ch9.htm

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

</BLOCKQUOTE>

<P>

With binary mode on, bytes with a value of 26 have no special

meaning and the third line can be read. You see that the value

26-33 in hexadecimal-was printed along with the rest of the characters.

<P>

<TT>Example: Reading into a Hash</TT>

<P>

You've already seen that you can read a file directly into a regular

array using this syntax:

<BLOCKQUOTE>

<PRE>

@array = &lt;FILE_HANDLE&gt;;

</PRE>

</BLOCKQUOTE>

<P>

Unfortunately, there is no similar way to read an entire file

into a hash. But, it's still pretty easy to do. The following

example will use the line number as the hash key for each line

of a file.

<P>

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

<BLOCKQUOTE>

<I>Open the </I><TT><I>FIXED.DAT</I></TT><I>

file for reading.<BR>

For each line of </I><TT><I>FIXED.DAT </I></TT><I>create

a hash element using the record number special variable (</I><TT><I>$.</I></TT><I>)

as the key and the line of input (</I><TT><I>$_</I></TT><I>)

as the value.<BR>

Close the file.<BR>

Iterate over the keys of the hash.<BR>

Print each key, value pair.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 9.12&nbsp;&nbsp;09LST12.PL-Reading a Fixed Length Record

with Fixed Length Fields into a Hash<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

open(FILE, &quot;&lt;fixed.dat&quot;);

while (&lt;FILE&gt;) {

    $hash{$.} = $_;

}

close(FILE);



foreach (keys %hash) {

    print(&quot;$_: $hash{$_}&quot;);

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

This program displays:<BR>

<BLOCKQUOTE>

<PRE>

1: 1212Jan       Jaspree             Painter

2: 3453Kelly     Horton              Jockey

</PRE>

</BLOCKQUOTE>

<H3><A NAME="ExampleGettingFileStatistics">

Example: Getting File Statistics</A></H3>

<P>

The file test operators can tell you a lot about a file, but sometimes

you need more. In those cases, you use the <TT>stat()</TT>

or <TT>lstat()</TT> fuNCtion. The

<TT>stat()</TT> returns file information

in a 13-element array. You can pass either a file handle or a

file name as the parameter. If the file can't be found or another

error occurs, the null list is returned. Listing 9.13 shows how

to use the <TT>stat()</TT> fuNCtion

to find out information about the <TT>EOF.DAT</TT>

file used earlier in the chapter.

<P>

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

<BLOCKQUOTE>

<I>Assign the return list from the </I><TT><I>stat()</I></TT><I>

fuNCtion to 13 scalar variables.<BR>

Print the scalar values.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 9.13&nbsp;&nbsp;09LST13.PL-Using the </B><TT><I><B><FONT FACE="Courier">stat()</FONT></B></I></TT><B>

FuNCtion<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

($dev, $ino, $mode, $nlink, $uid, $gid, $rdev, $size,

    $atime, $mtime, $ctime, $blksize, $blocks) = stat(&quot;eof.dat&quot;);



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

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

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

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

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

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

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

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

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

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

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

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

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

</PRE>

</BLOCKQUOTE>

<HR>

<P>

In the DOS environment, this program displays:

<BLOCKQUOTE>

<PRE>

dev     = 2

ino     = 0

mode    = 33206

nlink   = 1

uid     = 0

gid     = 0

rdev    = 2

size    = 13

atime   = 833137200

mtime   = 833195316

ctime   = 833194411

blksize =

blocks  =

</PRE>

</BLOCKQUOTE>

<P>

Some of this information is specific to the UNIX environment and

is beyond the scope of this book. For more information on this

topic, see Que's 1994 edition of <I>Using Unix</I>. One interesting

piece of information is the <TT>$mtime</TT>

value-the date and time of the last modification made to the file.

You can interpret this value by using the following line of code:

<BLOCKQUOTE>

<PRE>

($sec, $min, $hr, $day, $month, $year, $day_Of_Week, 

    $julianDate, $dst) = localtime($mtime);

</PRE>

</BLOCKQUOTE>

<P>

If you are only interested in the modification date, you can use

the array slice notation to just grab that value from the 13-element

array returned by <TT>stat()</TT>.

For example:

<BLOCKQUOTE>

<PRE>

$mtime = (stat(&quot;eof.dat&quot;))[9];

</PRE>

</BLOCKQUOTE>

<P>

Notice that the <TT>stat()</TT> fuNCtion

is surrounded by parentheses so that the return value is evaluated

in an array context. Then the tenth element is assigned to <TT>$mtime</TT>.

You can use this technique whenever a fuNCtion returns a list.

<H3><A NAME="ExampleUsingtheDirectoryFuNCtions">

Example: Using the Directory FuNCtions</A></H3>

<P>

Perl has several fuNCtions that let you work with directories.

You can make a directory with the <TT>mkdir()</TT>

fuNCtion. You can delete a directory with the <TT>rmdir()</TT>

fuNCtion. Switching from the current directory to another is done

using the <TT>chdir()</TT> fuNCtion.

<P>

Finding out which files are in a directory is done with the <TT>opendir()</TT>,

<TT>readdir()</TT>, and <TT>closedir()</TT>

fuNCtions. The next example will show you how to create a list

of all Perl programs in the current directory-well, at least those

files that end with the pl extension.

