<?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>

<idx>definite loop</idx>

A <term>definite loop</term>, also known as a <term><c>for</c> loop</term>, is a loop that is executed for a specific or <em>definite</em> number of times. In Python, the easiest way to write a definite loop is using the <c>for</c> loop structure in conjunction with the <c>range</c> function. <xref ref="python-range-options" text="type-global"/> shows the syntax for the <c>range</c> function.

</p>

<listing xml:id="python-definite-loop">

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

<code>

for i in range(10): # range(10) is a list of integers from 0 to 9

print(i)

</code>

</program>

</listing>

<p>

<xref ref="java-definite-loop" text="type-global"/> shows how the <c>for</c> loop is written in Java.

</p>

<listing xml:id="java-definite-loop">

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

<code>

public class DefiniteLoopExample {

public static void main(String[] args) {

for (Integer i = 0; i &lt; 10; i++ ) { // notice how the initialization, condition, and update are all on the same line.

System.out.println(i);

}

}

}

</code>

</program>

</listing>

<p>

Recall that the <c>range</c> function provides you with a wide variety of options for controlling the value of the loop variable as shown in <xref ref="python-range-options" text="type-global"/>.

</p>

<listing xml:id="python-range-options">

<program>

<code>

range(stop)

range(start,stop)

range(start,stop,step)

</code>

</program>

</listing>

<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.

<xref ref="java-for-loop-syntax" text="type-global"/> shows how the Java <c>for</c> loop is written.

</p>

<listing xml:id="java-for-loop-syntax">

<program>

<code>

for (start clause; stop clause; step clause) {

statement1

statement2

...

}

</code>

</program>

</listing>

<p>

If you want to start at 100, stop at 0 and count backward by 5, <xref ref="python-definite-decrement" text="type-global"/> shows how the Python <c>for</c> loop is written.

</p>

<listing xml:id="python-definite-decrement">

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

<code>

for i in range(100, -1, -5): # start at 100, stop at 0, decrement by 5

print(i)

</code>

</program>

</listing>

<p>

<xref ref="java-definite-decrement" text="type-global"/> shows how the <c>for</c> loop is written in Java.

</p>

<listing xml:id="java-definite-decrement">

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

<code>

public class DefiniteLoopBackward {

public static void main(String[] args) {

for (Integer i = 100; i &gt;= 0; i -= 5) { // start at 100, stop at 0, decrement by 5

System.out.println(i);

}

}

}

</code>

</program>

</listing>

<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>

<xref ref="python-list-iteration" text="type-global"/> shows how the <c>for</c> loop can be used to iterate over a list in Python.

</p>

<listing xml:id="python-list-iteration">

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

<code>

l = [1, 1, 2, 3, 5, 8, 13, 21] # create a list of integers

for fib in l: # iterate over the list

print(fib)

</code>

</program>

</listing>

<p>

<xref ref="java-list-iteration" text="type-global"/> shows how the <c>for</c> loop can be used to iterate over an <c>ArrayList</c> of integers in Java.

</p>

<listing xml:id="java-list-iteration">

<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; // create an ArrayList of integers

l.add(1); // add the first integer to the list

l.add(1); // add the second integer to the list

l.add(2); // keep going

l.add(3);

l.add(5);

l.add(8);

l.add(13);

l.add(21); // add the last integer to the list

for (Integer i : l) { // iterate over the list

System.out.println(i);

}

}

}

</code>

</program>

</listing>

<p>

<xref ref="java-list-iteration" text="type-global"/> 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.

<xref ref="java-for-each" text="type-global"/> shows how the <c>for</c> loop can be used to iterate over all elements in a primitive array in Java.

</p>

<listing xml:id="java-for-each">

<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}; // create an array of integers using primitive syntax

for(int i : l) { // iterate over the array

System.out.println(i);

}

}

}

</code>

</program>

</listing>

<p>

<xref ref="java-for-each" text="type-global"/> shows how the <c>for</c> loop can be used to iterate over all elements in a string in Java.

