var myHashTable = new Dictionary<TKey, TValue>

Where TKey is the type of your key (possibly an int to represent the Id of your object or string for a username) and TValue is the type of the value. I hope that saves you some time!

Sorting by a Hash Key

The answer is really quite simple and very elegant. Let’s build up an example.

my @data;
push @data, { name => "Item A", price => 9.99 };
push @data, { name => "Item B", price => 4.99 };
push @data, { name => "Item C", price => 7.5};

We now have an array with 3 hashes in. Suppose we want to sort by price, how do we do it?

my @sorted =  sort { $a->{price} <=> $b->{price} } @data;

Perl’s built in sort function allows us to specify a custom sort order. Within the curly braces Perl gives us 2 variables, $a and $b, which reference 2 items to compare. In our case, these are hash references so we can access the elements of the hash and sort by any key we want. A description of the <=> operator can be found on Perlfect. Suppose we want to sort in reverse order?

my @sorted =  sort { $b->{price} <=> $a->{price} } @data;

Simple. You can test this by adding the following line to the bottom of the script.

print join "n", map {$_->{name}." - ".$_->{price}} @sorted;

Which gives us:

Item B - 4.99
Item C - 7.5
Item A - 9.99

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Not terribly helpful or enlightening – so what do you do? Well, you can sit with a terminal window open tailing the http error log as you refresh the page to find out:

-$ tail -f error.log

But that’s a slow and painful experience. What we really want is to print the error message straight to the browser whilst we are developing. In steps the CGI::Carp module:

#!/usr/bin/perl
use strict;
use warnings
use CGI::Carp qw( fatalsToBrowser );

print "Content-type: text/html/n/n";
print "<h1>Hello World </h1
print "I wont work";

The fatalsToBrowser function will take over and print the error message straight to the browser window:

Software error:

syntax error at dummy.cgi line 9, near "print
Execution of dummy.cgi aborted due to compilation errors.

This is a much more useful error message and will help with debugging. Remember to remove the fatalsToBrowser call before releasing your application to production else you could potentially expose sensitive details to hackers.

You can read the manual pages for cron or type ‘man crontab’ at the command line for more information.

Using crontab

To view the current state of a crontab you need to specify the -l option. You can view another users crontab by specifying -u username (you’ll need to be root to do this):

user$ crontab -l [-u username]

To edit the state of a crontab you need to use the -e flag:

user$ crontab -e [-u username]

The Format of Crontab

When you edit a user’s crontab for the first time you will presented with an empty file – this is a daunting prospect for the cron newbie! Here’s a minimal example crontab to get you started:

MAILTO=adam@example.com

#minute hour monthday month weekday command
30 12 * * * echo "hello world!"

The first line sets the MAILTO environment variable. Any output that would normally have been printed to STDOUT by the processes will be emailed to this address. We then have a comment so we can remember the order of the variables. Finally, the 3rd line defines what to run and when. In this case, at 12:30 every day of every month we will echo “Hello World!”.

Scheduling a Task

Crontab gives us a very flexible syntax for defining when our commands will run. For each of the 5 periods we can use any of the following notations:

• * – every value
• # – one given value e.g. 5
• */# – every value that divides by the number e.g. */5 will run at 5,10,15,20…
• #,#,# – we can specify a comma separated list of values e.g. 5,20,24
• #-# – we can also use ranges of values e.g. 2-5

For each field the following values are valid:

• minute = 0-59
• hour = 0-23
• monthday = 1-31
• month = 1-12
• weekday = 0-7 (0 or 7 is Sun)

Installing a Crontab

When you save and exit from editing the crontab the system will parse the contents and schedule the tasks to run.

Further Options

As well as setting the MAILTO flag, you can set any environment variables you need within the crontab file. For example, if you need to add to the path:

PATH=$PATH:/my/new/dir

These declarations are read in order and can be redefined to create sections within the crontab. For example, if you have 1 job that needs to email sales@example.com and another that needs to email sysadmin@example.com, then you can use the following file structure:

MAILTO=sales@example.com

#minute hour monthday month weekday command
30 12 * * * /path/to/my/command
MAILTO=sales@example.com
1 * 1 * * /path/to/my/othercommand

The Rand() Function

The simplest use of rand(), with no parameters ( This is analogous to calling rand(1) ), generates a double precision number between 0 and 1.

#!/usr/bin/perl
use strict
use warnings;

my $rndnum = rand()
print $rndnum;

Will give you a result like: 0.186135607379587

Rand() with a Range

You can generate a bigger random number if you call rand($int) where $int is the maximum value you want. For example, to generate a number between 0 and 10 we could use:

#!/usr/bin/perl
use strict
use warnings;

my $rndnum = rand(10)
print $rndnum;

This will give a result like: 5.9358180787331.

Alternatively, we could call rand() and multiply by the maximum value:

#!/usr/bin/perl
use warnings
use strict;

my $rndnum = rand() * 10
print $rndnum;

Which returns the result: 1.65611195917339.

Generating a Random Integer

If we want to generate a random integer, to use an array index for example, then we need to use another function to round our double. We have 3 choices: int(), ceil() and floor(). int() will return the integer part of an expression, ceil() will round up to the next integer and floor() will round down.

#!/usr/bin/perl
use strict
use warnings;

my $rndnum = rand(10)
print "$rndnum => ". int($rndnum);

Gives us: 4.03471924205409 => 4

Ceil and Round require the POSIX module:

#!/usr/bin/perl
use strict
use warnings;
use POSIX;

my $rndnum = rand(10)
print "$rndnum => ". ceil($rndnum);

Gives us: 4.03471924205409 =>5

<MTSetVarBlock name="category"><MTEntryCategory></MTSetVarBlock>

<h3>Historic Data</h3>

<$MTSetVar name="year" value="0"$>

<MTEntries sort_order="descend" category="$category">

<MTIfNotEqual a="[MTEntryDate format='%Y']" b="[MTGetVar name='year']">
  <MTIfNotEqual a="[MTGetVar name='year']" b="0"></ul></MTIfNotEqual>
  <h4><$MTEntryDate format="%Y"$></h4><ul>
  <MTSetVarBlock name="year"><$MTEntryDate format="%Y"$></MTSetVarBlock>
</MTIfNotEqual>

<li><a href="<$MTEntryPermalink$>"><$MTEntryTitle$></a></li>

</MTEntries>

This code sets a variable, $category, containing the name of the category associated with the current entry. It then selects all the entries in this category in descending date order. A $year variable then tracks the year of the last entry, printing out a heading when the year changes.