The easiest way to think of map is to reduce it to something a bit
more familiar.
@result = map SOME_EXPRESSION, @input;
can be replaced with:
@TEMP = ();
foreach $_ (@input) {
push @TEMP, SOME_EXPRESSION;
}
@result = @TEMP;
In other words, evaluate SOME_EXPRESSION for each @input item, and take
the result(s) of that, concatenated together, to make an output. While
the expression is being evaluated, $_ contains the "current item" from @input.
-- Randal L. Schwartz, Perl hacker | [reply] [Watch: Dir/Any] [d/l] [select] |
This should probably be a Q&A but, "I understand your pain" in regards to map. Up until _very_ recently I didn't really understand it either (probably still don't -not like the true monks) but here goes:
Map executes a block of code for each element in a list and returns a list of the results. The magical variable $_ is set (actually localized so it isn't clobbered outside the block) to each element within the block. Map is _extremely_ powerful when in the hands of the likes of merlyn and friends. (i.e. Schwartzian Transform)
A couple (simple) examples to illustrate:
copy a list (i.e. do nothing same as @b = @a;)
@b = map {$_} @a;
Take a list of numbers (a vector) square them, add them together and take the square root (Euclidian N-dimensional distance):
$dist = sqrt(eval join("+",map {$_**2} @a));
Explanation of that last one:
map squares each element in @a (say 1,2,3,4,5) and returns a new list (1,4,9,16,25) then join makes a scalar "1+4+9+16+25", the eval makes it 55 and finally sqrt returns 7.41619... which is assigned to $dist.
For another example see A minor epiphany with grep and map as well as every piece of documentation on the topic you can get your hands on. I think what did it for me (finally) was
Effective Perl Programming by Joseph N. Hall with Randal L. Schwartz. The section (Item 12) is only 4 pages long and also covers grep and foreach but, it was the lightswitch for me.
-ase
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