Java

TOPIC 19 – WHILE LOOPS

LESSON NOTE

LOOP

In programming, a loop is a structure that permits the repetition of statements.

PSEUDOCODE EXAMPLE

Here is a pseudocode example of a loop that would display “Hello” one hundred times.

Loop 100 times

Display “Hello”

End Loop

THREE TYPES OF LOOPS IN JAVA

There are three types of loops in Java:

• while loop
• do-while loop
• for loop (next topic)

We will focus on the while loop in this lesson. We will also briefly visit the do-while loop.

ITERATION

Loops allow us to repeat a section a code multiple times. Each repetition is referred to as an iteration (or simply as a “pass”).

WHILE LOOP TEMPLATE

Here is the template for the while loop:

while (condition)

{

//statement block

}

The statement block continues to be executed as long as the condition is true.

If the condition is false to start, the statement block is not executed at all.

INFINITE LOOP

If the condition is such that it always remains true, the program will loop forever. This is called an infinite loop.

Example 1 – An infinite loop

while (true)

{

System.out.println("Forever");

}

Explanation

In the above code, the condition inside the while loop is always true and never changes. So the statement block gets executed over and over forever (or until you stop it from running).

Example 2 – Another infinite loop

int x = 10;
while (x == 10)

{

System.out.println("Still forever");

}

Explanation

In the above code, the condition x == 10 is always true since x never changes. So we have an infinite loop. It is likely that something like this comes up as an error. For example, if we changed the value of x inside the statement block, we would avoid an infinite loop.

EXAMPLES OF PROGRAMS WITH LOOPS

Example 1 – Write a program that will display “Mr. Campeau rocks” ten times on screen. (You can make it display 100 times if you really want to.) Use a loop.

int x = 1;
while (x <= 10)

{

System.out.println("Mr. Campeau rocks");
x++;

}

Example 2 – Write a program that will output numbers 1 to 100 to screen.

int x = 1;

while (x <= 100)

{

System.out.println(x);

x++;

}

Example 2B – Write a program that will output numbers 1 to n to screen. The value of n is to be provided for the user.

Scanner scr = new Scanner(System.in);
System.out.println("How many numbers do you want to see?");

int max = scr.nextInt();

int x = 1;

while (x <= max)

{

System.out.println(x);

x++;

}

Example 3 – Write a program that will output all even numbers starting at 20 going down to 0.

Solution 1

int y = 20;

while (y >= 0)

{

System.out.println(y);

y = y – 2;

}

Solution 2

int y = 20;

while (y >= 0)

{

if (y % 2 == 0)

{

System.out.println(y);

}

y = y – 1;

}

Example 4 – Write a program that outputs the sequence tn = 3n + 1 for n ranging from 0 to 10. So, the program should output 1, 4, 7, 10, …, 31

int n = 0;

while (n <= 10)

{

tn = 3 * n + 1;

System.out.println(tn);

n++;

}

Example 5 – Write a program that outputs all numbers between 0 and 1000 that are divisible by 17 or by 29.

int x = 0;

while (x <= 1000)

{

if (x % 17 == 0 || x % 29 == 0)

{

System.out.println(x);

}

x++; //increment by 1

}