Re: Plpgsql function to compute "every other Friday" - Mailing list pgsql-general

It is a very simplistic approach since you do not take into account
holidays.  But if it meets your needs what you want is the modulo operator (
"%"; "mod(x,y)" is the equivalent function ) which performs division but
returns only the remainder.

N % 14 = [a number between 0 and (14 - 1)]

N = 7; 7 % 14 = 7 (0, 7 remainder)
N = 14; 14 % 14 = 0 (1, 0 remainder)
N = 28; 28 % 14 = 0  (2, 0 remainder)
N = 31; 31 % 14 = 3 (2, 3 remainder)

If you KNOW the epoch date you are using is a Friday then you have no need
for CURRENT_DATE since you are passing in a date to check as a function
parameter.

I'll have to leave it to you or others to address the specific way to
integrate the modulo operator/function into the algorithm.

David J.

-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of C. Bensend
Sent: Monday, April 04, 2011 8:12 PM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Plpgsql function to compute "every other Friday"


Hey folks,

   So, I'm working on a little application to help me with my budget.  Yeah,
there are apps out there to do it, but I'm having a good time learning some
more too.  :)

   I get paid every other Friday.  I thought, for scheduling purposes in
this app, that I would take a stab at writing a plpgsql function to
determine if a given date is a payday.  Here is what I have so far:


CREATE OR REPLACE FUNCTION is_payday( d DATE ) RETURNS BOOLEAN AS $$

        DECLARE epoch DATE;
                days_since_epoch INTEGER;
                mult FLOAT8;
                ret BOOLEAN := FALSE;

BEGIN

        SELECT INTO epoch option_value
                FROM options WHERE option_name = 'payroll_epoch';

        SELECT INTO days_since_epoch ( SELECT CURRENT_DATE - d);

        *** here's where I'm stuck ***

        RETURN ret;

END;
$$ LANGUAGE plpgsql;


   OK.  So, I have a "starting" payday (payroll_epoch) in an options table.
That is the first payday of the year.  I then calculate the number of days
between that value and the date I pass to the function.
Now I need to calculate whether this delta (how many days since
epoch) is an even multiple of 14 days (the two weeks).

   I have no idea how to do that in plpgsql.  Basically, I need to figure
out if the date I pass to the function is a payday, and if it is, return
TRUE.

   I would very much appreciate any help with this last bit of math and
syntax, as well as any advice on whether this is a reasonable way to attack
the problem.  And no - this isn't a homework assignment.  :)

Thanks folks!

Benny


--
"Hairy ape nads."        -- Colleen, playing Neverwinter Nights





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