then increments x to x = 1; Then it checks the while condition which is x < 2, it satisfies the condition to be true, so it executes the statement again. And then increments x to x =2; this is where the while condition fails because x == 2 the condition is x < 2. So the loop is terminated at this point.

for loop

“The for statement provides a compact way to iterate over a range of values. Programmers often refer to it as the "for loop" because of the way in which it repeatedly
loops until a particular condition is satisfied. (citation)”

The for-loop has the following syntax.

for ( initialization; termination; incrementation; ) { <"statement(s)"> }

<incrementation>

<statement(s)>

<Initialization>

Example:

for (int x = 0; x < 10; x++){

System.out.println("Hello!");

}

Output: the for statement executes ten times because the initial state of x = 0; each time for statement

checks for the termination value if it is false or true. It return true ten times before the termination

state of x < 10 is met. Meaning that “Hello!” is printed ten times to the screen before the conditions

returns false.
(Alphonce & Decker)

for-each loop

The for-each-loop has the following syntax.

for ( <Object> <task/identifier>: collection) {<"statement(s)">}

For-each loop is useful to iterate through a collection. For instance,

String[] numbers = {"1","2","3","4","5"};

for (String item : numbers) {

System.out.println("String : " + item);

}

Output:

String : 1

String : 2

String : 3

String : 4

String : 5