ch4.htm

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

</TD><TD WIDTH=437>This operator returns true if <TT>op1</TT> is greater than or equal to <TT>op2</TT>. For example, 7 &gt;= 7 is true.

</TD></TR>

<TR><TD WIDTH=153><CENTER><TT>op1 &lt;=&gt; op2</TT></CENTER>

</TD><TD WIDTH=437>This operator returns 1 if <TT>op1</TT> is greater than <TT>op2</TT>, 0 if <TT>op1</TT> equals <TT>op2</TT>, and -1 if <TT>op1</TT> is less than <TT>op2</TT>. 

</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

You will see many examples of these operators when you read about

controlling program flow in <A HREF="ch7.htm" >Chapter 7</A> &quot;Control Statements.&quot;

Therefore, I'll show only an example of the <TT>&lt;=&gt;</TT>

comparison operator here.

<H3><A NAME="ExampleUsingtheltgtOperator">

Example: Using the &lt;=&gt; Operator</A></H3>

<P>

The <I>number comparison </I>operator is used to quickly tell

the relationship between one operand and another. It is frequently

used during sorting activities.<BR>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

You may sometimes see the &lt;=&gt; operator called the spaceship operator because of the way that it looks.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

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

<BLOCKQUOTE>

<I>Set up three variables.<BR>

Print the relationship of each variable to the variable </I><TT><I>$midVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$lowVar =  8;

$midVar = 10;

$hiVar  = 12;



print($lowVar &lt;=&gt; $midVar, &quot;\n&quot;);

print($midVar &lt;=&gt; $midVar, &quot;\n&quot;);

print($hiVar  &lt;=&gt; $midVar, &quot;\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

The program produces the following output:

<BLOCKQUOTE>

<PRE>

-1

0

1

</PRE>

</BLOCKQUOTE>

<P>

The -1 indicates that <TT>$lowVar (8)</TT>

is less than <TT>$midVar (10)</TT>.

The 0 indicates that <TT>$midVar</TT>

is equal to itself. And the 1 indicates that <TT>$hiVar

(12)</TT> is greater than <TT>$midVar

(10)</TT>.

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

The String Relational Operators</FONT></A></H2>

<P>

The <I>string relational operators</I>, listed in Table 4.10,

are used to test the relationship between two operands. You can

see if one operand is equal to another, if one operand is greater

than another, or if one operator is less than another.<BR>

<P>

<CENTER><B>Table 4.10&nbsp;&nbsp;The String Relational Operators</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=153><CENTER><I>Operator</I></CENTER></TD><TD WIDTH=437><I>Description</I>

</TD></TR>

<TR><TD COLSPAN=2 WIDTH=590><B>The Equality Operators</B></TD>

</TR>

<TR><TD WIDTH=153><CENTER><TT>op1 eq op2</TT></CENTER>

</TD><TD WIDTH=437>This operator returns true if <TT>op1</TT> is equal to <TT>op2</TT>. For example, &quot;b&quot; eq &quot;b&quot; is true.

</TD></TR>

<TR><TD WIDTH=153><CENTER><TT>Op1 ne op2</TT></CENTER>

</TD><TD WIDTH=437>This operator returns true if <TT>op1</TT> is not equal to <TT>op2</TT>. For example, &quot;b&quot; ne &quot;c&quot; is true.

</TD></TR>

<TR><TD COLSPAN=2 WIDTH=590><B>The Comparison Operators</B></TD>

</TR>

<TR><TD WIDTH=153><CENTER><TT>op1 lt op2</TT></CENTER>

</TD><TD WIDTH=437>This operator returns true if <TT>op1</TT> is less than <TT>op2</TT>. For example, &quot;b&quot; lt &quot;c&quot; is true.

</TD></TR>

<TR><TD WIDTH=153><CENTER><TT>Op1 le op2</TT></CENTER>

</TD><TD WIDTH=437>This operator returns true if <TT>op1</TT> is less than or equal to <TT>op2</TT>. For example, &quot;b&quot; le &quot;b&quot; is true.

