Ha ha, nysus just reminded me of an old interview question I used to ask. Implement a simple dispatch table.

Let's start with a specification:

• The key of the dispatch table is a string \w+
• The name of the callback function is the key name with _callback appended
• Each callback function takes a single string parameter and returns a positive number

You must write the invoker function, which takes two arguments (the name and the string argument to be passed to the callback):

• If the name is invalid (e.g. "fred" below), invoker must return a negative number
• Otherwise, invoker must pass its second argument to the callback function and return what the callback function returns

To clarify, here is a sample implementation.

use strict;
use warnings;

# Callback functions ---------------------------------------

sub first_callback {
   my $z = shift;
   print "in first_callback, z=$z\n";
   return 1;
}

sub last_callback {
   my $z = shift;
   print "in last_callback, z=$z\n";
   return 2;
}

# Implementation of dispatch table -------------------------
# (You need to write this code)

my %op_table = ( first  => \&first_callback,
                 last   => \&last_callback,
               );

sub invoker
{
   my ($name, $z) = @_;
   exists($op_table{$name}) or return -1;
   $op_table{$name}->($z);
}

# Main program for testing ---------------------------------

for my $name ( "first", "last", "fred" ) {
   my $rc = invoker( $name, $name . '-arg' );
   print "$name: rc=$rc\n";
}
[download]

Running the above test program produces:

in first_callback, z=first-arg
first: rc=1
in last_callback, z=last-arg
last: rc=2
fred: rc=-1
[download]

Points to consider:

• Is a hash the recommended way to implement a dispatch table in Perl?
• How many other ways can you think of to implement it in Perl? (working demonstration code would be good)

For more fun, feel free to implement the above specification in another language of your choice.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.