While and do-while loops.md

While

block of code and condition (a Boolean expresion)

if the condition is true, the code within the block is executed. This code repeats until the condition becomes false. the condition is check before the block is executed, is also known as a pre-test loop

Syntax

while (condition) { // body: do something repetitive 
}

infinite loop if the condition is invariably true:

while (true) {
	// body: do something indefinitely
}

Example letters in English

public class WhileDemo {

	public static void main(String[] args){
		char letter = 'A';
		while (letter <= 'Z'){
			System.out.printnl(letter);
			letter++;
		}

	}
}

do-while loop

in this loop the body is executed first, while the condition is tested afterwards. if the condition is true, statements within the block are executed again. This repeats until the condition becomes false. because do-while check the condition after is executed, the control structure is often also known as a post-test loop Syntax

do { // body: do something 
} while (condition);

public class DoWhileDemo {
	public static void main(String[] args) {
		 Scanner scanner = new Scanner(System.in);
		 int value; do { value = scanner.nextInt(); 
		 System.out.println(value); } while (value != 0); 
	} 
}

In practice, the do-while loop is used less than the while loop. It is used when code inside the loop must be executed at least once.

Reading a sequence with an unknown length

The while loop can also be used to read a sequence of characters of an arbitrary length. For that, we can invoke the hasNextInt() method of Scanner inside the condition. The method returns true if the next element is an integer number and false otherwise.

Here is a code that calculates the sum of all elements from the provided sequence:

Scanner scanner = new Scanner(System.in);
int sum = 0;
while (scanner.hasNextInt()) {
	int elem = scanner.nextInt();
	sum += elem;
} 
System.out.println(sum);