</TD></TR>

<TR><TD WIDTH=153><CENTER><TT>Op1 gt op2</TT></CENTER>

</TD><TD WIDTH=437>This operator returns true if <TT>op1</TT> is greater than <TT>op2</TT>. For example, &quot;b&quot; gt &quot;a&quot; is true.

</TD></TR>

<TR><TD WIDTH=153><CENTER><TT>Op1 ge op2</TT></CENTER>

</TD><TD WIDTH=437>This operator returns true if <TT>op1</TT> is greater than or equal to <TT>op2</TT>. For example, &quot;b&quot; ge &quot;b&quot; is true.

</TD></TR>

<TR><TD WIDTH=153><CENTER><TT>Op1 cmp op2</TT></CENTER>

</TD><TD WIDTH=437>This operator returns 1 if <TT>op1</TT> is greater than <TT>op2</TT>, 0 if <TT>op1</TT> equals <TT>op2</TT>, and -1 if <TT>op1</TT> is less than <TT>op2</TT>. 

</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

String values are compared using the ASCII values of each character

in the strings. You will see examples of these operators when

you read about control program flow in <A HREF="ch7.htm" >Chapter 7</A> &quot;Control

Statements.&quot; So, we'll only show an example of the <TT>cmp</TT>

comparison operator here. You may want to glaNCe at Appendix E,

&quot;ASCII Table,&quot; to see all of the possible ASCII values.

<H3><A NAME="ExampleUsingthecmpOperator">

Example: Using the cmp Operator</A></H3>

<P>

The string comparison operator acts exactly like the &lt;<I>=</I>&gt;

operator except that it is designed to work with string operands.

This example will compare the values of three different strings.

<P>

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

<BLOCKQUOTE>

<I>Set up three variables.<BR>

<I>Print the relationship of each variable to the variable </I></I><TT><I>$midVar</I></TT><I><FONT FACE="MCPdigital-BI">.</FONT></I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$lowVar = &quot;AAA&quot;;

$midVar = &quot;BBB&quot;;

$hiVar  = &quot;ccC&quot;;



print($lowVar cmp $midVar, &quot;\n&quot;);

print($midVar cmp $midVar, &quot;\n&quot;);

print($hiVar  cmp $midVar, &quot;\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

The program produces the following output:

<BLOCKQUOTE>

<PRE>

-1

0

1

</PRE>

</BLOCKQUOTE>

<P>

Notice that even though strings are being compared, a numeric

value is returned. You may be wondering what happens if the strings

have spaces in them. Let's explore that for a moment.

<BLOCKQUOTE>

<PRE>

$firstVar = &quot;AA&quot;;

$secondVar = &quot; A&quot;;

print($firstVar cmp $secondVar, &quot;\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

The program produces the following output:

<BLOCKQUOTE>

<PRE>

1

</PRE>

</BLOCKQUOTE>

<P>

which means that &quot;<TT>AA</TT>&quot;

is greater than &quot; <TT>A</TT>&quot;

according to the criteria used by the <TT>cmp</TT>

operator.

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

The Ternary Operator</FONT></A></H2>

<P>

The <I>ternary</I> is actually a sequeNCe of  operators. The operator

is used like this:

<BLOCKQUOTE>

<PRE>

CONDITION-PART ? TRUE-PART : FALSE-PART

</PRE>

</BLOCKQUOTE>

<P>

which is shorthand for the following statement:

<BLOCKQUOTE>

<PRE>

if (CONDITION-PART) {

    TRUE-PART

} else {

    FALSE-PART

}

</PRE>

</BLOCKQUOTE>

<P>

You can find more information about <TT>if</TT>

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

<P>

The value of the entire operation depends on the evaluation of

the CONDITION-PART section of the statement. If the CONDITION-PART

evaluates to true, then the TRUE-PART is the value of the entire

operation. If the CONDITION-PART evaluates to false, then the

FALSE-PART is the value of the entire operation.<BR>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

The ternary operator is also referred to as the conditional operator by some refereNCes.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="ExampleUsingtheTernaryOperatortoAssignValues">

Example: Using the Ternary Operator to Assign Values</A></H3>

<P>

I frequently use the ternary operator to assign a value to a variable

when it can take one of two values. This use of the operator is

fairly straightforward.

