ch7.htm

this is a book on pearl , simple example with explanation is given here. it could be beneficial for

things.

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Specify a label called </I><TT><I>OUTER_LOOP</I></TT><I>.

<BR>

Loop from 0 to 10 using </I><TT><I>$index</I></TT><I>

as the loop variable.<BR>

If </I><TT><I>$index</I></TT><I> is

equal to 5, then exit the loop.<BR>

Start an inner loop that repeats while </I><TT><I>$index</I></TT><I>

is less than 10.<BR>

If </I><TT><I>$index</I></TT><I> is

4, then exit out of both inner and outer loops.<BR>

INCrement </I><TT><I>$index</I></TT><I>.

<BR>

Print the value of </I><TT><I>$index</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

OUTER_LOOP:

    for ($index = 0; $index &lt; 10; $index++) {

        if ($index == 5) {

            last;

        }

        while ($index &lt; 10) {

            if ($index == 4) {

                last OUTER_LOOP;

            }

            print(&quot;inner: index = $index\n&quot;);

            $index++;

        }

       print(&quot;outer: index = $index\n&quot;);

}

print(&quot;index = $index\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

inner: index = 0

inner: index = 1

inner: index = 2

inner: index = 3

index = 4

</PRE>

</BLOCKQUOTE>

<P>

The inner <TT>while</TT> loop iNCrements

<TT>$index</TT> while it is less than

10. However, before it can reach 10 it must pass 4, which triggers

the <TT>if</TT> statement and exits

both loops. You can tell that the outer loop also was exited because

the outer print statement is never executed.

<H3><A NAME="ExampleTheInextIKeyword">

Example: The <I>next</I> Keyword</A></H3>

<P>

The <TT>next</TT> keyword lets you

skip the rest of the statement block and start the next iteration.

One use of this behavior could be to select specific array elements

for processing and ignoring the rest. For example:

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Create an array of 10 elements.<BR>

Print the array.<BR>

Iterate over the array.<BR>

Ignore the third and fifth element.<BR>

Change the current element to an asterisk.<BR>

Print the array to verify that it has been changed.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

@array = (0..9);

print(&quot;@array\n&quot;);

for ($index = 0; $index &lt; @array; $index++) {

    if ($index == 3 || $index == 5) {

        next;

    }

    $array[$index] = &quot;*&quot;;

}

print(&quot;@array\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

0 1 2 3 4 5 6 7 8 9

* * * 3 * 5 * * * *

</PRE>

</BLOCKQUOTE>

<P>

This example changes every array element, except the third and

fifth, to asterisks regardless of their former values. The next

keyword forces Perl to skip over the assignment statement and

go directly to the iNCrement/decrement expression. You also can

use the next keyword in nested loops.

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Define a label called </I><TT><I>OUTER_LOOP</I></TT><I>.

<BR>

Start a for loop that iterates from 0 to 3 using </I><TT><I>$row</I></TT><I>

as the loop variable.<BR>

Start a for loop that iterates from 0 to 3 using </I><TT><I>$col</I></TT><I>

as the loop variable.<BR>

Display the values of </I><TT><I>$row</I></TT><I>

and </I><TT><I>$col</I></TT><I> and

mention that the code is inside the inner loop.<BR>

If </I><TT><I>$col</I></TT><I> is

equal to 1, start the next iteration of loop near the label </I><TT><I>OUTER_LOOP</I></TT><I>.

<BR>

Display the values of </I><TT><I>$row</I></TT><I>

and </I><TT><I>$col</I></TT><I> and

mention that the code is inside the outer loop.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

OUTER_LOOP: for ($row = 0; $row &lt; 3; $row++) {

                for ($col = 0; $col &lt; 3; $col++) {

                    print(&quot;inner: $row,$col\n&quot;);

                    if ($col == 1) {

                        next OUTER_LOOP;

                    }

                }

                print(&quot;outer: $row,$col\n\n&quot;);

            }

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

inner: 0,0

inner: 0,1

inner: 1,0

inner: 1,1

inner: 2,0

inner: 2,1

</PRE>

</BLOCKQUOTE>

<P>

You can see that the <TT>next</TT>

statement in the inner loop causes Perl to skip the <TT>print</TT>

statement in the outer loop whenever <TT>$col</TT>

is equal to 1.

