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copied!<p>Perl doesn't have <code>null</code>, but it has <code>undef</code> – which incidentally is the default value for unassigned elements in arrays:</p> <pre><code>my @array = (undef, undef, 32, 54, undef, 59); </code></pre> <p>creates the same structure as:</p> <pre><code>my @array; $array[2] = 32; $array[3] = 54; $array[5] = 59; </code></pre> <p>There is no need to assign the other fields to <code>undef</code> yourself.</p> <p>If you are trying to assign a default value to elements which are currently <code>undef</code>, you could write something like this (but note the limitations below):</p> <pre><code>defined or $_ = "default" for @array; </code></pre> <p>On Perl 5.10 or later, you can use the <code>//</code> defined-or operator:</p> <pre><code>$_ //= "default" for @array </code></pre> <p>To assign defaults for multiple arrays:</p> <pre><code>$_ //= "default" for @array, @other_array </code></pre> <p>If you want to set an array to a specific length, you can do <code>$#array = $length - 1</code>, so this actually specifies the highest index. This removes entries from longer arrays. For shorter arrays, the newly created entries will all be <code>undef</code>.</p> <hr> <p>There is a small problem with this: Perl has two kinds of <code>undef</code>:</p> <ul> <li><p>Scalars can contain the value “undef” like <code>my $foo = undef</code>.</p> <p>This is the case if we initialize the whole array at once like <code>my @array = (undef, undef, 32, 54, undef, 59)</code>.</p></li> <li><p>Unassigned values in arrays or hashes <em>share</em> their <code>undef</code> scalar, which is read-only.</p> <p>This is the case when we initialize the array by assigning some indices only, like <code>$array[4] = 2</code>.</p></li> </ul> <p>Usually this is no problem, but in a for-loop, the <code>$_</code> is an <em>alias</em> to the current scalar, which in our case can be readonly. Therefore we can't always do <code>$_ //= "default" for @array</code> but must either:</p> <ul> <li><p>Assign directly into the array element, which creates an assignable scalar in that slot:</p> <pre><code>$array[$_] //= "default" for 0 .. $#array; </code></pre> <p>For multiple arrays:</p> <pre><code>for my $ref (\@array, \@other_array) { $ref->[$_] //= "default" for 0 .. $#$ref; } </code></pre></li> <li><p>Don't assign to the element and create a copy of the array with defaults instead:</p> <pre><code>@array = map { $_ // "default" } @array; </code></pre> <p>For multiple arrays:</p> <pre><code>@$_ = map { $_ // "default" } @$_ for \@array, \@other_array; </code></pre> <p>While I perceive this as more elegant, there can be some problems with this as each array element is copied.</p></li> </ul>

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