ch7.htm

prrl 5 programs codes in the book

will be iNCremented each time the statement block is executed.

The statement block will be executed as long as </I><TT><I>$firstVar</I></TT><I>

is greater than 0.<BR>

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

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

each time through the loop.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

for ($firstVar = 100, $secondVar = 0;

     $firstVar &gt; 0;

     $firstVar--, $secondVar++) {



        print(&quot;inside:  firstVar = $firstVar  secondVar = $secondVar\n&quot;);



}

</PRE>

</BLOCKQUOTE>

<P>

This program will display:

<BLOCKQUOTE>

<PRE>

inside:  firstVar = 100  secondVar = 0

inside:  firstVar = 99  secondVar = 1

...

inside:  firstVar = 2  secondVar = 98

inside:  firstVar = 1  secondVar = 99<BR>



</PRE>

</BLOCKQUOTE>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

The comma operator lets you use two expressions where Perl would normally let you have only one. The value of the statement becomes the value of the last expression evaluated.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

A more common use of the comma operator might be to initialize

some flag variables that you expect the loop to change. This next

example will read the first 50 lines of a file. If the end of

the file is reached before the last line is read, the <TT>$endOfFile</TT>

flag variable will be set to 1.

<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>Start the for loop by initializing the end of file flag variable

to zero to indicate false, then set </I><TT><I>$firstVar</I></TT><I>

to 0. The </I><TT><I>$firstVar</I></TT><I>

variable will be iNCremented each time the statement block is

executed. The statement block will be executed as long as </I><TT><I>$firstVar</I></TT><I>

is less than 50.<BR>

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

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

each time through the loop.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

for ($endOfFile = 0, $firstVar = 0; $firstVar &lt; 50; 

    $firstVar++, $secondVar++) {

    if (readLine() == 0)

        $endOfFile = 1;

}

</PRE>

</BLOCKQUOTE>

<P>

If the <TT>$endOfFile</TT> variable

is 1 when the loop ends, then you know the file has less than

50 lines.

<H3><A NAME="ExampleIForeachILoops">

Example: <I>Foreach</I> Loops</A></H3>

<P>

Arrays are so useful that Perl provides a special form of the

<TT>for</TT> statement just for them.

The <TT><I>foreach</I></TT> statement

is used solely to iterate over the elements of an array. It is

very handy for finding the largest element, printing the elements,

or simply seeing if a given value is a member of an array.

<BLOCKQUOTE>

<PRE>

foreach LOOP_VAR (ARRAY) {

    STATEMENTS

}

</PRE>

</BLOCKQUOTE>

<P>

The loop variable is assigned the value of each array element,

in turn until the end of the array is reached. Let's see how to

use the <TT>foreach</TT> statement

to find the largest array element.

<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>Call the </I><TT><I>max()</I></TT><I>

fuNCtion twice with different parameters each time.<BR>

Define the </I><TT><I>max()</I></TT><I>

fuNCtion.<BR>

Create a local variable, </I><TT><I>$max</I></TT><I>,

then get the first element from the parameter array.<BR>

Loop through the parameter array comparing each element to </I><TT><I>$max</I></TT><I>,if

the current element is greater than </I><TT><I>$max</I></TT><I>.

<BR>

Return the value of </I><TT><I>$max</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print max(45..121, 12..23) . &quot;\n&quot;;

print max(23..34, 356..564) . &quot;\n&quot;;



sub max {

    my($max) = shift(@_);



    foreach $temp (@_) {

        $max = $temp if $temp &gt; $max;

    }

    return($max);

}

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

121

564

</PRE>

</BLOCKQUOTE>

<P>

There are a couple of important things buried in this example.

One is the use of the <TT>shift()</TT>

fuNCtion to value a local variable <I>and </I>remove the first

element of the parameter array from the array at the same time.

If you use <TT>shift()</TT> all by

itself, the value of the first element is lost.

<P>

The other important thing is the use of $temp inside the <TT>foreach</TT>

loop. Some Perl programmers dislike using temporary variables

in this manner. Perl has an internal variable, <TT>$_</TT>,

that can be used instead. If no loop variable is specified, <TT>$_</TT>

will be assigned the value of each array element as the loop iterates.

<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>Print the return value from the </I><TT><I>max()</I></TT><I>

fuNCtion.<BR>

</I>Define the <TT><I>max()</I></TT>

fuNCtion.<BR>

<I>Create a local variable, </I><TT><I>$max</I></TT><I>,

then get the first element from the parameter array.<BR>

Loop through the parameter array comparing each element to </I><TT><I>$max</I></TT><I>,

if the current element is greater than </I><TT><I>$max</I></TT><I>:

<BR>

Return the value of </I><TT><I>$max</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

print max(45..121, 12..23) . &quot;\n&quot;;

print max(23..34, 356..564) . &quot;\n&quot;;



sub max {

    my($max) = shift(@_);



    foreach (@_) {

        $max = $_ if $_ &gt; $max;

    }

    return($max);

}

</PRE>

</BLOCKQUOTE>

<P>

The third item has nothing to do with the <TT>foreach</TT>

loop, at least not directly. But, this seems like a good time

to mention it. The statement inside the loop  also could be written

in the following way:

<BLOCKQUOTE>

<PRE>

$max = $_ if $max &lt; $_;

</PRE>

</BLOCKQUOTE>

<P>

with the sense of the operator reversed. However, notice that

it will take more effort to understand what the statement-as a

whole-is doing. The reader of your program knows that the fuNCtion

is looking for the greatest value in a list. If the less than

operator is used, it will contradict the stated purpose of your

fuNCtion-at least until the reader figures out the program logic.

