On Mar 1, 2005, at 2:48 AM, Mark Wheeler wrote:
Hi Sherm,
That works perfectly. Could you give me a brief rundown on how the sort works with Date::Manip, so I can understand what is going on?
On Feb 28, 2005, at 1:03 AM, Sherm Pendley wrote:
my @sorted = sort { Date_Cmp(ParseDate($b), ParseDate($a)); } @dates;
I'm not certain how much detail you need, so I'll start at the beginning, with sort(). Sort() allows you to provide a block of code that's used as a comparison function. Two arguments are passed to that function - $a and $b - and the return value should mimic that of <=> or cmp - that is, < 0 if $a < $b, 0 if $a == $b, and > 0 if $a > $b.
So, suppose we have a list of hashes, like this:
my @garbled = (
{ 'foo'=>'trouble', 'sort_by'=>3 },
{ 'foo'=>'hubble', 'sort_by'=>0 },
{ 'foo'=>'toil', 'sort_by'=>2 },
{ 'foo'=>'bubble', 'sort_by'=>1 },
);We could sort it according to the "sort_by" field like this:
my @spell = sort { $a->{'sort_by'} <=> $b->{'sort_by'} } @garbled;Note that there's no comma between the comparison function and @garbled. If there were, Perl would treat the block as the first element in the list to be sorted. Also, $a and $b are magic - you don't have to declare them with my() or our(), or shift them off of @_. They're just there. Also, strict knows about them, although not in detail; you can use an undeclared $a or $b *anywhere*, not just with a sort(), without hearing a peep from strict.
So anyway, how's that work with Date::Manip?
The ParseDate() function takes a date in a variety of formats, and returns it in a normalized format: "yyyymmddhh:mm:ss". Date_Cmp() compares two of these normalized date strings, and returns a value that's consistent with <=> or cmp - that is, <0, 0, or >0 - as expected by sort.
You *could* simply use cmp to compare these strings - in fact that's basically all Date_Cmp() does right now - but the Date::Manip docs indicate that the normalized format will be extended at some future point, to include various flags such as the time zone. At that point, Date_Cmp() will also be updated to take the flags into account, whereas cmp will no longer work. So, using Date_Cmp() is optional at the moment, but it's more future-proof than cmp.
sherm--
Cocoa programming in Perl: http://camelbones.sourceforge.net Hire me! My resume: http://www.dot-app.org