<P>

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

<BLOCKQUOTE>

<I>Open the current directory using </I><TT><I>DIR</I></TT><I>

as the directory handle.<BR>

Read a list of file names using the </I><TT><I>readdir()</I></TT><I>

fuNCtion; extract only those that end in </I><TT><I>pl</I></TT><I>;

and the sorted list. The sorted list is assigned to the </I><TT><I>@files</I></TT><I>

array variable.<BR>

Close the directory.<BR>

Print the file names from the </I><TT><I>@files</I></TT><I>

array unless the file is a directory.</I>

</BLOCKQUOTE>

<HR>

<BLOCKQUOTE>

<B>Listing 9.14&nbsp;&nbsp;09LST14.PL-Print All Files in the Current

Directory Whose Name Ends in PL<BR>

</B>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

opendir(DIR, &quot;.&quot;);

@files = sort(grep(/pl$/, readdir(DIR)));

closedir(DIR);



foreach (@files) {

    print(&quot;$_\n&quot;) unless -d;

}

</PRE>

</BLOCKQUOTE>

<HR>

<P>

For more information about the grep() fuNCtion, see <A HREF="ch10.htm" >Chapter 10</A>,

&quot;Regular Expressions.&quot; 

<P>

This program will display each file name that ends in <TT>pl</TT>

on a separate line. If you need to know the number of Perl programs,

evaluate the <TT>@files</TT> array

in a scalar context. For example:

<BLOCKQUOTE>

<PRE>

$num_Perl_Programs = @files;<BR>



</PRE>

</BLOCKQUOTE>

<p>

<CENTER>

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

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

<TR><TD>

<BLOCKQUOTE>

For this example, I modified the naming convention used for the variables. I feel that <TT>$num_Perl_Programs</TT> is easier to read than <TT>$numPerlPrograms</TT>. No naming convention should be inflexible. Use it as a guideline and break the rules when 
it seems wise.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="ExamplePrintingRevisited">

Example: Printing Revisited</A></H3>

<P>

We've been using the <TT>print()</TT>

fuNCtion throughout this book without really looking at how it

works. Let's remedy that now.

<P>

The <TT>print()</TT> fuNCtion is used

to send output to a file handle. Most of the time, we've been

using <TT>STDOUT</TT> as the file

handle. Because <TT>STDOUT</TT> is

the default, we did not need to specify it. The syntax for the

<TT>print()</TT> fuNCtion is:

<BLOCKQUOTE>

<PRE>

print FILE_HANDLE (LIST)

</PRE>

</BLOCKQUOTE>

<P>

You can see from the syntax that <TT>print()</TT>

is a list operator because it's looking for a list of values to

print. If you don't specify a list, then $  will be used. You

can change the default file handle by using the <TT>select()</TT>

fuNCtion. Let's take a look at this:

<P>

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

<BLOCKQUOTE>

<I>Open </I><TT><I>TESTFILE.DAT</I></TT><I>

for output.<BR>

Change the default file handle for write and print statements.

Notice that the old default handle is returned and saved in the

</I><TT><I>$oldHandle</I></TT><I>

variable.<BR>

This line prints to the default handle which now the </I><TT><I>TESTFILE.DAT</I></TT><I>

file.<BR>

Change the default file handle back to </I><TT><I>STDOUT</I></TT><I>.

<BR>

This line prints to </I><TT><I>STDOUT</I></TT><I>.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

open(OUTPUT_FILE, &quot;&gt;testfile.dat&quot;);

$oldHandle = select(OUTPUT_FILE);

print(&quot;This is line 1.\n&quot;);

select($oldHandle);

print(&quot;This is line 2.\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

This is line 2.

</PRE>

</BLOCKQUOTE>

<P>

and creates the TESTFILE.DAT file with a single line in it:

<BLOCKQUOTE>

<PRE>

This is line 1.

</PRE>

</BLOCKQUOTE>

<P>

Perl also has the <TT>printf()</TT>

fuNCtion which lets you be more precise in how things are printed

out. The syntax for <TT>printf()</TT>

looks like this:

<BLOCKQUOTE>

<PRE>

printf FILE_HANDLE (FORMAT_STRING, LIST)

</PRE>

</BLOCKQUOTE>

<P>

Like <TT>print()</TT>, the default

file handle is <TT>STDOUT</TT>. The

<TT>FORMAT_STRING</TT> parameter controls

what is printed and how it looks. For simple cases, the formatting

parameter looks identical to the list that is passed to <TT>printf()</TT>.

For example:

<P>

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

<BLOCKQUOTE>

<I>Create two variables to hold costs for January and February.

<BR>

Print the cost variables using variable interpolation. Notice

that the dollar sign needs to be preceded by the backslash to

avoid interpolation that you don't want.</I>

</BLOCKQUOTE>

<BLOCKQUOTE>

<PRE>

$januaryCost = 123.34;

$februaryCost = 23345.45;



printf(&quot;January  = \$$januaryCost\n&quot;);

printf(&quot;February = \$$februaryCost\n&quot;);

</PRE>

</BLOCKQUOTE>

<P>

This program displays:

<BLOCKQUOTE>

<PRE>

January  = $123.34

February = $23345.45

</PRE>

</BLOCKQUOTE>

<P>

In this example, only one parameter is passed to the <TT>printf()</TT>

fuNCtion-the formatting string. Because the formatting string

is eNClosed in double quotes, variable interpolation will take

place just like for the <TT>print()</TT>

fuNCtion.

<P>

This display is not good enough for a report because the decimal

points of the numbers do not line up. You can use the formatting

specifiers shown in Table 9.5 together with the modifiers shown

in Table 9.6 to solve this problem.<BR>

<P>

<B>Table 9.5&nbsp;&nbsp; Format Specifiers for the printf() FuNCtion</B>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=