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List::Util & Friends - 2011-12-10

Perl Is Not Enough

Perl has lots of useful built-in functions. Everybody loves grep, right?


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my @coal_kids = grep { $_->is_naughty } @children;
 

Maybe we only want the first element, though. There's more than one way to do it – here are two:


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# This works just fine, but you might be waiting a while, since it's going to
# evaluate the block for every item in the list.
my ($first_coal_goes_to) = grep { $_->is_naughty } @children;

# This is a lot more efficient to run, but not to write. Worse, it's not
# anywhere near as efficient to skim.
my $first_coal_goes_to;
for (@children) {
  next unless $_->is_naughty;
  $first_coal_goes_to = $_;
  last;
}

 

Fortunately, we've got List::Util:


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use List::Util qw(first);

my $first_coal_goes_to = first { $_->is_naughty } @children;

 

It's easy to read, easy to write, and works efficiently, stopping at the first hit.

List::Util has a bunch of useful stuff like this. It has routines for summing up numbers, or finding maxima and minima. It's got a shuffle routine for randomizing lists, which you should use. I can't tell you how many horrible reimplementations of List::Util's shuffle I've deleted over the years.

Even more importantly, though, it provides a reduce method. reduce is a really common higher-order function, often called fold. You can use it to build lots of other really useful behavior. For example, we might use it to reimplement sum, since the one that comes with List::Util stinks¹.

reduce is called with a function (usually written as a block) and a list of inputs. If there's only one item in the list, it returns that. Otherwise, it calls the function with the first two items and makes the result the new head of the list. Confused?


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my @numbers = get_numbers;
my $sum = reduce { $a + $b } (0, @numbers);

 

If @numbers is empty, we return 0. If @numbers contains (1, 2, 3), this is what happens:

  1. We have more than one item in the list! Our input is (0, 1, 2, 3)

  2. Shift off 0 and 1, and unshift the result back onto the input.

  3. Now our input is (1, 2, 3)

  4. Shift off 1 and 2, and unshift the result back onto the input.

  5. Now our input is (3, 3)

  6. Shift off 3 and 3, and unshift the result back onto the input.

  7. Now our input is (6)

  8. There's only one input! Return it!

Lots of stuff can get implemented in terms of reduce. Knowing how (and when) to use it is really useful. In fact, the List::Util documentation points out how several of its functions could have been written as folding constructs instead.

List::Util is Not Enough

List::Util's documentation also lists a few things that people have proposed for inclusion over the years, like any:


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sub any { $_ && return 1 for @_; 0 }
 

The docs say that any has been omitted because it's so easy to write inline (especially if you have first). Still, why not make them available? List::MoreUtils does just that, providing all the routines List::Util declined to and much more.


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# Note that you couldn't do this as easily with first, because it would
# return undef on both success and failure. Also note that List::MoreUtil's
# "any" takes a block, not just a list.
if (any { ! defined } @input_values) {
  ...
}

# Get every element until the block returns true.
my @first_half_alphabet = before { $_->last_name =~ /^N/i } @children;

 

List::MoreUtils also helps get rid of many occurances of %seen, one of my least favorite often-seen variables:


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# Don't write:
my @unique_guesses = do { my %seen = map {; $_ => 1 } @guesses; keys %seen };

# Write:
my @unique_guesses = uniq @guesses;

 

And it provides the terribly-named natatime for processing n elements at a time:


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my $iterator = natatime 2, @pairs;
while (my ($k, $v) = $iterator->()) {
  print "$key has value $v";
}

 

There's a Lot More

I've only given a very thin gloss over List::Util, and barely scratched the surface of List::MoreUtils. The point is that these two libraries are useful all the time. If you haven't used them yet, it's almost certain that you could've saved time by using them a few times. Learn what's in them, and consider them a tool you can reach for without thinking about it.

In fact, to help you not have to think about it, don't use either of them! Instead, use List::AllUtils, which provides all the routines of the other two, combined into one library so you don't have to try to remember whether minmax is from Util or MoreUtils².

Footnotes

  1. List::Util's sum returns undef, instead of 0, for an empty input list.

  2. min and max are from Util, but minmax is from MoreUtils

See Also

Gravatar Image This article contributed by: Ricardo Signes <rjbs@cpan.org>