ch4.htm

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

<TR><TD WIDTH=175><CENTER><TT>op1</TT>++</CENTER>

</TD><TD WIDTH=217>Post-iNCrement operand by one</TD></TR>

<TR><TD WIDTH=175><CENTER><TT>op1</TT>--</CENTER>

</TD><TD WIDTH=217>Post-decrement operand by one</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

Arithmetic operators start to get complicated when unary operators

are introduced. Just between you and me, I didn't get the hang

of negative numbers until someone said: &quot;If you have five

pieces of chocolate, and add negative two pieces&#133;&quot;

<P>

You might think that adding negative numbers is strange. Not so.

I know that you will never write a mathematics statement such

as the following: <TT>345 + -23</TT>.

However, you might use <TT>354 + $gasBill</TT>,

where <TT>$gasBill</TT> represents

a 23-dollar debit-in other words, a negative number.

<P>

Using the unary plus operator does nothing, and Perl ignores it.

The unary negative operator, however, changes the meaning of a

value from positive to negative or vice versa. For instaNCe, if

you had a variable called <TT>$firstVar</TT>

equal to 34, then printing <TT>-$firstVar</TT>

would display -34.

<P>

The <TT>++</TT> and <TT>--</TT>

operators are examples of the Perl shorthand notation. If the

<TT>++</TT> or <TT>--</TT>

operators appear in front of the operand, the operand is iNCremented

or decremented before its value is used. If the <TT>++</TT>

or <TT>--</TT> operators appear after

the operand, then the value of the operand is used and then the

operand is iNCremented or decremented as required.

<H3><A NAME="ExampleThePreiNCrementOperator">

Example: The Pre-iNCrement Operator</A></H3>

<P>

This example shows how to use the pre-iNCrement operator (<I><B>++</B></I>).

<P>

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

<BLOCKQUOTE>

<I>The </I><TT><I>$numPages</I></TT><I>

variable is assigned a value of 5.<BR>

The </I><TT><I>$numPages</I></TT><I>

variable is iNCremented by 1.<BR>

The </I><TT><I>$numPages</I></TT><I>

variable is printed.<BR>

The </I><TT><I>$numPages</I></TT><I>

variable is assigned a value of 5.<BR>

The </I><TT><I>$numPages</I></TT><I>

variables are iNCremented using the pre-iNCrement operator and

then printed.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 4.2&nbsp;&nbsp;04LST02.PL-Using Pre-iNCrement Operator

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

# Original Way

$numPages = 5;

$numPages = $numPages + 1;

print($numPages, &quot;\n&quot;);



# New Way

$numPages = 5;

print(++$numPages, &quot;\n&quot;);

</PRE>

</BLOCKQUOTE>

<HR>

<P>

This program produces the following output:

<BLOCKQUOTE>

<PRE>

6

6

</PRE>

</BLOCKQUOTE>

<P>

You can see that the new way of coding is shorter than the original

way. The statement <TT>print(++$numPages,

&quot;\n&quot;);</TT> will first iNCrement the <TT>$numPages</TT>

variable and then allow the print command to use it.

<H3><A NAME="ExampleThePredecrementOperator">

Example: The Pre-decrement Operator</A></H3>

<P>

This example shows how to use the pre-decrement operator (<TT>--</TT>).

<P>

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

<BLOCKQUOTE>

<I>The </I><TT><I>$numPages</I></TT><I>

variable is assigned a value of 5.<BR>

The </I><TT><I>$numPages</I></TT><I>

variable is decremented by 1.<BR>

The </I><TT><I>$totalPages</I></TT><I>

variable is assigned the value of </I><TT><I>$numPages

+ 5</I></TT><I>.<BR>

The </I><TT><I>$numPages</I></TT><I>

and </I><TT><I>$totalPages</I></TT><I>

variables are printed.<BR>

The </I><TT><I>$numPages</I></TT><I>

variable is assigned a value of 5.<BR>

The </I><TT><I>$numPages</I></TT><I>

variable is decremented and then </I><TT><I>$numPages

+ 5</I></TT><I> is assigned to </I><TT><I>$totalPages</I></TT><I>.

<BR>

The </I><TT><I>$numPages</I></TT><I>

and </I><TT><I>$totalPages</I></TT><I>

variables are printed.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 4.3&nbsp;&nbsp;04LST03.PL-Using Pre-iNCrement Operator

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

# Original Way

$numPages = 5;

$numPages = $numPages - 1;

$totalPages = $numPages + 5;

print(&quot;$numPages $totalPages \n&quot;);



# New Way

$numPages = 5;

$totalPages = --$numPages + 5;

print(&quot;$numPages $totalPages \n&quot;);

</PRE>

</BLOCKQUOTE>

<HR>

<P>

This program produces the following output:

<BLOCKQUOTE>

<PRE>

4 9

4 9

</PRE>

</BLOCKQUOTE>

<P>

The statement <TT>$totalPages = --$numPages

+ 5;</TT> will first decrement the <TT>$numPages</TT>

variable and then allow the plus operator to use it.

