ch4.htm

prrl 5 programs codes in the book

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

</TABLE>

</CENTER>

<P>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

You might be tempted to try the following:</BLOCKQUOTE>

<BLOCKQUOTE>

<TT>if ($firstVar == (9 || 10)) {<BR>

&nbsp;&nbsp;&nbsp;&nbsp;print(&quot;Error!&quot;);<BR>

};</TT>

</BLOCKQUOTE>

<BLOCKQUOTE>

to determine <TT>if $firstVar</TT> is equal to either 9 or 10. Don't do it. Perl doesn't work this way. First, the expression (<TT>9 </TT><FONT FACE="Palatino">||</FONT><TT> 10</TT>) will be evaluated to be equal to 9. And then, Perl will evaluate 
<TT>$firstVar == 9</TT>. The correct method for testing <TT>$firstVar</TT> is to explicitly state each sub-condition that needs to be met in order for the entire condition to return true. The correct way is:

</BLOCKQUOTE>

<BLOCKQUOTE>

<TT>if ($firstVar == 9 || $firstVar == 10) {<BR>

&nbsp;&nbsp;&nbsp;&nbsp;print(&quot;Error!&quot;);<BR>

};</TT>

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

Example: The &quot;NOT&quot; Operator (!)</FONT></A></H2>

<P>

The <TT>!</TT> operator is used to

convert true values to false and false values to true. In other

words, it inverts a value. Perl considers any non-zero value to

be true-even string values. Table 4.7 shows the results of using

the <TT>!</TT> operator on true and

false values.<BR>

<P>

<CENTER><B>Table 4.7&nbsp;&nbsp;The ! Result Table</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=91><CENTER><I>Op1</I></CENTER></TD><TD WIDTH=108><CENTER><I>!Op1</I></CENTER>

</TD></TR>

<TR><TD WIDTH=91><CENTER>0</CENTER></TD><TD WIDTH=108><CENTER>1</CENTER>

</TD></TR>

<TR><TD WIDTH=91><CENTER>1</CENTER></TD><TD WIDTH=108><CENTER>0</CENTER>

</TD></TR>

</TABLE>

</CENTER>

<P>

<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>Assign a value of 10 to </I><TT><I>$firstVar</I></TT><I>.

<BR>

Negate </I><TT><I>$firstVar-!10</I></TT><I>

is equal to </I><TT><I>0-</I></TT><I>and

assign the new value to </I><TT><I>$secondVar</I></TT><I>.

<BR>

If the </I><TT><I>$secondVar</I></TT><I>

variable is equal to zero, then print the string &quot;zero.&quot;</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = 10;

$secondVar = !$firstVar;



if ($secondVar == 0) {

   print(&quot;zero\n&quot;);

};

</PRE>

</BLOCKQUOTE>

<P>

The program produces the following output:

<BLOCKQUOTE>

<PRE>

zero

</PRE>

</BLOCKQUOTE>

<P>

You could replace the 10 in the first line with &quot;ten,&quot;

'ten,' or any non-zero, non-null value.

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

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

<P>

The <I>bitwise </I>operators, listed in Table 4.8, are similar

to the logical operators, except that they work on a smaller scale.

<BR>

<P>

<CENTER><B>Table 4.8&nbsp;&nbsp;The Bitwise Operators</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=127><CENTER><I>Operator</I></CENTER></TD><TD WIDTH=463><I>Description</I>

</TD></TR>

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

</TD><TD WIDTH=463>The AND operator compares two bits and generates a result of 1 if both bits are 1; otherwise, it returns 0.

</TD></TR>

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

</TD><TD WIDTH=463>The OR operator compares two bits and generates a result of 1 if the bits are complementary; otherwise, it returns 0.

</TD></TR>

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

</TD><TD WIDTH=463>The EXCLUSIVE-OR operator compares two bits and gener-ates a result of 1 if either or both bits are 1; otherwise, it returns 0.

</TD></TR>

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

</TD><TD WIDTH=463>The COMPLEMENT operator is used to invert all of the bits of the operand. I've never found this useful, so we'll skip looking at an example of it.

</TD></TR>

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

</TD><TD WIDTH=463>The SHIFT RIGHT operator moves the bits to the right, discards the far right bit, and assigns the leftmost bit a value of 0. Each move to the right effectively divides <TT>op1</TT> in half.

