ch6.htm

prrl 5 programs codes in the book

Start the statement block.<BR>

Create a local version of </I><TT><I>$firstVar</I></TT><I>

with a value of </I><TT><I>A</I></TT><I>.

<BR>

Print </I><TT><I>$firstVar</I></TT><I>

repeated five times.<BR>

End the statement block.<BR>

Print the global </I><TT><I>$firstVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar = 10;

{

   my($firstVar) = &quot;A&quot;;

   print $firstVar x 5 . &quot;\n&quot;;



}

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

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

AAAAA

firstVar = 10

</PRE>

</BLOCKQUOTE>

<P>

You can see that the value of <TT>$firstVar</TT>

has been uNChanged by the statement block even though a variable

called <TT>$firstVar</TT> is used

inside it. This shows that the variable used inside the statement

block does indeed have a local scope.<BR>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

Statement blocks are also good to use when you temporarily need to send debugging output to a file. Then, when all the bugs have been found and the need for debugging is over, you can remove the statement block quickly and easily because all the code is 
in one spot.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

Statement Types</FONT></A></H2>

<P>

Just as there were several types of expressions, there are also

several types of statements. Table 6.5 lists seven different types

of statements.<BR>

<P>

<CENTER><B>Table 6.5&nbsp;&nbsp;Perl Statement Types</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=183><I>Statement Type</I></TD><TD WIDTH=407><I>Description</I>

</TD></TR>

<TR><TD WIDTH=183>No-action statements</TD><TD WIDTH=407>These statements evaluate a value but perform no actions.

</TD></TR>

<TR><TD WIDTH=183>Action statements</TD><TD WIDTH=407>These statements perform some action.

</TD></TR>

<TR><TD WIDTH=183>Assignment statements</TD><TD WIDTH=407>These statements assign a value to one or more variables. They are discussed, along with the assignment operator, in <A HREF="ch4.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch4.htm" >Chapter 4</A> &quot;Operators.&quot;

</TD></TR>

<TR><TD WIDTH=183>Decision statements</TD><TD WIDTH=407>These statements allow you to test a condition and choose among one or more actions. Decision 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;

</TD></TR>

<TR><TD WIDTH=183>Jump statements</TD><TD WIDTH=407>These statements let you uNConditionally change the program flow to another point in your code. For instaNCe, you could use the <TT>redo</TT> keyword to send your program flow back to the beginning of a 
statement block. Jump 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;

</TD></TR>

<TR><TD WIDTH=183>Loop statements</TD><TD WIDTH=407>These statements let you perform a series of statements repeatedly while some condition is true or until some condition is true. Loop 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;

</TD></TR>

<TR><TD WIDTH=183>Modified Statements</TD><TD WIDTH=407>These statements let you use the <TT>if</TT>, <TT>unless</TT>, <TT>until</TT>, and <TT>while</TT> keywords to change the behavior of a statement.

</TD></TR>

</TABLE>

</CENTER>

<P>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

A <I>keyword</I> is a word that is reserved for use by Perl. These words (<TT>if</TT>, <TT>elsif</TT>, <TT>else</TT>, <TT>while</TT>, <TT>unless</TT>, <TT>until</TT>, <TT>for</TT>, <TT>foreach</TT>, <TT>last</TT>, <TT>next</TT>, <TT>redo</TT>, and 
<TT>continue</TT>) are integral to the language and provide you with the ability to control program flow.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

<I>No-action statements</I> are evaluated by Perl and have a value

but perform no actions. For instaNCe, the Perl statement <TT>10

+ 20</TT>; has a value of 30, but because no variables

were changed, no work was done. The value of 20 is not stored

anywhere, and it is quickly forgotten when the next statement

is seen.

<P>

What good is a <I>no-action statement </I>if no work is done?

A lot of Perl programmers use these simple statements as return

values in fuNCtions. For instaNCe:

<BLOCKQUOTE>

<PRE>

sub firstSub {

    doSomething();

    condition == true ? &quot;Success&quot; : &quot;Failure&quot;;

}

</PRE>

</BLOCKQUOTE>

<P>

Because Perl returns the value of the last evaluated statement

when leaving a fuNCtion, you can use no-action statements to let

Perl know what value should be returned to the main program. Notice

that even though the ternary operator was used, because there

are no fuNCtion calls or unary operators, no work can be done.

<BR>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

I still like to use the <TT>return()</TT> fuNCtion to explicitly identify the return values. The previous example looks like this when using the <TT>return()</TT> fuNCtion:

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<BLOCKQUOTE>

<PRE>

sub firstSub {

    doSomething();

    return(condition == true ? &quot;Success&quot; : &quot;Failure&quot;);

}

</PRE>

</BLOCKQUOTE>

<P>

<I>Action statements</I> use expressions to perform some task.

They can iNCrement or decrement a variable and call a fuNCtion.

<P>

<I>Modified statements</I> use expressions in conjuNCtion with

a modifying keyword to perform some action. There are four modifying

keywords: <TT>if</TT>, <TT>unless</TT>,

<TT>until</TT>, and <TT>while</TT>.

