Re: Re: DT::Duration overloads
I said:
On the other hand, maybe these should be DateTime::Set methods:
my $mean = $set-mean( $sunrise, $sunset );
my $median = $set-median( $sunrise, $sunset );
On Thu, 10 Jun 2004 18:04:36 -0700, Bruce Van Allen wrote
Huh? I'm confused by your usage of 'mean' and 'median'. In the case in
question, I think 'midpoint' is a much clearer term to use.
I suggest placing mean and median in a module rather than just the FAQ because the
current solution is just the midpoint of two datetimes. The mean can give this, but
can
also give results for more than two datetimes.
Consider that an FAQ entry for calculating the average time you start work from a list
of
DateTimes would be a lot longer so should we put it in the FAQ? I don't think so. I
think
it should be a library method placed either in a Util module or in the DateTime::Set
module. Then, once we do that, we can change the FAQ entry for midpoint to Either
use the 'mean' function in DateTime::XXX::XXX or use the following algorithm
As for my usage of 'mean' and 'median':
'mean' and 'median' are mathematical constructs. The 'mean' is often called the
'average' while the 'median' would be the middle value.
mean(1,2,3) == 2
median(1,2,3) == 2
But here's the difference:
mean(1,2,6) == 3
median(1,2,6) == 2
And the 'median' value with an even number of arguments:
median(1,2,3,4) == 2.5
(middle is both 2 and 3, therefore use their mean (2 + 3) / 2 == 2.5)
And the maths involved:
sub mean{
return sum(@_) / scalar(@_);
}
sub median{
my $middle = (scalar(@_) + 1) / 2;
return ($middle == int(middle))
? $_[ $middle ]
: mean( $_[ int($middle) ], $_[ int($middle)+1 ]);
}
sub sum {
my $return = 0;
$return += $_ foreach @_;
return $return;
}
Of course, the DateTime code would be more difficult. I'd probably cheat by getting
rd_secs and using the above mathematical functions to get mean and median rd_secs
and then turning them back into DateTime objects.
Hope that clears it up.
Cheers!
Rick
Re: DT::Duration overloads
Quoting Dave Rolsky [EMAIL PROTECTED]: With DateTime::Span: $mid_point = $span-start-add_duration( seconds = $span-duration-seconds / 2 ); With DateTime: $mid_point = $start-add_duration( seconds = $end-subtract_datetime_absolute( $start )-seconds / 2 ); Oh, duh! Why didn't I think of that. That's nice simple. If the operator overloads did that sort of thing behind the scene (and yes, with spans also), then it would be nicer and simpler. :) But thanks, Flavio and Dave, for the recipes and dealing with my questions. How about a DateTime::Span-midpoint method? Let's wait and see if others ask for it. For now, let's just add those recipes to the faq. Midpoints are compelling, but also arbitrary. I'd like to see the general problem of portioning a span, or the delta between two arbitrary datetimes (expressed as a span or otherwise, transparently), solved in a general way. After all, if midpoint is that common, sure, provide a method, but we don't need methods for thirds, quarters, eighths, forty-thirds, etc. Thanks again, Matt (in the Big Easy for a wedding...man I should get some sleep) P.S. Sorry for the wonky quoting in my earlier messages -- I plead inadequate mail client that can't properly convert between markup and plain text.
Re: DT::Duration overloads
On Fri, 11 Jun 2004, Matt Sisk wrote: Quoting Dave Rolsky [EMAIL PROTECTED]: With DateTime::Span: $mid_point = $span-start-add_duration( seconds = $span-duration-seconds / 2 ); With DateTime: $mid_point = $start-add_duration( seconds = $end-subtract_datetime_absolute( $start )-seconds / 2 ); Oh, duh! Why didn't I think of that. That's nice simple. If the operator overloads did that sort of thing behind the scene (and yes, with spans also), then it would be nicer and simpler. :) Uh, exactly what operator would you overload for DateTime? I certainly don't see and obvious candidate. -dave /*=== House Absolute Consulting www.houseabsolute.com ===*/
Re: DT::Duration overloads
On Thu, 10 Jun 2004, Matt Sisk wrote: What I'd like to do is simply find the midpoint, more or less, between two arbitrary datetimes. Off the cuff, knowing nothing about the internals (which I do, but I'm pretending not to) I'd think this: $mid = $dt1 + ($dt2 - $dt1)/2 to dwim. However, as you say, things aren't really well defined the way durations are defined internally at the moment. So the question becomes -- if the above is not the datetime idiom for finding a midpoint between two datetimes, then what is? Well, if you just want the _date_, it's pretty easy. my $dur = $dt1-delta_days($dt2); # or use Math::Round if you want my $mid = $dt1-add( days = int( $dur-delta_days / 2 ) ); If you want to account for the time then it gets funkier. -dave /*=== House Absolute Consulting www.houseabsolute.com ===*/
Re: DT::Duration overloads
