>>>>> "sw" == shawn wilson <ag4ve...@gmail.com> writes:
sw> On Tue, Jan 11, 2011 at 4:57 AM, Sean Murphy <mhysnm1...@gmail.com> wrote:
>> Hi all.
>>
>> I have read the explaination of the Map function and it is still a mystry
to
>> myself on what it is for and when would you use it?
>>
>> All explainations I have seen in books and blogs don't make it clear.
>>
sw> my @new = map {$_} @arr;
sw> is the same as:
sw> foreach (@arr) {
sw> push $_, @new;
sw> }
sw> in other words, do something to every element of an array.
that is a poor explanation IMO.
map transforms one list into another. it is that simple but it is very
useful. you can alway write map in terms of a perl loop but it will be
slower and clunkier in most cases.
map take an input list, it aliases $_ to each element (an implied loop)
and then executes the block or expression. it collects the results of
that code and builds up a list which it returns when the input elements
are exhausted. the key to understanding map is that the expression can
return 0, 1 or more elements EACH time through. so the complete map call
can return ANY number of elements, regardless of the number of input
elements. if the expression returns 0 or 1 elements, it is similar to
perl's grep:
these are equivilent:
my @out = grep /foo/, @in ;
my @out = map { /foo/ ? $_ : () } @in ;
but map can do much more than grep. grep can ONLY return its input list
or some elements of it. map can generate more elements:
this builds a hash of 3 keys, each which has the value 1. it can be used
to test if something is one of the keys with a simple boolean.
my %is_a_foo = map { $_ => 1 } qw( foo bar baz) ;
if ( $is_a_foo( $stuff ) { ....
map is very simple when you really get it and very useful. it should be
in all perl hacker's skill sets. for some reason newbies have trouble
with it but that is usually due to poor explanations and not knowing
about functional programming (map comes from lisp which has similar map
funcs). map's name comes from its role in mapping one list into
another. that should help too.
uri
--
Uri Guttman ------ u...@stemsystems.com -------- http://www.sysarch.com --
----- Perl Code Review , Architecture, Development, Training, Support ------
--------- Gourmet Hot Cocoa Mix ---- http://bestfriendscocoa.com ---------
--
To unsubscribe, e-mail: beginners-unsubscr...@perl.org
For additional commands, e-mail: beginners-h...@perl.org
http://learn.perl.org/