</p>

<listing xml:id="java-string-iteration">

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

<code>

public class StringIterationExample {

public static void main(String[] args) {

String t = "Hello World"; // create a string

for (char c : t.toCharArray()) { // iterate over the characters in the string

System.out.println(c);

}

}

}

</code>

</program>

</listing>

<exercise label="java_loops_sum_evens_parsons">

<statement>

<p>

Rearrange the blocks to create a Java method that accepts an upper bound integer <c>limit</c> and calculates the sum of all even numbers from 2 up to and including <c>limit</c>.

</p>

</statement>

<blocks>

<block order="1">

<cline>public int sumEvens(int limit) {</cline>

</block>

<block order="2">

<choice correct="yes">

<cline> int total = 0;</cline>

</choice>

<choice correct="no">

<cline> int total;</cline>

</choice>

</block>

<block order="3">

<choice correct="yes">

<cline> for (int i = 2; i &lt;= limit; i += 2) {</cline>

</choice>

<choice correct="no">

<cline> for (int i = 2; i &lt; limit; i =+ 2) {</cline>

</choice>

</block>

<block order="4">

<choice correct="yes">

<cline> total += i;</cline>

<cline> }</cline>

</choice>

<choice correct="no">

<cline> total = i;</cline>

<cline> }</cline>

</choice>

</block>

<block order="5">

<choice correct="yes">

<cline> return total;</cline>

<cline>}</cline>

</choice>

<choice correct="no">

<cline> return i;</cline>

<cline>}</cline>

</choice>

</block>

</blocks>

</exercise>

</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.

<xref ref="python-countdown" text="type-global"/> shows a simple example in Python that counts down from 5.

</p>

<listing xml:id="python-countdown">

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

<code>

i = 5

while i &gt; 0: # while i is greater than 0

print(i)

i = i - 1

</code>

</program>

</listing>

<p>

In Java, we add parentheses and curly braces. <xref ref="java-countdown" text="type-global"/> shows the same countdown loop in Java.

</p>

<listing xml:id="java-countdown">

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

<code>

public class WhileLoopExample {

public static void main(String[] args) {

int i = 5;

while (i &gt; 0) { // while i is greater than 0

System.out.println(i);

i = i - 1;

}

}

}

</code>

</program>

</listing>

<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, <xref ref="java-do-while" text="type-global"/> shows how loop will execute once even though the condition is initially false.

</p>

<listing xml:id="java-do-while">

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

<code>

public class DoWhileExample {

public static void main(String[] args) {

int i = 10;

do { // do-while loop, will run at least once no matter the condition

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

} while (i &lt; 5); // while i is less than 5

}

}

</code>

</program>

</listing>

<exercise label="java_loops_indefinite_parsons">

<statement>

<p>

Rearrange the blocks to create a Java method that accepts a starting balance and a target amount, then calculates how many years it takes for the balance to reach or exceed the target by doubling each year.

</p>

</statement>

<blocks>

<block order="1">

<choice correct="yes">

<cline>public int yearsToTarget(double balance, double target) {</cline>

<cline> int years = 0;</cline>

</choice>

<choice correct="no">

<cline>public int yearsToTarget(double balance, double target) {</cline>

<cline> int years;</cline>

</choice>

</block>

<block order="2">

<choice correct="yes">

<cline> while (balance &lt; target) {</cline>

</choice>

<choice correct="no">

<cline> while (balance &gt;= target) {</cline>

</choice>

</block>

<block order="3">

<choice correct="yes">

<cline> balance *= 2;</cline>

<cline> years++;</cline>

<cline> }</cline>

</choice>

<choice correct="no">

<cline> balance * 2;</cline>

<cline> years++;</cline>

<cline> }</cline>

</choice>

</block>

<block order="4">

<choice correct="yes">

<cline> return years;</cline>

<cline>}</cline>

</choice>

<choice correct="no">

<cline> return balance;</cline>

<cline>}</cline>

</choice>

</block>

</blocks>

</exercise>

</section>

<section xml:id="chapter5_summary">

<title>Summary &amp; Reading Questions</title>

<p>

<ol label="1">

<li>

<p>Java’s <c>for</c> loop syntax allows you to control loop initialization, condition, and update in one line using the format <c>for (init; condition; update)</c>.</p>

