Undefined values

For languages which have an explicit notion of an undefined value, (as opposed to a NULL value), identify and exercise those language's mechanisms for identifying and manipulating a variable's value's status as being undefined

Undefined values
You are encouraged to solve this task according to the task description, using any language you may know.

Perl

<lang perl>#!/usr/bin/perl -w use strict;

1. Declare the variable

our $var;

1. Check to see whether it is defined

print "var is undefined at first check\n" unless defined $var;

1. Give it a value

$var = "Chocolate";

1. Check to see whether it is defined after we gave it the
2. value "Chocolate"

print "var is undefined at second check\n" unless defined $var;

1. Give the variable an undefined value.

$var = undef;

1. or, equivalently:

undef $var;

1. Check to see whether it is defined after we've explicitly
2. given it an undefined value.

print "var is undefined at third check\n" unless defined $var;

1. Give the variable a value of 42

$var = 42;

1. Check to see whether the it is defined after we've given it
2. the value 42.

print "var is undefined at fourth check\n" unless defined $var;

1. Because most of the output is conditional, this serves as
2. a clear indicator that the program has run to completion.

print "Done\n";</lang>

Results in:

var is undefined at first check
var is undefined at third check
Done

PHP

<lang php><?php // Check to see whether it is defined if (!isset($var))

   echo "var is undefined at first check\n";

// Give it a value $var = "Chocolate";

// Check to see whether it is defined after we gave it the // value "Chocolate" if (!isset($var))

   echo "var is undefined at second check\n";

// Give the variable an undefined value. unset($var);

// Check to see whether it is defined after we've explicitly // given it an undefined value. if (!isset($var))

   echo "var is undefined at third check\n";

// Give the variable a value of 42 $var = 42;

// Check to see whether the it is defined after we've given it // the value 42. if (!isset($var))

   echo "var is undefined at fourth check\n";

// Because most of the output is conditional, this serves as // a clear indicator that the program has run to completion. echo "Done\n"; ?></lang>

Results in:

var is undefined at first check
var is undefined at third check
Done