Exercise (Learning Perl Objects, References & Modules)

6.8.1. Exercise [30 min]

The Professor modified some files on Monday afternoon, and now he's forgotten which ones they were. This happens all the time. He wants you to make a subroutine called gather_mtime_between, which, given a starting and ending timestamp, returns a pair of coderefs. The first one will be used with File::Find to gather the names of only the items that were modified between those two times; the second one should return the list of items found.

Here's some code to try; it should list only items that were last modified on the most recent Monday, although you could easily change it to work with a different day. (You don't have to type all of this code. This program should be available as the file named ex6-1.plx in the downloadable files, available on the O'Reilly web site.)

Hint: You can find a file's timestamp (mtime) with code such as:

 my $timestamp = (stat $file_name)[9];

Because it's a slice, remember that those parentheses are mandatory. Don't forget that the working directory inside the callback isn't necessarily the starting directory in which find was called.

use File::Find;
use Time::Local;

my $target_dow = 1;        # Sunday is 0, Monday is 1, ... 
my @starting_directories = (".");

my $seconds_per_day = 24 * 60 * 60;
my($sec, $min, $hour, $day, $mon, $yr, $dow) = localtime;
my $start = timelocal(0, 0, 0, $day, $mon, $yr);        # midnight today
while ($dow != $target_dow) {
  # Back up one day
  $start -= $seconds_per_day;        # hope no DST! :-)
  if (--$dow < 0) {
    $dow += 7;
  }
}
my $stop = $start + $seconds_per_day;

my($gather, $yield)  = gather_mtime_between($start, $stop);
find($gather, @starting_directories);
my @files = $yield->( );

for my $file (@files) {
  my $mtime = (stat $file)[9];        # mtime via slice
  my $when = localtime $mtime;
  print "$when: $file\n";
}

Note the comment about DST. In many parts of the world, on the days when daylight savings time or summer time kicks in and out, the day is no longer 86,400 seconds long. The program glosses over this issue, but a more pedantic coder might take it into consideration appropriately.

6.8. Exercise

6.8.1. Exercise [30 min]