ch4.htm

prrl 5 programs codes in the book

</PRE>

</BLOCKQUOTE>

<P>

Notice that every tenth item is printed. By changing the value

on the right side of the modulus operator, you can affect how

many items are processed before the message is printed. Changing

the value to 15 means that a message will be printed every 15

items. <A HREF="ch7.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch7.htm" >Chapter 7</A> &quot;Control Statements,&quot; describes the

<TT>if</TT> and <TT>for</TT>

statement in detail.

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

The Unary Arithmetic Operators</FONT></A></H2>

<P>

The unary arithmetic operators act on a single operand. They are

used to change the sign of a value, to iNCrement a value, or to

decrement a value. <I>INCrementing</I> a value means to add one

to its value. <I>Decrementing</I> a value means to subtract one

from its value. Table 4.3 lists Perl's unary operators.<BR>

<P>

<CENTER><B>Table 4.3&nbsp;&nbsp;The Unary Arithmetic Operators</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=175><CENTER><I>Operator</I></CENTER></TD><TD WIDTH=217><I>Description</I>

</TD></TR>

<TR><TD COLSPAN=2 WIDTH=392><B>Changing the sign of op1</B></TD>

</TR>

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

</TD><TD WIDTH=217>Positive operand</TD></TR>

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

</TD><TD WIDTH=217>Negative operand</TD></TR>

<TR><TD COLSPAN=2 WIDTH=392><B>Changing the value of op1 before usage</B>

</TD></TR>

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

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

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

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

<TR><TD COLSPAN=2 WIDTH=392><B>Changing the value of op1 after usage</B>

</TD></TR>

<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" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/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" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/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" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/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" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/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>