in reply to parsing

#!/usr/bin/perl -w
use strict;
$|++;

use Time::Local;

my $online = [
              map { timelocal( (split /[:\/\s]+/)[2,1,0,4,3,5] ) } <DA
+TA>
             ];

my $total = 0;

for ( local $_ = 0; $_ <= $#{$online}; $_ += 2 ) {
  $total += $online->[$_+1] - $online->[$_];
}

print "$total seconds online\n"

__DATA__
10:09:55        05/28/00        Sunday
10:37:45        05/28/00        Sunday
09:27:02        05/29/00        Monday
09:34:05        05/29/00        Monday
20:48:00        05/29/00        Monday
20:57:19        05/29/00        Monday
21:38:32        05/29/00        Monday
21:40:14        05/29/00        Monday
09:32:50        05/30/00        Tuesday
09:37:29        05/30/00        Tuesday
13:03:23        05/30/00        Tuesday
13:09:45        05/30/00        Tuesday

[download]
Enjoy! 
--
Casey

Replies are listed 'Best First'.
RE: Re: parsing
by toadi (Chaplain) on Jul 27, 2000 at 11:52 UTC
    I never used map before, I read the docs but don't really understand it. Can you make it a bit more clear what it just does?

    --
    My opinions may have changed,
    but not the fact that I am right

[reply]
      The easiest way to think of map is to reduce it to something a bit more familiar. 
      @result = map SOME_EXPRESSION, @input;

      [download]
      can be replaced with: 
      @TEMP = ();
foreach $_ (@input) {
  push @TEMP, SOME_EXPRESSION;
}
@result = @TEMP;

      [download]
      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

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      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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