Learning Perl Objects, References amp;amp; Modules [Electronic resources] نسخه متنی

7.4 The Schwartzian Transform

The
intermediate variables between each of these steps were not
necessary, except as input to the next step. You can save yourself
some brainpower by just stacking all the steps together:

my @names =
map $_->[0],
sort { $b->[1] <=> $a->[1] }
map [ $_, ask_monkey_about($_) ],
@castaways;

Because the map and sort
operators are right to left, you have to read this construct from the
bottom up. Take a list of @castaways, create some
arrayrefs by asking the monkey a simple question, sort the list of
arrayrefs, and then extract the names from each arrayref. This gives
you the list of names in the desired order.

This construct is commonly called the Schwartzian
Transform, which was named after me (but not by me),
thanks to a Usenet posting I made many years ago. It has proven to be
a very nice thing to have in your bag of sorting tricks.

If this transform looks like it might be too complex to memorize or
come up with from first principles, it might help to look at the
flexible and constant parts:

my @output_data =
map $_->[0],
sort { SORT COMPARISON USING $a->[1] AND $b->[1] }
map [ $_, EXPENSIVE FUNCTION OF $_ ],
@input_data;

The basic structure maps the
original list into a list of arrayrefs, computing the expensive
function only once for each; sorts those array refs, looking at the
cached value of each expensive function invocation;[5] and then extracts the original values back out in the new
order. All you have to do is plug in the proper two operations, and
you're done.

[5] An expensive operation is one that takes a
relatively long time or a relatively large amount of memory.