<H3><A NAME="ExampleThePostiNCrementOperator">

Example: The Post-iNCrement Operator</A></H3>

<P>

This example shows how to use the ++ and -- post-iNCrement operators.

<P>

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

<BLOCKQUOTE>

<I>The </I><TT><I>$numPages</I></TT><I>

variable is assigned a value of 5.<BR>

The </I><TT><I>$totalPages</I></TT><I>

variable is assigned the value of </I><TT><I>$numPages</I></TT><I>.

<BR>

The </I><TT><I>$numPages</I></TT><I>

variable is iNCremented by one.<BR>

The </I><TT><I>$numPages</I></TT><I>

and </I><TT><I>$totalPages</I></TT><I>

variables are printed.<BR>

The </I><TT><I>$numPages</I></TT><I>

variable is assigned a value of 5.<BR>

The </I><TT><I>$totalPages</I></TT><I>

variable is assigned the value of </I><TT><I>$numPages</I></TT><I>

and then the </I><TT><I>$numPages</I></TT><I>

variable is iNCremented.<BR>

The </I><TT><I>$numPages</I></TT><I>

and </I><TT><I>$totalPages</I></TT><I>

variables are printed.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 4.4&nbsp;&nbsp;04LST04.PL-Using Pre-iNCrement Operator

<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

# Original Way

$numPages = 5;

$totalPages = $numPages;

$numPages = $numPages + 1;

print(&quot;$numPages $totalPages \n&quot;);



# New Way

$numPages = 5;

$totalPages = $numPages++;

print(&quot;$numPages $totalPages \n&quot;);

</PRE>

</BLOCKQUOTE>

<HR>

<P>

The program produces the following output:

<BLOCKQUOTE>

<PRE>

6 5

6 5

</PRE>

</BLOCKQUOTE>

<P>

The statement <TT>$totalPages = $numPages++;</TT>

will first assign the value of <TT>$numPages</TT>

to <TT>$totalPages</TT> and then iNCrement

the <TT>$numPages</TT> variable. It

may help to know that post-iNCrement and post-decrement operators

do not affect the value of the variable on the left side of the

assignment operator. If you see post-iNCrement or post-decrement

operators, evaluate the statement by ignoring them. Then, when

done, apply the post-iNCrement and post-decrement operators as

needed.<BR>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

The Perl programming language has many ways of achieving the same objective. You will become a more efficient programmer if you decide on one approach to iNCrementing/decrementing and use it consistently.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

The Logical Operators</FONT></A></H2>

<P>

<I>Logical operators </I>are mainly used to control program flow.

Usually, you will find them as part of an <TT>if</TT>,

a <TT>while</TT>, or some other control

statement. Control statements are discussed in <A HREF="ch7.htm" >Chapter 7</A> &quot;Control

Statements.&quot;<BR>

<P>

<CENTER><B>Table 4.4&nbsp;&nbsp;The Logical Operators</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=127><CENTER><I>Operator</I></CENTER></TD><TD WIDTH=336><I>Description</I>

</TD></TR>

<TR><TD WIDTH=127><CENTER><TT>op1 &amp;&amp; op2</TT></CENTER>

</TD><TD WIDTH=336>Performs a logical AND of the two operands.

</TD></TR>

<TR><TD WIDTH=127><CENTER><TT>op1 </TT>||<TT> op2</TT></CENTER>

</TD><TD WIDTH=336>Performs a logical OR of the two operands.

</TD></TR>

<TR><TD WIDTH=127><CENTER><TT>!op1</TT></CENTER>

</TD><TD WIDTH=336>Performs a logical NOT of the operand.</TD>

</TR>

</TABLE>

</CENTER>

<P>

<P>

The coNCept of logical operators is simple. They allow a program

to make a decision based on multiple conditions. Each operand

is considered a condition that can be evaluated to a true or false

value. Then the value of the conditions is used to determine the

overall value of the <TT>op1 operator op2</TT>

or <TT>!op1</TT> grouping. The following

examples demonstrate different ways that logical conditions can

be used.

<H3><A NAME="ExampleThequotANDquotOperatorampamp">

Example: The &quot;AND&quot; Operator (&amp;&amp;)</A></H3>

<P>

The <TT>&amp;&amp;</TT> operator is

used to determine whether both operands or conditions are true.