<H3><A NAME="ExampleTheIredoIKeyword">

Example: The <I>redo</I> Keyword</A></H3>

<P>

The <TT>redo</TT> keyword causes Perl

to restart the current statement block. Neither the iNCrement/decrement

expression nor the conditional expression is evaluated before

restarting the block. This keyword is usually used when getting

input from outside the program, either from the keyboard or from

a file. It is essential that the conditions that caused the <TT>redo</TT>

statement to execute can be changed so that an endless loop does

not occur.

<P>

This example will demonstrate the <TT>redo</TT>

keyword with some keyboard input:

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Start a statement block.<BR>

Print a prompt asking for a name.<BR>

Read a string from the keyboard. Control is returned to the program

when the user of the program presses the Enter key.<BR>

Remove the newline character from the end of the string.<BR>

If the string has zero length, it means the user simply pressed

the Enter key without entering a name, so display an error message

and redo the statement block.<BR>

Print a thank-you message with the name in uppercase characters.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print(&quot;What is your name? &quot;);

    $name = &lt;STDIN&gt;;

    chop($name);



    if (! length($name)) {

        print(&quot;Msg: Zero length input. Please try again\n&quot;);

        redo;

    }



    print(&quot;Thank you, &quot; . uc($name) . &quot;\n&quot;);

}<BR>



</PRE>

</BLOCKQUOTE>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD><B>Tip</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

It's worth noting that the statement block in this example acts like a single-time loop construct. You can use any of the jump keywords inside the statement block.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

The <TT>redo</TT> statement helps

you to have more straightforward program flow. Without it, you

would need to use a <TT>do...until</TT>

loop. For example:

<P>

<IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p>

<BLOCKQUOTE>

<I>Start a </I><TT><I>do...until</I></TT><I>

statement.<BR>

Print a prompt asking for a name.<BR>

Read a string from the keyboard. Control is returned to the program

when the user of the program presses the enter key.<BR>

Remove the newline character from the end of the string.<BR>

If the string has zero length, it means the user simply pressed

the Enter key without entering a name, so display an error message.

<BR>

Evaluate the conditional expression. If true, then the user entered

a name and the loop can end.<BR>

Print a thank you message with the name in uppercase characters.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

do {

    print(&quot;What is your name? &quot;);

    $name = &lt;STDIN&gt;;

    chomp($name);



    if (! length($name)) {

        print(&quot;Msg: Zero length input. Please try again\n&quot;);

    }



} until (length($name));





print(&quot;Thank you, &quot; . uc($name) . &quot;\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

The <TT>do...until</TT> loop is less

efficient because the length of <TT>$name</TT>

needs to be tested twice. Because Perl has so many ways to do

any given task, it pays to think about which method is more efficient

before implementing your ideas.

<H3><A NAME="ExampleTheIgotoIKeyword">

Example: The <I>goto</I> Keyword</A></H3>

<P>

The <TT>goto</TT> statement lets your

program jump directly to any label. However, because Perl also

provides the loop statements and other jump keywords, its use

is looked down on by most programmers. Using the <TT>goto</TT>

in your programs frequently causes your program logic to become

convoluted. If you write a program that you feel needs a <TT>goto</TT>

in order to run, then use it-but first, try to restructure the

program to avoid it.

<H2><A NAME="Summary"><FONT SIZE=5 COLOR=#FF0000>

Summary</FONT></A></H2>

<P>

This chapter was devoted to learning about three types of statements:

decision, loop, and jump. Decision statements use the <TT>if</TT>

keyword to execute a statement block depending on the evaluation

of conditional expressions. Loop statements also execute a statement

block based on a given condition, but they will repeatedly execute

the block until the condition is true or while the condition is

true. Jump statements are used to restart statement blocks, skip

to the next iteration in a loop, and exit loops prematurely.