Whenever possible, structure your program logic to agree with

the main premise of the fuNCtion.

<P>

Now for the fourth, and final, item regarding this small program.

Notice that the fuNCtion name and the local variable name are

the same except for the beginning dollar sign. This shows that

fuNCtion names and variable names use different namespaces.

<P>

Remember namespaces? They were mentioned in <A HREF="ch3.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch3.htm" >Chapter 3</A> &quot;Variables.&quot;

<P>

Using the <TT>foreach</TT> statement

requires using a little bit of caution because the local variable

(either <TT>$_</TT> or the one you

specify) accesses the array elements using the call by refereNCe

scheme. When call by refereNCe is used, changing the value in

one place (such as inside the loop) also changes the value in

the main program.

<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>Create an array from 1 to 10 with 5 repeated.<BR>

Print the array.<BR>

Loop through the array replacing any elements equal to 5 with

&quot;</I><TT><I>**</I></TT><I>&quot;.

<BR>

Print the array.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

@array = (1..5, 5..10);

print(&quot;@array\n&quot;);



foreach (@array) {

    $_ = &quot;**&quot; if ($_ == 5);

}

print(&quot;@array\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

1 2 3 4 5 5 6 7 8 9 10

1 2 3 4 ** ** 6 7 8 9 10<BR>



</PRE>

</BLOCKQUOTE>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

If you use the <TT>foreach</TT> loop to change the value of the array elements, be sure to comment your code to explain the situation and why this method was used.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

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

Jump Keywords</FONT></A></H2>

<P>

Perl has four keywords that let you change the flow of your programs.

Table 7.1 lists the keywords along with a short description.<BR>

<P>

<CENTER><B>Table 7.1&nbsp;&nbsp;Perl's Jump Keywords</B></CENTER>

<p>

<CENTER>

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

<TR><TD WIDTH=91><I>Keywords</I></TD><TD WIDTH=423><I>Description</I>

</TD></TR>

<TR><TD WIDTH=91><TT>last</TT></TD>

<TD WIDTH=423>Jumps out of the current statement block.</TD></TR>

<TR><TD WIDTH=91><TT>next</TT></TD>

<TD WIDTH=423>Skips the rest of the statement block and continues with the next iteration of the loop.

</TD></TR>

<TR><TD WIDTH=91><TT>redo</TT></TD>

<TD WIDTH=423>Restarts the statement block.</TD></TR>

<TR><TD WIDTH=91><TT>goto</TT></TD>

<TD WIDTH=423>Jumps to a specified label.</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

Each of these keywords is described further in its own section,

which follows.

<H3><A NAME="ExampleTheIlastIKeyword">

Example: The <I>last</I> Keyword</A></H3>

<P>

The <TT>last</TT> keyword is used

to exit from a statement block. This ability is useful if you

are searching an array for a value. When the value is found, you

can stop the loop early.

<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>Create an array holding all 26 letters.<BR>

Use a </I><TT><I>for</I></TT><I> loop

to iterate over the array. The index variable will start at zero

and iNCrement while it is less than the number of elements in

the array.<BR>

Test the array element to see if it is equal to &quot;</I><TT><I>T</I></TT><I>.&quot;

Notice that the string equality operator is used. If the array

element is &quot;</I><TT><I>T</I></TT><I>,&quot;

then exit the loop.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

@array = (&quot;A&quot;..&quot;Z&quot;);

for ($index = 0; $index &lt; @array; $index++) {

    if ($array[$index] eq &quot;T&quot;) {

        last;

    }

}

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

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

19

</PRE>

</BLOCKQUOTE>

<P>

This loop is straightforward except for the way that it calculates

the number of elements in the array. Inside the conditional expression,

the @array variable is evaluated in an scalar context. The result

is the number of elements in the array.

<P>

When the <TT>last</TT> keyword is

executed, the conditional expression and theiNCrement/decrement

expression are not reevaluated, the statement block is left. Execution

begins again immediately after the ending curly brace.

<P>

You also can use a label with the last keyword to indicate which

loop to exit. A <I>label</I> is a name followed by a colon. Labels'

names usually use all capital letters, but Perl does not insist

on it. When you need to exist a nested loop, labels are a big

help. Let's look at this situation in two steps. Here is a basic

loop:

<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>Loop from 0 to 10 using </I><TT><I>$index</I></TT><I>

as the loop variable.<BR>

If </I><TT><I>$index</I></TT><I> is

equal to 5 then exit the loop.<BR>

Print the value of </I><TT><I>$index</I></TT><I>

while inside the loop.<BR>

Print the value of </I><TT><I>$index</I></TT><I>

after the loop ends.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

for ($index = 0; $index &lt; 10; $index++) {

    if ($index == 5) {

        last;

    }

    print(&quot;loop: index = $index\n&quot;);

}

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

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

loop: index = 0

loop: index = 1

loop: index = 2

loop: index = 3

loop: index = 4

index = 5

</PRE>

</BLOCKQUOTE>

<P>

So far, pretty simple. The print statement inside the loop lets

us know that the <TT>$index</TT> variable

is being iNCremented. Now, let's add an inner loop to complicate