<?xml version="1.0"?>

<chapter xml:id="loops-and-iteration">

<title>Loops and Iteration</title>

<section xml:id="definite-loop">

<title>Definite Loop</title>

<p>

You have already seen a couple of examples of iteration and looping in Java.

So this section will just serve as a reference for the differences in syntax.

</p>

<p><idx><c>for</c> loop</idx>

A <term>definite loop</term> is a loop that is executed a specific or definite number of times. In Python, the easiest way to write a definite loop is using the <c>for</c> loop in conjunction with the <c>range</c> function.

For example:

</p>

<program interactive="activecode" language="python">

<code>

for i in range(10):

print(i)

</code>

</program>

<p>

In Java, we would write this as:

</p>

<program interactive="activecode" language="java">

<code>

public class DefiniteLoopExample {

public static void main(String[] args) {

for (Integer i = 0; i &lt; 10; i++ ) {

System.out.println(i);

}

}

}

</code>

</program>

<p>

Recall that the <c>range</c> function provides you with a wide variety of options for controlling the value of the loop variable.

</p>

<pre>range(stop)

range(start,stop)

range(start,stop,step)</pre>

<p>

The Java <c>for</c> loop is really analogous to the last option giving you explicit control over the starting, stopping, and stepping in the three clauses inside the parenthesis.

You can think of it this way:

</p>

<pre>for (start clause; stop clause; step clause) {

statement1

statement2

...

}</pre>

<p>

If you want to start at 100, stop at 0 and count backward by 5, the Python loop would be written as:

</p>

<program interactive="activecode" language="python">

<code>

for i in range(100, -1, -5):

print(i)

</code>

</program>

<p>

In Java, we would write this as:

</p>

<program interactive="activecode" language="java">

<code>

public class DefiniteLoopBackward {

public static void main(String[] args) {

for (Integer i = 100; i &gt;= 0; i -= 5) {

System.out.println(i);

}

}

}

</code>

</program>

<p>

In Python, the <c>for</c> loop can also iterate over any sequence such as a list, a string, or a tuple.

Java also provides a variation of its <c>for</c> loop that provides the same functionality in its so-called for each loop.

</p>

<p>

In Python, we can iterate over a list as follows:

</p>

<program interactive="activecode" language="python">

<code>

l = [1, 1, 2, 3, 5, 8, 13, 21]

for fib in l:

print(fib)

</code>

</program>

<p>

In Java we can iterate over an <c>ArrayList</c> of integers too. Note that this requires importing the <c>ArrayList</c> class.

</p>

<program interactive="activecode" language="java">

<code>

import java.util.ArrayList;

public class ForEachArrayListExample {

public static void main(String[] args) {

ArrayList&lt;Integer&gt; l = new ArrayList&lt;Integer&gt;();

l.add(1);

l.add(1);

l.add(2);

l.add(3);

l.add(5);

l.add(8);

l.add(13);

l.add(21);

for (Integer i : l) {

System.out.println(i);

}

}

}

</code>

</program>

<p>

This example stretches the imagination a bit, and in fact points out one area where Java's primitive arrays are easier to use than an array list.

In fact all primitive arrays can be used in a for each loop.

</p>

<program interactive="activecode" language="java">

<code>

public class ForEachArrayExample {

public static void main(String[] args) {

int l[] = {1,1,2,3,5,8,13,21};

for(int i : l) {

System.out.println(i);

}

}

}

</code>

</program>

<p>

To iterate over the characters in a string in Java do the following:

</p>

<program interactive="activecode" language="java">

<code>

public class StringIterationExample {

public static void main(String[] args) {

String t = "Hello World";

for (char c : t.toCharArray()) {

System.out.println(c);

}

}

}

</code>

</program>

</section>

<section xml:id="indefinite-loops">

<title>Indefinite Loops</title>

<p><idx><c>while</c> loop</idx>

Both Python and Java support the <c>while</c> loop, which continues to execute as long as a condition is true.

Here is a simple example in Python that counts down from 5:

</p>

<program interactive="activecode" language="python">

<code>

i = 5

while i &gt; 0:

print(i)

i = i - 1

</code>

</program>

<p>

In Java we add parenthesis and curly braces. Here is the same countdown loop in Java:

</p>

<program interactive="activecode" language="java">

<code>

public class WhileLoopExample {

public static void main(String[] args) {

int i = 5;

while (i &gt; 0) {

System.out.println(i);

i = i - 1;

}

}

}

</code>

</program>

<p><idx><c>do-while</c> loop</idx>

Java adds an additional, if seldom used variation of the <c>while</c> loop called the <c>do-while</c> loop.

The <c>do-while</c> loop is very similar to <c>while</c> except that the condition is evaluated at the end of the loop rather than the beginning.

This ensures that a loop will be executed at least one time.

Some programmers prefer this loop in some situations because it avoids an additional assignment prior to the loop.

For example, the following loop will execute once even though the condition is initially false.

</p>

<program interactive="activecode" language="java">

<code>

public class DoWhileExample {

public static void main(String[] args) {

int i = 10;

do {

System.out.println("This runs once, i = " + i);

} while (i &lt; 5);

}

}

</code>

</program>

</section>

</chapter>