</li>

<li>

<p>To loop over lists or arrays in Java, the enhanced for-each loop uses the syntax <c>for (type var : collection)</c>, which is similar to Python’s <c>for item in list</c>.</p>

</li>

<li>

<p>Python’s <c>range()</c> function supports start, stop, and step, which directly maps to the three components of a Java <c>for</c> loop.</p>

</li>

<li>

<p>Java also supports the <c>while</c> loop, which executes a block of code while a condition is true, similar to Python’s <c>while</c> loop.</p>

</li>

<li>

<p>Java’s <c>do-while</c> loop guarantees the loop body executes at least once, because the condition is evaluated after the loop body.</p>

</li>

<li>

<p>To iterate through characters in a Java String, use <c>toCharArray()</c> along with a for-each loop.</p>

</li>

</ol>

</p>

<reading-questions xml:id="rqs-5">

<exercise label="summary-for-loop-structure">

<statement>

<p>Which of the following is the correct format for a definite loop in Java that runs 10 times?</p>

</statement>

<choices>

<choice>

<statement><p>for i in range(10):</p></statement>

<feedback><p>No, that is Python syntax.</p></feedback>

</choice>

<choice correct="yes">

<statement><p>for (int i = 0; i &lt; 10; i++)</p></statement>

<feedback><p>Correct! That’s the proper Java syntax for a definite loop.</p></feedback>

</choice>

<choice>

<statement><p>loop i from 0 to 10</p></statement>

<feedback><p>No, this is not valid syntax in Java.</p></feedback>

</choice>

<choice>

<statement><p>for (i &lt; 10; i++)</p></statement>

<feedback><p>No, the initialization part is missing in this Java loop.</p></feedback>

</choice>

</choices>

</exercise>

<exercise label="summary-for-each-array">

<statement>

<p>Which loop correctly iterates through all elements in a Java array of integers?</p>

</statement>

<choices>

<choice correct="yes">

<statement><p>for (int i : array)</p></statement>

<feedback><p>Yes! This is Java's enhanced for-each loop for arrays.</p></feedback>

</choice>

<choice>

<statement><p>for (i in array)</p></statement>

<feedback><p>No, that's closer to Python syntax.</p></feedback>

</choice>

<choice>

<statement><p>for (int i = 0; i &lt; array; i++)</p></statement>

<feedback><p>No, <c>array</c> is not a valid condition; use <c>array.length</c>.</p></feedback>

</choice>

<choice>

<statement><p>foreach i in array:</p></statement>

<feedback><p>No, this is not valid Java syntax.</p></feedback>

</choice>

</choices>

</exercise>

<exercise label="summary-do-while">

<statement>

<p>What is a unique characteristic of the Java <c>do-while</c> loop compared to the <c>while</c> loop?</p>

</statement>

<choices>

<choice>

<statement><p>It checks the condition before the loop body runs.</p></statement>

<feedback><p>No, that describes a regular <c>while</c> loop.</p></feedback>

</choice>

<choice>

<statement><p>It always runs infinitely.</p></statement>

<feedback><p>No, a <c>do-while</c> loop will stop when its condition becomes false.</p></feedback>

</choice>

<choice correct="yes">

<statement><p>It guarantees the loop body runs at least once.</p></statement>

<feedback><p>Correct! The <c>do-while</c> loop checks the condition after running the loop body.</p></feedback>

</choice>

<choice>

<statement><p>It is not supported in Java.</p></statement>

<feedback><p>No, Java does support <c>do-while</c> loops.</p></feedback>

</choice>

</choices>

</exercise>

</reading-questions>

</section>

</chapter>