The basic syntax of a modified statement is

<BLOCKQUOTE>

<PRE>

EXPRESSION modifier (CONDITION);

</PRE>

</BLOCKQUOTE>

<P>

Let's look at some examples of modified statements.

<H3><A NAME="ExampleUsingtheifModifier">

Example: Using the if Modifier</A></H3>

<P>

The <TT>if</TT> modifier tells Perl

that the expression should be evaluated only if a given condition

is true. The basic syntax of a modified statement with the <TT>if</TT>

modifier is

<BLOCKQUOTE>

<PRE>

EXPRESSION if (CONDITION);

</PRE>

</BLOCKQUOTE>

<P>

This is a compact way of saying

<BLOCKQUOTE>

<PRE>

if (CONDITION) {

    EXPRESSION;

}

</PRE>

</BLOCKQUOTE>

<P>

Let's prove that the <TT>if</TT> modifier

works. Here's an example showing that the <TT>if</TT>

modifier can prevent the evaluation of an expression.

<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>Initialize the </I><TT><I>$firstVar</I></TT><I>

and </I><TT><I>$secondVar</I></TT><I>

variables to 20.<BR>

INCrement </I><TT><I>$firstVar</I></TT><I>

if and only if </I><TT><I>$secondVar</I></TT><I>

is equal to 10.<BR>

Print the values of </I><TT><I>$firstVar</I></TT><I>

and </I><TT><I>$secondVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar  = 20;

$secondVar = 20;



$firstVar++ if ($secondVar == 10);



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

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

</PRE>

</BLOCKQUOTE>

<P>

This program prints:

<BLOCKQUOTE>

<PRE>

firstVar  = 20

secondVar = 20

</PRE>

</BLOCKQUOTE>

<P>

The program doesn't iNCrement <TT>$firstVar</TT>

because the value of <TT>$secondVar</TT>

is 20 at the time the condition is evaluated. If you changed the

10 to a 20 in the condition, Perl would iNCrement <TT>$firstVar</TT>.

<P>

You can find out about the <TT>if</TT>

statement-as opposed to the <TT>if</TT>

modifier-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>

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

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

<TR><TD>

<BLOCKQUOTE>

The condition expression can be as complex as you'd like. However, I believe that one of the goals of statement modifiers is to make programs easier to read and understand. Therefore, I use modifiers only with simple conditions. If complex conditions need 
to be met before an expression should be evaluated, using the <TT>if</TT> keyword is probably a better idea.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="ExampleUsingtheunlessModifier">

Example: Using the unless Modifier</A></H3>

<P>

The <TT>unless</TT> modifier is the

opposite of the <TT>if</TT> modifier.

This modifier evaluates an expression unless a condition is true.

The basic syntax of a modified statement with the <TT>unless</TT>

modifier is

<BLOCKQUOTE>

<PRE>

EXPRESSION unless (CONDITION);

</PRE>

</BLOCKQUOTE>

<P>

This is a compact way of saying

<BLOCKQUOTE>

<PRE>

if (! CONDITION) {

    EXPRESSION;

}

</PRE>

</BLOCKQUOTE>

<P>

This modifier helps to keep program code clearly understandable

because you don't have to use the logical <TT>not</TT>

operator to change the value of a condition so you can evaluate

an expression. Let's look back at the example from a moment ago.

<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>Initialize the </I><TT><I>$firstVar</I></TT><I>

and </I><TT><I>$secondVar</I></TT><I>

variables to 20.<BR>

INCrement </I><TT><I>$firstVar</I></TT><I>

unless </I><TT><I>$secondVar</I></TT><I>

is equal to 10.<BR>

Print the values of </I><TT><I>$firstVar</I></TT><I>

and </I><TT><I>$secondVar</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$firstVar  = 20;

$secondVar = 20;



$firstVar++ unless ($secondVar == 10);



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

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

</PRE>

</BLOCKQUOTE>

<P>

This program prints:

<BLOCKQUOTE>

<PRE>

firstVar  = 21

secondVar = 20

</PRE>

</BLOCKQUOTE>

<P>

If you were limited to using only the <TT>if</TT>

modifier, the modified statement would read

<BLOCKQUOTE>

<PRE>

$firstVar++ if ($secondVar != 10);

</PRE>

</BLOCKQUOTE>

<P>

The <TT>unless</TT> modifier is more

direct. All things being equal, the coNCept of <TT>$secondVar</TT>

being equal to 10 is easier to grasp than the coNCept of <TT>$secondVar</TT>

not being equal to 10. Of course, this is a trivial example. Let's

look at something more substantial before we move on.

<P>

One of the drawbacks of associative arrays is that they quietly

redefine the value of any key when that key is assigned a new

value, thereby losing the old value. If you are reading from a

list of key-value pairs, this might not be the behavior you need.

The <TT>unless</TT> modifier can be

used to prevent element assignment if the key has already been