</TD></TR>

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

</TD><TD WIDTH=463>The SHIFT LEFT operator moves the bits to the left, discards the far left bit, and assigns the rightmost bit a value of 0. Each move to the left effectively multiplies <TT>op1</TT> by 2.

</TD></TR>

</TABLE>

</CENTER>

<P>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

Both operands associated with the bitwise operator must be integers.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

Bitwise operators are used to change individual bits in an operand.

A single byte of computer memory-when viewed as 8 bits-can signify

the true/false status of 8 flags because each bit can be used

as a boolean variable that can hold one of two values: true or

false. A <I>flag</I> variable is typically used to indicate the

status of something. For instaNCe, computer files can be marked

as read-only. So you might have a <TT>$fReadOnly</TT>

variable whose job would be to hold the read-only status of a

file. This variable is called a flag variable because when <TT>$fReadOnly</TT>

has a true value, it's equivalent to a football referee throwing

a flag. The variable says, &quot;Whoa! Don't modify this file.&quot;

<P>

When you have more than one flag variable, it might be more efficient

to use a single variable to indicate the value of more than one

flag. The next example shows you how to do this.

<H3><A NAME="ExampleUsingtheampandOperators">

Example: Using the &amp;, |, and ^ Operators</A></H3>

<P>

The first step to using bitwise operators to indicate more than

one flag in a single variable is to define the meaning of the

bits that you'd like to use. Figure 4.1 shows an example of 8

bits that could be used to control the attributes of text on a

display.

<P>

<A HREF="f4-1.gif" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/f4-1.gif"><B>Figure 4.1 : </B><I>The bit definition of a text attribute control

variable</I>.</A>

<P>

If you assume that <TT>$textAttr</TT>

is used to control the text attributes, then you could set the

italic attribute by setting <TT>$textAttr</TT>

equal to 128 like this:

<BLOCKQUOTE>

<PRE>

$textAttr = 128;

</PRE>

</BLOCKQUOTE>

<P>

because the bit pattern of 128 is 10000000. The bit that is turned

on corresponds to the italic position in <TT>$textAttr</TT>.

<P>

Now let's set both the italic and underline attributes on at the

same time. The underline value is 16, which has a bit pattern

of 00010000. You already know the value for italic is 128. So

we call on the <TT>OR</TT> operator

to combine the two values.

<BLOCKQUOTE>

<PRE>

$textAttr = 128 </FONT><FONT SIZE=2 FACE="Palatino">|</FONT><FONT SIZE=2 FACE="Courier"> 16;

</PRE>

</BLOCKQUOTE>

<P>

or using the bit patterns (this is just an example-you can't do

this in Perl)

<BLOCKQUOTE>

<PRE>

$textAttr = 10000000 </FONT><FONT SIZE=2 FACE="Palatino">|</FONT><FONT SIZE=2 FACE="Courier"> 00010000;

</PRE>

</BLOCKQUOTE>

<P>

If you look back at Table 4.8 and evaluate each bit, you will

see that <TT>$textAttr</TT> gets assigned

a value of 144 (or 10010000 as a bit pattern). This will set both

italic and underline attributes on.

<P>

The next step might be to turn the italic attribute off. This

is done with the EXCLUSIVE-OR operator, like so:

<BLOCKQUOTE>

<PRE>

$textAttr = $textAttr ^ 128;

</PRE>

</BLOCKQUOTE>

<H3><A NAME="ExampleUsingthegtgtandltltOperators">

Example: Using the &gt;&gt; and &lt;&lt; Operators</A></H3>

<P>

The <I>bitwise shift </I>operators are used to move all of the

bits in the operand left or right a given number of times. They

come in quite handy when you need to divide or multiply integer

values.

<P>

This example will divide by 4 using the <TT>&gt;&gt;</TT>

operator.

<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>Assign a value of 128 to the </I><TT><I>$firstVar</I></TT><I>

variable.<BR>

Shift the bits inside </I><TT><I>$firstVar</I></TT><I>

two places to the right and assign the new value to </I><TT><I>$secondVar</I></TT><I>.

<BR>

Print the </I><TT><I>$secondVar</I></TT><I>

variable.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = 128;

$secondVar = $firstVar &gt;&gt; 2;

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

</PRE>

</BLOCKQUOTE>

<P>

The program produces the following output:

<BLOCKQUOTE>

<PRE>

32

</PRE>

</BLOCKQUOTE>

<P>

Let's look at the bit patterns of the variables before and after

the shift operation. First, <TT>$firstVar</TT>