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Loops

Loops are basic programming constructs that let you handle tasks by executing specific code over and over. For example, you might want to handle the items in a set of data by working with each item in succession, or keep performing a task until a particular condition becomes true.

There are a number of different loop statements in Java, and we'll take a look at them all here, starting with the for loop.

The for Loop

In Java, the basic loop statement is the for statement, which is a very general loop statement. It's usually used to let you execute code using a loop index. Each time through the loop, the loop index will have a different value, and you can use the loop index to access a different data item in your data set; such as when you use the loop index as an index into an array.

Here's what the for loop looks like in general:

for (initialization_expression; end_condition; iteration_expression) {
  statement;
}

The initialization_expression is executed before the loop is executed the first time; end_condition is checked every time through the loop, and when it becomes false, the loop ends; and iteration_expression is executed after each time through the loop. statement makes up the body of the loop, holding the code you want executed each time through the loop (note that statement can be a compound statement, including several single statements inside curly braces).

For example, you can initialize a loop index in the initialization_expression, provide a test for ending the loop when that test becomes false in the end_condition, and provide some way of changing—usually incrementing—the loop index in the iteration_expression.

Let's make this concrete with an example. In this case, the code will execute a for loop 10 times. Here, the code assigns a loop index variable named loopIndex the value 1 to start (as you can see in the initialization expression loopIndex = 1) and end the loop when that loop index exceeds 10 (as you can see in the end condition loopIndex <= 10, which becomes false when loopIndex is greater than 10). This means that the loop body will execute exactly 10 times. Each time after the code in the body of the loop executes, the code increments loopIndex by one (in the iteration expression, which is loopIndex++ here), as you see in Listing 3.8.

Listing 3.8 Using the for Statement (ch03_08.jsp)

<HTML>
 <HEAD>
  <TITLE>Using the for Statement</TITLE>
 </HEAD>

 <BODY>
  <H1>Using the for Statement</H1>
  <%
    int loopIndex;

    for (loopIndex = 1; loopIndex <= 10; loopIndex++) {
      out.println("This is iteration number "
        + loopIndex + "<BR>");
    }
  %>
 </BODY>
</HTML>

You can see the results of this code in Figure 3.7.

Figure 3.7 Using a for loop.

Here's another example, showing how for loops and arrays naturally go together. In this case, the code will use a for loop to sum up the grades of a number of students in an array and compute the students' average grade. In this case, the code starts loopIndex at 0 and increments it each time through the loop, so you can access a new student's grade each time through the loop. Note that in addition to initializing the loop index to 0 in the initialization expression of the for loop, the code also declares the loop index at the same time via int loopIndex = 0, which is a handy shortcut that Java allows (otherwise, you'd have to declare loopIndex before using it in the loop), as you see in Listing 3.9.

Listing 3.9 Finding Average Grades (ch03_09.jsp)

<HTML>
 <HEAD>
  <TITLE>Finding Average Grades</TITLE>
 </HEAD>

 <BODY>
  <H1>Finding Average Grades</H1>
  <%
    double grades[] = {88, 98, 73, 56, 86, 64};
    double sum, average;

    sum = 0;

    for (int loopIndex = 0; loopIndex < grades.length; loopIndex++) {
      sum += grades[loopIndex];
    }

    average = sum / grades.length;

    out.println("Average grade = " + average);
  %>
 </BODY>
</HTML>

Take a look at the for statement in this example:

for (int loopIndex = 0; loopIndex < grades.length; loopIndex++) {
  sum += grades[loopIndex];
}

This code loops over all items in the grades array and adds them, leaving the result in the variable named sum. The loop index starts at 0 (because of the initialization expression int loopIndex = 0) and is steadily incremented each time through the loop, ending after the last item in the array is processed (because of the end condition loopIndex < grades.length—as we saw in Day 2, you can use the length data member to find the length of an array). You can see the results of this code in Figure 3.8.

Figure 3.8 Using a for loop on an array.

You can use very general expressions in the expressions in a for loop. For example, Java lets you separate expressions in the parentheses in a for loop with commas, as you see in Listing 3.10, where the code supports two loop indexes.