Table 4.5 shows the results of using the <TT>&amp;&amp;</TT>

operator on the four sets of true/false values.<BR>

<P>

<CENTER><B>Table 4.5&nbsp;&nbsp;The &amp;&amp; Result Table</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=67><CENTER><I>Op1</I></CENTER></TD><TD WIDTH=70><CENTER><I>Op2</I></CENTER>

</TD><TD WIDTH=140><CENTER><I>Op1 &amp;&amp; Op2</I></CENTER>

</TD></TR>

<TR><TD WIDTH=67><CENTER>0</CENTER></TD><TD WIDTH=70><CENTER>0</CENTER>

</TD><TD WIDTH=140><CENTER>0</CENTER></TD></TR>

<TR><TD WIDTH=67><CENTER>1</CENTER></TD><TD WIDTH=70><CENTER>0</CENTER>

</TD><TD WIDTH=140><CENTER>0</CENTER></TD></TR>

<TR><TD WIDTH=67><CENTER>0</CENTER></TD><TD WIDTH=70><CENTER>1</CENTER>

</TD><TD WIDTH=140><CENTER>0</CENTER></TD></TR>

<TR><TD WIDTH=67><CENTER>1</CENTER></TD><TD WIDTH=70><CENTER>1</CENTER>

</TD><TD WIDTH=140><CENTER>1</CENTER></TD></TR>

</TABLE>

</CENTER>

<P>

<P>

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

<BLOCKQUOTE>

<I>If the value of </I><TT><I>$firstVar</I></TT><I>

is 10 AND the value of </I><TT><I>$secondVar</I></TT><I>

is 9, then print the error message.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

if ($firstVar == 10 &amp;&amp; $secondVar == 9) {

    print(&quot;Error!&quot;);

};

</PRE>

</BLOCKQUOTE>

<P>

If either of the two conditions is false or iNCorrect, then the

print command is bypassed.

<H3><A NAME="ExampleThequotORquotOperator">

Example: The &quot;OR&quot; Operator (||)</A></H3>

<P>

The || operator is used to determine whether either of the conditions

is true. Table 4.6 shows the results of using the || operator

on the four sets of true/false values.<BR>

<P>

<CENTER><B>Table 4.6&nbsp;&nbsp;The || Result Table</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=73><CENTER><I>0p1 </I></CENTER></TD><TD WIDTH=94><CENTER><I>0p2 </I></CENTER>

</TD><TD WIDTH=104><CENTER><I>0p1 || 0p2</I></CENTER></TD></TR>

<TR><TD WIDTH=73><CENTER>0</CENTER></TD><TD WIDTH=94><CENTER>0</CENTER>

</TD><TD WIDTH=104><CENTER>0</CENTER></TD></TR>

<TR><TD WIDTH=73><CENTER>1</CENTER></TD><TD WIDTH=94><CENTER>0</CENTER>

</TD><TD WIDTH=104><CENTER>1</CENTER></TD></TR>

<TR><TD WIDTH=73><CENTER>0</CENTER></TD><TD WIDTH=94><CENTER>1</CENTER>

</TD><TD WIDTH=104><CENTER>1</CENTER></TD></TR>

<TR><TD WIDTH=73><CENTER>1</CENTER></TD><TD WIDTH=94><CENTER>1</CENTER>

</TD><TD WIDTH=104><CENTER>1</CENTER></TD></TR>

</TABLE>

</CENTER>

<P>

<P>

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

<BLOCKQUOTE>

<I>If the value of </I><TT><I>$firstVar</I></TT><I>

is 9 OR the value of </I><TT><I>$firstVar</I></TT><I>

is 10, then print the error message.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

if ($firstVar == 9 </FONT><FONT SIZE=2 FACE="Palatino">||</FONT><FONT SIZE=2 FACE="Courier"> $firstVar == 10) {

    print(&quot;Error!&quot;);

</PRE>

</BLOCKQUOTE>

<P>

If either of the two conditions is true, then the print command

is run.<BR>

<p>

<CENTER>

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

<TR><TD><B>Caution</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

If the first operand of the || operator evaluates to true, the second operand will not be evaluated. This could be a source of bugs if you are not careful. For instaNCe, in the following code fragment:</BLOCKQUOTE>

<BLOCKQUOTE>

<TT>if ($firstVar++ || $secondVar++) { print(&quot;\n&quot;); }</TT>

</BLOCKQUOTE>

<BLOCKQUOTE>

variable <TT>$secondVar</TT> will not be iNCremented if <TT>$firstVar++</TT> evaluates to true.

</BLOCKQUOTE>



</TD></TR>