Perl array dumping

Question

I do this:

my @items = [];

sub pushItem {
   my $itemId = "i" . sprintf("%d", $_[0]);
   push(@items, $itemId);
}

sub popItems {
   $itemsXml = "<key>items</key>\n<array>\n";

   foreach (@items) {
      $itemsXml .= "<string>$_</string>\n";
   }

   $itemsXml .= "</array>\n";

   return $itemsXml;
}

pushItem(0);
pushItem(1);
pushItem(2);
print popItems();

I get this:

 <key>items</key>
 <array>
 <string>ARRAY(0x7fa1730070d0)</string>
 <string>i0</string>
 <string>i1</string>
 <string>i2</string>
 </array>

The problem of course being:

<string>ARRAY(0x7fa1730070d0)</string>

Miguel Prz · Accepted Answer · 2013-03-06 22:09:04Z

5

my @items = [];

should be

my @items = ();

answered Mar 6, 2013 at 22:09

Miguel Prz

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ikegami · Accepted Answer · 2013-03-06 22:21:42Z

3

[] creates an array and a reference to that array, and returns the latter. This means the following populates @items with a reference to a second array in addition to creating @items:

 my @items = [];

You simply want

 my @items;

If you insist on assigning something to it, you'd want to assign a list of zero scalars to it, and you can do that as follows:

 my @items = ();

But really, that's just a waste (since you're clearing an empty array) and it adds needless complexity (code that does nothing).

Just to be clear, @items = (); (as opposed to my @items = ();) is still useful, though only rarely. I think the only time I've used it is in code of the following form:

my @buf;
while (my $item = get()) {
   if (check($item)) {
      something(@buf);
      @buf = ();
   }

   push @buf, $item;
}

something(@buf) if @buf;

edited Mar 6, 2013 at 22:21

answered Mar 6, 2013 at 22:12

ikegami

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4 Comments

QED Over a year ago

I am clearing the array later, so @items = () comes in handy. Thanks! I'm accepting yours for the explanation and because Miguel has lots up of upvotes already. Thanks to both!

ikegami Over a year ago

Yes, @items = (); is handy (though it's very very very rare to need that). my @items = (); is not handy since it clears an array you've just created (so it's already clear).

ikegami Over a year ago

Added some clarification to my answer.

QED Over a year ago

Thank you! Now my plist is beautiful. Just in time to deprecate it and go to JSON...

theta-fish · Accepted Answer · 2013-03-06 22:15:28Z

2

Your problem is your first line:

my @items = [];

It should be:

my @items = ();

The problem is that you have assigned an array reference to the first element of the array @items, and then later you push a bunch of other values onto it.

If you initialize it using the parens instead, it does what you want, which is create an empty array. This becomes more apparent if you comment out your calls to pushItem and run the script.

answered Mar 6, 2013 at 22:15

theta-fish

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