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Programming in Java Netbeans - A Step by Step Tutorial for Beginners: Lesson 10

Danson Wachira is a certified Trainer in Computer Science, Information Technology and related studies.

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Lesson 10: Control Structures in Java – The FOR Loop

programming-in-java-a-step-by-step-tutorial-for-beginners-lesson-10

When we started with control structures in Lesson 7, I started by saying that control structures help us to write our code in non-linear approach, that is, we can control the flow of our program.

Another way of controlling the flow of our program is by forcing it to repeat execution of statements a number of times.

This in programming is referred to as looping. Java has several looping control structures. In this lesson, we’ll learn about another important control structure in Java that is used to force the program to repeat execution of statements a number of times.

The FOR loop is one of the most common looping controls used in Java. The FOR loop has the following structure:

for(initial value; condition; increment/decrement){ // Type as one line

//Statements to be repeated if condition is TRUE

}

From the above structure, you can see FOR loop has three parts. After the keyword "for" (in lowercase) there are round brackets. Inside the round brackets you include the three parts of the FOR loop: the initial value part, condition part, and increment/decrement part. Notice that we separate these parts by a semi colon (;).

Let us understand these three parts

Initial value – This part sets at what number should the repeat process start, usually it starts at 1 but it can be any other starting number.

Condition – Because the repeat should occur only if a certain condition hold TRUE, this part keep on checking whether the condition hold for every loop.

Increment/decrement – The purpose of this part is to move the counter either forward or backward depending on what type of repeat we want to do.

After the round brackets there is a pair of curly braces. The curly braces are used to hold the code that you want to execute repeatedly.

As usual, let us work with a practical example. Create another class, call it JavaLoops. Suppose we want a program to count from 1 to 10 and output the values. We can use the FOR loop as shown below:

programming-in-java-a-step-by-step-tutorial-for-beginners-lesson-10

Write the program and run it yourself. What we’ve done is to set an int variable called count. We have set the count variable to start at 1, that is, the initial value, count = 1.

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Next we have set the condition of the count to be less of equal to 10, i.e. the count can move from 1, 2, 3 … up to 10 but not beyond. Count <=10

To make the counter move from one step to another, we have count++. The operator (++), two plus signs, is called an increment operator. It moves the count by one step for every single loop until the condition turns out FALSE. The opposites of increment (++) operator is the decrement operator ( --), two minus signs.

As the count move from 1 to 10, the statement: System.out.println("Counting: "+count); output the numbers on the output window.

Caution: Always check your condition in loops to make sure it has an end. A condition line Count >=0 is endless since count which is set to 1 will always be greater than zero. Such endless loops may cause your computer to hang up.

Now let us work with some few examples of FOR loop.

1. A FOR loop program to sum numbers 1 to 10. We just count the number as we did earlier but as we do so, we also sum them. See the code below, notice the statement to output the total sum is outside the FOR loop.

programming-in-java-a-step-by-step-tutorial-for-beginners-lesson-10

1. A program to output all odd numbers between 1 and 20. Odd numbers have a remainder greater than zero when divided by 2. So as we count from 1 to 20, we’ll divide the number by 2 and use IF statement inside the FOR loop to check if remainder is greater than zero and if so, we output the number.

Notice the use of remainder symbol (%). A statement like 5%2 means the remainder of 5 when divided by 2, which is 1. Below is the code, take care of curly braces as you type. Try it yourself.

programming-in-java-a-step-by-step-tutorial-for-beginners-lesson-10

Exercise:

Write a program using FOR loop to output even numbers between 60 and 100.

That is all about the FOR loop for now. In the next lesson, we shall look at WHILE loop.


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Comments

FG on September 13, 2012:

NICE

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