neilwebber

Friday the 13th and NumerousApp

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neilwebber
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I made a new NumerousApp metric that will tell you when the next Friday the 13th is: http://n.numerousapp.com/m/1w4c4tk4uh29j

Nerd info: I wrote this brute-force python program to find the next Friday the 13th:

from datetime import datetime, date

dt = datetime.now()
while dt.day != 13 or dt.weekday() != 4:
    dt = date.fromordinal(dt.toordinal()+1)

It gets today via  datetime.now()and loops until it is a Friday and a 13th. The weekday() method returns 0 .. 6 with Monday as 0; hence Friday is 4. If “dt” isn’t a Friday the 13th, I just bump it to the next day. The toordinal() method converts a date into a number of days since some arbitrary long-ago time. So I do that, add one, and convert back to a date to get the next day.

I didn’t optimize this algorithm because it just doesn’t matter. I could have added code to compute exactly how many days to add to get to the next 13th and then just test if that is a Friday. Even more generally we could write an algorithm that would directly compute when the next Friday the 13th is (has to take month lengths and leap years into account).

But: We know that at most we’ll have to iterate no more than a few hundred times because every year has at least one Friday the 13th (proving this is left as an exercise for the reader). Speed just doesn’t matter here at all and this works and it is simple.

Once a night I run the above code and use my python NumerousApp API class to update the metric. This too could have been optimized; there’s no reason to run the code at all except on a 13th (indeed, except on a Friday the 13th) but it was just easier to stick it into my nightly updates I do for my other metrics.

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