Listing 3.10 Using Two Loop Indexes (ch03_10.jsp)

<HTML>
 <HEAD>
  <TITLE>Using Two Loop Indexes</TITLE>
 </HEAD>

 <BODY>
  <H1>Using Two Loop Indexes</H1>
  <%
    for (int loopIndex = 0, doubleIndex = 0; loopIndex <= 10;
      loopIndex++, doubleIndex = 2 * loopIndex) {
      out.println("loopIndex: " + loopIndex +
        " doubleIndex: " + doubleIndex + "<BR>");
    }
  %>
 </BODY>
</HTML>

You can see the results of this code in Figure 3.9.

Figure 3.9 Using two loop indexes.

You can be even trickier if you want—for instance, you don't have to give a for loop any body at all. You can see an example in which the code averages all the elements of an array in a for loop without any code in its body in Listing 3.11.

Listing 3.11 Using the for Loop (ch03_11.jsp)

<HTML>
 <HEAD>
  <TITLE>Using the for Statement</TITLE>
 </HEAD>

 <BODY>
  <H1>Using the for Statement</H1>
  <%
    int array[] = {1, 2, 3, 4, 5}, sum = 0;

    for (int loopIndex = 0;
      loopIndex < array.length;
      sum += array[loopIndex++]);

    out.println("The average = " + sum / array.length);
  %>
 </BODY>
</HTML>

Expertise with this loop comes in time; for now, we have what we need to know. That takes care of the for loop—next up is the while loop.

The while Loop

A while loop keeps executing the statement in its body (which may be a compound statement, with a number of single statements inside curly braces) while a particular condition evaluates to true. Here's what a while loop looks like in general:

while(condition)
  statement

Note that if the condition is not true, the body of the loop is not even executed once.

Here's an example putting the while loop to work; in this case, the code will display a value, then subtract one from it the next time through the loop, as long as that value stays positive. When the value becomes 0, the while loop stops, because the condition used (value < 0) has become false, as you see in Listing 3.12.

Listing 3.12 Using a while Loop (ch03_12.jsp)

<HTML>
 <HEAD>
  <TITLE>Using the while Loop</TITLE>
 </HEAD>

 <BODY>
  <H1>Using the while Loop</H1>
  <%
    int value = 5;

    while (value > 0) {
      out.println("The value is now " + value-- + ".<BR>");
    }
  %>
 </BODY>
</HTML>

You can see the countdown in Figure 3.10.

Figure 3.10 Using the while loop.

Here's another while loop example, this time calculating factorials. The factorial of a positive integer is the product of that integer with all the positive integers less than it—for example, the factorial of 6 is 6 x 5 x 4 x 3 x 2 x 1 = 720.

You can see how to find the factorial of 6 with a while loop—each time through the loop's body, the code finds the current product and moves on to the next lower integer until it has found the whole factorial, as you see in Listing 3.13.

Listing 3.13 Finding a Factorial (ch03_13.jsp)

<HTML>
 <HEAD>
  <TITLE>Finding a Factorial</TITLE>
 </HEAD>

 <BODY>
  <H1>Finding a Factorial</H1>
  <%
    int value = 6, factorial = 1, temporaryValue = value;

    while (temporaryValue > 0) {
      factorial *= temporaryValue;
      temporaryValue--;
    }

    out.println("The factorial of " + value + " is " + factorial + ".");
  %>
 </BODY>
</HTML>

You can see the results of this code in Figure 3.11.

Figure 3.11 Finding a factorial.

There's another type of while loop in Java—the do-while loop, and that's next.