On Thu, 10 Jun 2004, Matt Sisk wrote: What I'd like to do is simply find the midpoint, more or less, between two arbitrary datetimes. Off the cuff, knowing nothing about the internals (which I do, but I'm pretending not to) I'd think this: Also, I'd like to point out that this really doesn't have as much to do with the internals as it does with the nature of date time math. You cannot expect to understand date math without understanding that it's not possible to convert between various units of date/time, in particular from months to days or vice versa. This would be a problem regardless of the internals, right? -dave /*=== House Absolute Consulting www.houseabsolute.com ===*/
Re: DT::Duration overloads
Dave Rolsky wrote: Well, if you just want the _date_, it's pretty easy. my $dur = $dt1-delta_days($dt2); # or use Math::Round if you want my $mid = $dt1-add( days = int( $dur-delta_days / 2 ) ); If you want to account for the time then it gets funkier. Hmm, indeed. I need to approximate solar noon by deriving the midpoint between sunrise and sunset. I also need to approximate solar midnight by looking at the sunset from the prior day. As a general problem, finding a fractional time between two points in time is not that unusual. I *suppose* I could convert to epoch, take the diff, and use that to create a new duration. But it seems unfortunate that I'd have to step out of the datetime API like that. The two endpoints are known quantities -- there is no abiguity in selecting a midpoint. Is this sort of thing something that should exist in spans rather than expecting regular date math to handle? Matt
Re: DT::Duration overloads
Dave Rolsky wrote: Also, I'd like to point out that this really doesn't have as much to do with the internals as it does with the nature of date time math. You cannot expect to understand date math without understanding that it's not possible to convert between various units of date/time, in particular from months to days or vice versa. This would be a problem regardless of the internals, right? I do understand the ambiguities when you are dealing with various forms of durations that are not anchored to an actual point in time. As I said in my last message, however, we're talking about two absolute points in time -- no ambiguity. Rephrased, the question is again: what is the midpoint of a span? (and if we come up with a good approach to this problem, it should solve the general class of problems having to do with portioning out spans). It's not immediately apparent to me what the solution is -- I know what I want to do, and would prefer it to be fairly easy and straightforward. This seems to be an area of the interface that could use some brushing up rather than some intrinsic ambiguity of date math. (I'm willing to meditate on possible solutions myself -- I'm not merely asking you to solve it for me. However, I just thought I might be missing something obvious) Thanks again, Matt
Re: DT::Duration overloads
Matt Sisk wrote: what is the midpoint of a span? With DateTime::Span: $mid_point = $span-start-add_duration( seconds = $span-duration-seconds / 2 ); With DateTime: $mid_point = $start-add_duration( seconds = $end-subtract_datetime_absolute( $start )-seconds / 2 ); (you may want to account for fractional seconds too) How about a DateTime::Span-midpoint method? - Flavio S. Glock
Re: DT::Duration overloads
On Thu, 10 Jun 2004 [EMAIL PROTECTED] wrote: Matt Sisk wrote: what is the midpoint of a span? With DateTime::Span: $mid_point = $span-start-add_duration( seconds = $span-duration-seconds / 2 ); With DateTime: $mid_point = $start-add_duration( seconds = $end-subtract_datetime_absolute( $start )-seconds / 2 ); Oh, duh! Why didn't I think of that. That's nice simple. How about a DateTime::Span-midpoint method? Let's wait and see if others ask for it. For now, let's just add those recipes to the faq. -dave /*=== House Absolute Consulting www.houseabsolute.com ===*/
Re: Re: DT::Duration overloads
On Thu, 10 Jun 2004 [EMAIL PROTECTED] wrote: How about a DateTime::Span-midpoint method? On 11 Jun 2004, at 6:55 AM, Dave Rolsky replied: Let's wait and see if others ask for it. For now, let's just add those recipes to the faq. I'm not sure of the best namespace, but I can see a Util namespace would be a good place for FRSs (Frequently Required Solutions) use DateTime::Util::Stats qw/:all/; # Solar Noon is the mean of sunrise and sunset: my $mean = mean_datetime( @dt_list ) # The mean is: round(scalar(@_) + 1 / 2) # IIRC # therefore $median == $sunset my $median = median_datetime( @dt_list ) On the other hand, maybe these should be DateTime::Set methods: my $mean = $set-mean( $sunrise, $sunset ); my $median = $set-median( $sunrise, $sunset ); I'm not sure that DateTime::Span is a good place for these as it would only apply to getting the mean of two DateTime objects whereas in DateTime::Set it's scope increases to allow any number of DateTimes. Would/Could there be any use for other statistical functions such as Standard Deviations? I can't think of any possible need for StdDev at the moment, but maybe there getting rid of outliers will be of benefit to someone? Cheers! Rick Senior Developer PrintSupply - Print Procurement Supply Management 18 Greenaway Street VIC 3105 Tel: (03) 9850 3255 Fx: (03) 9850 3277 http://www.printsupply.com.au/