<P>

The <TT>if</TT> statement can be used

with an <TT>else</TT> clause to choose

one of two statement blocks to execute. Or, you can use the <TT>elsif</TT>

clause to choose from among more than two statement blocks.

<P>

Both the <TT>while </TT>and <TT>until</TT>

loop statements have two forms. One form (the <TT>do...</TT>

form) executes a statement block and then tests a conditional

expression, and the other form tests the condition before executing

the statement block.

<P>

The <TT>for</TT> loops are the most

complicated type of loop because they involve three expressions

in addition to a statement block. There is an initialization expression,

a conditional expression, and an iNCrement/decrement expression.

The initialization expression is evaluated first, then the conditional

expression. If the conditional expression is false, the statement

block is executed. Next, the iNCrement/decrement expression is

evaluated and the loop starts again with the conditional expression.

<P>

<TT>Foreach</TT> loops are used to

iterate through an array. Each element in the array is assigned

to a local variable as the loop progresses through the array.

If you don't specify a local variable, Perl will use the <TT>$</TT>

special variable. You need to be careful when changing the value

of the local variable because it uses the call by refereNCe scheme.

Therefore, any change to the local variable will be reflected

in the value of the array element outside the <TT>foreach</TT>

loop.

<P>

The <TT>last</TT> keyword is used

to jump out of the current statement block. The <TT>next</TT>

keyword is used to skip the rest of the statement block and continue

to the next iteration of the loop. The <TT>redo</TT>

keyword is used to restart the statement block. And finally, the

<TT>goto</TT> keyword should not be

used because the other jump keywords are more descriptive. All

of the jump keywords can be used with labels so they can be used

inside nested loops.

<H2><A NAME="ReviewQuestions"><FONT SIZE=5 COLOR=#FF0000>

Review Questions</FONT></A></H2>

<P>

Answers to Review Questions are in Appendix A.

<OL>

<LI>What are the four loop keywords?

<LI>What are the four jump keywords?

<LI>Which form of the <TT>until</TT>

statement is used when the statement block needs to be executed

at least oNCe?

<LI>What will be displayed when this program executes?<BR>

<BR>

<TT>$firstVar = 5;<BR>

{<BR>

&nbsp;&nbsp;&nbsp;&nbsp;if ($firstVar &gt; 10) {<BR>

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;last;

<BR>

&nbsp;&nbsp;&nbsp;&nbsp;}<BR>

&nbsp;&nbsp;&nbsp;&nbsp;$firstVar++;<BR>

&nbsp;&nbsp;&nbsp;&nbsp;redo;<BR>

}<BR>

print(&quot;$firstVar\n&quot;);</TT>

<LI>What is the default name of the local variable in the <TT>foreach</TT>

loop?

<LI>How is the <TT>next</TT> keyword

different from the <TT>redo</TT> keyword?

<LI>Why is the comma operator useful in the initialization expression

of a <TT>for</TT> loop?

<LI>What is the <TT>shift()</TT> fuNCtion

used for?

</OL>

<H2><A NAME="ReviewExercises"><FONT SIZE=5 COLOR=#FF0000>

Review Exercises</FONT></A></H2>

<OL>

<LI>Use the <TT>while</TT> loop in

a program to count from 1 to 100 in steps of 5.

<LI>Use the <TT>for</TT> loop in a

program to print each number from 55 to 1.

<LI>Use an <TT>until</TT> loop, the

<TT>next</TT> statement, and the modulus

operator to loop from 0 to 100 and print out &quot;AAA&quot; every

Sixteenth iteration.

<LI>Use the <TT>foreach</TT> loop

to determine the smallest element in an array.

<LI>Use a <TT>for</TT> loop to iterate

over an array and multiple each element by 3.

<LI>Use a <TT>do..until</TT> loop

and the <TT>each()</TT> fuNCtion to

iterate over an associative array looking for an value equal to

&quot;AAA.&quot; When the element is found, the loop should be

ended.

</OL>

<HR>



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