The do-while Loop

The do-while loop is just like a while loop, except that the test condition is evaluated at the end of the loop, not at the beginning. Here's what the do-while loop looks like (as before, statement can be a compound statement with a number of single statements inside curly braces):

do
  statement
while(condition);

One situation in which you would use a do-while loop instead of a while loop is when you need the body of the loop to be run at least once. For example, the following is a case in which the value the code is testing is not even available for testing until the end of the loop, which means a while loop would not be appropriate:

int values[] = {1, 2, 3, 0, 5}, test, index = 0;

do {
  test = 5 * values[index++];
} while (test < 15);

On the other hand, there are times when you should use a while loop instead of a do-while loop, as in cases in which the body of the loop shouldn't even run once if the test condition is not true. Take a look at this case, where a do-while loop evaluates the reciprocal of a value, but can only test if the value at the end of the loop is nonzero:

<HTML>
 <HEAD>
  <TITLE>Finding Reciprocals</TITLE>
 </HEAD>

 <BODY>
  <H1>Finding Reciprocals</H1>
  <%
    double values[] = {4, 3, 2, 1, 0, 1, 2, 3, 4};
    int loopIndex = 0;

    do {
      out.println("The reciprocal = " + 1 / values[loopIndex] + ".<BR>");
    } while (values[loopIndex++] != 0);
  %>
 </BODY>
</HTML>

This way we won't avoid attempting to find the reciprocal of 0, which will cause mathematical problems. It's better here to use a while loop to test for 0 before the body of the loop is executed, as you see in Listing 3.14.

Listing 3.14 Finding Reciprocals (ch03_14.jsp)

<HTML>
 <HEAD>
  <TITLE>Finding Reciprocals</TITLE>
 </HEAD>

 <BODY>
  <H1>Finding Reciprocals</H1>
  <%
    double values[] = {4, 3, 2, 1, 0, 1, 2, 3, 4};
    int loopIndex = 0;

    while (values[loopIndex] != 0){
     out.println("The reciprocal = " + 1 / values[loopIndex++] + ".<BR>");
    }
  %>
 </BODY>
</HTML>

Using the break Statement to End a Loop

You can also use the break statement to end a loop, as in Listing 3.15, where the code ends a loop if a sum becomes greater than 12.

Listing 3.15 Using the break Statement (ch03_15.jsp)

<HTML>
 <HEAD>
  <TITLE>Using the break Statement</TITLE>
 </HEAD>

 <BODY>
  <H1>Using the break Statement</H1>
  <%
    double array[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    int sum = 0;

    for(int loopIndex = 0; loopIndex <
      array.length; loopIndex++) {

      sum += array[loopIndex];
      if (sum > 12) break;
      out.println("Looping...<BR>");
    }
    out.println("The sum exceeded the maximum allowed value.");
  %>
 </BODY>
</HTML>

You can see the results of this code in Figure 3.12, where the loop keeps looping until the value in sum exceeds 12, when we use the break statement to end the loop.

Figure 3.12 Using the break statement.

As you can see, the break statement gives you control over the execution of a loop. In fact, Java also has another statement that gives you similar control—the continue statement.

Using the continue Statement to Skip Iterations

You can use the continue statement to skip to the next iteration of a loop. Why would you want to do that? You might encounter some data in the data set you're working on that you don't want to process, for example, so you can use the continue statement to move on to the next loop iteration.

Here's an example to show how that works—here, the code is printing out reciprocals, and it should avoid trying to print out the reciprocal of 0. If the current loop index equals 0, you can just skip the current iteration and move on to the next one, as you see in Listing 3.16.

Listing 3.16 Skipping an Iteration (ch03_16.jsp)

<HTML>
 <HEAD>
  <TITLE>Using the continue Statement</TITLE>
 </HEAD>

 <BODY>
  <H1>Using the continue Statement</H1>
  <%
    for(double loopIndex = 6; loopIndex > -6; loopIndex--) {
      if (loopIndex == 0) continue;
      out.println("The reciprocal of " + loopIndex +
        " is " + (1 / loopIndex) + ".<BR>");
    }
  %>
 </BODY>
</HTML>

You can see the results of this code in Figure 3.13—note that the display skips over the line where it would try to calculate the reciprocal of 0.

Figure 3.13 Using the continue statement.

And that completes our look at loops in Java. We've taken a look at for loops, while loops, and do-while loops, and we've seen the break and continue statements to break out of loops and continue on to the next iteration for extra power. Now let's turn to the last topic for today—creating methods.

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