Re: Re: DT::Duration overloads
On 6/11/04 Rick Measham wrote: On Thu, 10 Jun 2004 [EMAIL PROTECTED] wrote: How about a DateTime::Span-midpoint method? On 11 Jun 2004, at 6:55 AM, Dave Rolsky replied: Let's wait and see if others ask for it. For now, let's just add those recipes to the faq. I'm not sure of the best namespace, but I can see a Util namespace would be a good place for FRSs (Frequently Required Solutions) Flavio's solutions are brief and efficient. Why not just have it in the FAQ, as Dave suggests? use DateTime::Util::Stats qw/:all/; # Solar Noon is the mean of sunrise and sunset: my $mean = mean_datetime( @dt_list ) # The mean is: round(scalar(@_) + 1 / 2) # IIRC # therefore $median == $sunset my $median = median_datetime( @dt_list ) On the other hand, maybe these should be DateTime::Set methods: my $mean = $set-mean( $sunrise, $sunset ); my $median = $set-median( $sunrise, $sunset ); Huh? I'm confused by your usage of 'mean' and 'median'. In the case in question, I think 'midpoint' is a much clearer term to use. My $.02. 1; - Bruce __bruce__van_allen__santa_cruz__ca__
DT::Duration overloads
I understand that division can be expressed as multiplication, but is there any particular reason why division (/) is not overloaded but multiplication is for durations? Then you could say: $midpoint = ($dt2 - $dt1)/2; rather than $midpoint = ($dt2 - $dt1) * 0.5; Small thing. Just curious. Thanks, Matt
Re: DT::Duration overloads
On Wed, 9 Jun 2004, Matt Sisk wrote: I understand that division can be expressed as multiplication, but is there any particular reason why division (/) is not overloaded but multiplication is for durations? Then you could say: $midpoint = ($dt2 - $dt1)/2; rather than $midpoint = ($dt2 - $dt1) * 0.5; Small thing. Just curious. Well, division doesn't really work, whether you do it as multiplication or not ;) What is half a minute? How long is half a month? -dave /*=== House Absolute Consulting www.houseabsolute.com ===*/
Re: DT::Duration overloads
On 10 Jun 2004, at 9:25 AM, Dave Rolsky wrote: What is half a minute? How long is half a month? $dtd = DateTime::Duration-new( months = 1, minutes = 1, ); $half_dtd = $dtd / 2; print $half_dtd-months . \n; # 0.5 print $half_dtd-seconds . \n; # 0.5 print strfduration( normalise = 'ISO', pattern = '%Y years, %m months, %e days, %H hours, %M minutes, %S seconds', duration = $half_dtd ); # 0 years, 0 months, 15 days, 0 hours, 0 minutes, 30 seconds Senior Developer PrintSupply - Print Procurement Supply Management 18 Greenaway Street VIC 3105 Tel: (03) 9850 3255 Fx: (03) 9850 3277 http://www.printsupply.com.au/
Re: DT::Duration overloads
On Thu, 10 Jun 2004, Rick Measham wrote: On 10 Jun 2004, at 9:25 AM, Dave Rolsky wrote: What is half a minute? How long is half a month? $dtd = DateTime::Duration-new( months = 1, minutes = 1, ); $half_dtd = $dtd / 2; print $half_dtd-months . \n; # 0.5 print $half_dtd-seconds . \n; # 0.5 print strfduration( normalise = 'ISO', pattern = '%Y years, %m months, %e days, %H hours, %M minutes, %S seconds', duration = $half_dtd ); # 0 years, 0 months, 15 days, 0 hours, 0 minutes, 30 seconds Great, now what should DateTime.pm do in the add_duration method? -dave /*=== House Absolute Consulting www.houseabsolute.com ===*/
Re: DT::Duration overloads
On Wed, 9 Jun 2004, Matt Sisk wrote: I understand that division can be expressed as multiplication, but is there any particular reason why division (/) is not overloaded but multiplication is for durations? Then you could say: $midpoint = ($dt2 - $dt1)/2; rather than $midpoint = ($dt2 - $dt1) * 0.5; Small thing. Just curious. Thinking about this more, I'm considering maybe just requiring that multiplication be passed an integer, because if you do this: my $dur = DateTime::Duration-new( months = 1, days = 1, minutes = 1 ); $dur-multiple(.5); print DateTime-now-add_duration($dur)-datetime; The results are kind of weird, and certainly not what anyone would expect. -dave /*=== House Absolute Consulting www.houseabsolute.com ===*/
Re: DT::Duration overloads
Dave Rolsky wrote: Thinking about this more, I'm considering maybe just requiring that multiplication be passed an integer, because if you do this: What I'd like to do is simply find the midpoint, more or less, between two arbitrary datetimes. Off the cuff, knowing nothing about the internals (which I do, but I'm pretending not to) I'd think this: $mid = $dt1 + ($dt2 - $dt1)/2 to dwim. However, as you say, things aren't really well defined the way durations are defined internally at the moment. So the question becomes -- if the above is not the datetime idiom for finding a midpoint between two datetimes, then what is? Thanks, Matt
