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Re-active formatting
File Handling example from a Well House Consultants training course
More on File Handling [link]

This example is described in the following article(s):
   • Reactive (dynamic) formatting in Perl - [link]

Source code: wacsal Module: P207

=head1 Dynamically Altering a format to suite data

In this example program, we're reading a web access
log file and counting the number of visits from each
client. We want to produce a table of the results -
neatly in column, but with the columns as narrow as

Method ...

a) Loop through the data that WILL be printed in each
column, finding the longest string

b) use (s)printf with a format string that includes
information gathered in step (a)

Note - if you have a variable containing data that
you want to write up against a letter, you can use
curly braces to delimit the variable name. In this
example, I have a variable called $longhost containing
the value (say) 9, and I want to produce "%9s".
However, if I write "%$longhosts", my program will
look for a variable called $longhosts. The solution
id to write "%${longhost}s".


open (FH,"../Downloads/access_log.xyz") or die;
while ($line = <FH>) {
        # Host name is first field on line

# Find longest host name and longest number
# so that we can dynamically alter the format in printf
$longhost = 1; $longnumber = 1;
foreach $host (keys %counter) {
        if (length($host) > $longhost) {
                $longhost = length($host); }
        if (length("$counter{$host}") > $longnumber) {
                $longnumber = length("$counter{$host}"); }

# Sort hosts by count of accesses, largest number first
@hoho = sort {$counter{$b} <=> $counter{$a}} (keys %counter);

# Top ten hosts
foreach $host (@hoho[0..9]) {
        printf("%-${longhost}s %${longnumber}d\n",$host,

# And the rest
foreach $host (@hoho[10..$#hoho]) {
        printf("%-${longhost}s ",$host);
        # Add a \n every fourth host and at the very end
        print ("\n") if (++$n%4==0 or $host eq $hoho[-1]);


Sample output:

Dorothy:p82 grahamellis$ perl wacsal
skate 1995
whale 1044
o_whelk 458
perch 391
sardine 348
sealion 324
flipper 322
aviemore 318
seal 313
magnet 312
walrus trout mussel plaice
tuna clam lecht dogfish
dolphin catfish seaweed whiting
localhost manatee dab pilchard
Wed whelk
Dorothy:p82 grahamellis$

Learn about this subject
This module and example are covered on the following public courses:
 * Perl Programming
 * Perl bootcamp
 * Learning to program in Perl
 * Perl Programming
Also available on on site courses for larger groups

Books covering this topic
Yes. We have over 700 books in our library. Books covering Perl are listed here and when you've selected a relevant book we'll link you on to Amazon to order.

Other Examples
This example comes from our "File Handling" training module. You'll find a description of the topic and some other closely related examples on the "File Handling" module index page.

Full description of the source code
You can learn more about this example on the training courses listed on this page, on which you'll be given a full set of training notes.

Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

Other resources
• Our Solutions centre provides a number of longer technical articles.
• Our Opentalk forum archive provides a question and answer centre.
The Horse's mouth provides a daily tip or thought.
• Further resources are available via the resources centre.
• All of these resources can be searched through through our search engine
• And there's a global index here.

Purpose of this website
This is a sample program, class demonstration or answer from a training course. It's main purpose is to provide an after-course service to customers who have attended our public private or on site courses, but the examples are made generally available under conditions described below.

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This web site is written and maintained by Well House Consultants.

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Past attendees on our training courses are welcome to use individual examples in the course of their programming, but must check the examples they use to ensure that they are suitable for their job. Remember that some of our examples show you how not to do things - check in your notes. Well House Consultants take no responsibility for the suitability of these example programs to customer's needs.

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