<P>

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

<BLOCKQUOTE>

<I>If<FONT FACE="MCPdigital-BI"> </FONT></I><TT><I>$firstVar</I></TT><I>

is zero, then assign </I><TT><I>$secondVar</I></TT><I>

a value of zero. Otherwise, assign </I><TT><I>$secondVar</I></TT><I>

the value in the first element in the array </I><TT><I>@array</I></TT><I><FONT FACE="MCPdigital-BI">.</FONT></I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$secondVar = ($firstVar == 0) ? 0 : $array[0];

</PRE>

</BLOCKQUOTE>

<P>

The ternary operator can also be used to control which code sections

are performed. However, I recommend against this use because it

makes the program harder to read. I believe that operators should

affect variables, not program flow.

<P>

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

<BLOCKQUOTE>

<I>The CONDITION-PART evaluates to true so the </I><TT><I>$firstVar</I></TT><I>

variable is iNCremented.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

1 ? $firstVar++ : $secondVar++;

</PRE>

</BLOCKQUOTE>

<P>

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

<BLOCKQUOTE>

<I>The CONDITION-PART evaluates to false so the </I><TT><I>$secondVar</I></TT><I>

variable is iNCremented.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

0 ? $firstVar++ : $secondVar++;

</PRE>

</BLOCKQUOTE>

<P>

In this example, you get a chaNCe to see how the language can

be abused. When you have more than two actions to consider, you

can nest ternary operators inside each other. However, as you

can see the result is confusing code.

<P>

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

<BLOCKQUOTE>

<I>Assign one of four values to </I><TT><I>$firstVar</I></TT><I>

depending on the value of </I><TT><I>$temp</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = $temp == 0 ? 

                $numFiles++ : 

                ($temp == 1 ? 

                    $numRecords++ : 

                    ($temp == 3 ? $numBytes++ : $numErrors++));

<BR>



</PRE>

</BLOCKQUOTE>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

Abusing the language in this manner will make your programs difficult to understand and maintain. You can use the <TT>if</TT> statement for better looking and more maintainable code. See <A HREF="ch7.htm" >Chapter 7</A> &quot;Control Statements,&quot; for 
more information.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

If you'd like to see a really strange use of the ternary operator,

take a look at this next example. It uses the ternary operator

to determine which variable gets assigned a value.

<BLOCKQUOTE>

<PRE>

$firstVar = 1;

$secondVar = 1;



$thirdVar = 1;



($thirdVar == 0 ? $firstVar : $secondVar) = 10;



print &quot;$firstVar\n&quot;;

print &quot;$secondVar\n&quot;;

print &quot;$thirdVar\n&quot;;

</PRE>

</BLOCKQUOTE>

<P>

The program produces the following output:

<BLOCKQUOTE>

<PRE>

1

10

1

</PRE>

</BLOCKQUOTE>

<P>

The line <TT>($thirdVar == 0 ? $firstVar

: $secondVar) = 10;</TT> is equivalent to the following

control statement:

<BLOCKQUOTE>

<PRE>

if ($thirdVar ==0) {

    $firstVar = 10;

} else {

    $secondVar = 10;

}

</PRE>

</BLOCKQUOTE>

<P>

This use of the ternary operator works because Perl lets you use

the results of evaluations as <I>lvalues</I>. An lvalue is anything

that you can assign a value to. It's called an lvalue because

it goes on the left side of an assignment operator.<BR>

<p>

<CENTER>

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

<TR><TD><B>Note</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

Some programmers might think that this use of the ternary operator is as bad as using it to control program flow. However, I like this ability because it gives you the ability to coNCisely determine which variable is the target of an assignment. 
</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

The Range Operator (..)</FONT></A></H2>

<P>

The range operator was already introduced to you in <A HREF="ch3.htm" >Chapter 3</A>

&quot;Variables,&quot; when you read about arrays. I review its

use here-in an array context-in a bit more detail.

<H3><A NAME="ExampleUsingtheRangeOperator">

Example: Using the Range Operator</A></H3>

<P>