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Re: Arbitrary precision modulo operation

From: "Dann Corbit" <DCorbit(at)connx(dot)com>
To: "Paul Tillotson" <pntil(at)shentel(dot)net>,<pgsql-general(at)postgresql(dot)org>
Subject: Re: Arbitrary precision modulo operation
Date: 2004-04-27 00:46:23
Message-ID: D90A5A6C612A39408103E6ECDD77B829408D6A@voyager.corporate.connx.com (view raw or flat)
Thread:
Lists: pgsql-general
> -----Original Message-----
> From: Paul Tillotson [mailto:pntil(at)shentel(dot)net] 
> Sent: Monday, April 26, 2004 4:41 PM
> To: pgsql-general(at)postgresql(dot)org
> Subject: Re: [GENERAL] Arbitrary precision modulo operation
> 
> 
> I see there are a few misconceptions about numeric and 
> modulus on here:
> 
> (1) A modulus operation on a numeric type should NOT have 
> rounding errors.  The whole point of numeric is that it is an 
> arbitrary precision BASE 10 representation of your number.  

This is true

> The modulus returns the (whole
> number) remainder as a result of a division.

This is true if the numeric values are integers.

When the values are not integral, some non-integral results can be
returned.

2.50 % 2.50 is 0
But 13.89 modulo 3.50 is 3.39 
If you work it out on paper, you will see that 3.39 is the correct
remainder.
 
> (2) the modulus operator/function is, AFAIK, supposed to 
> return the modulus with the SAME SIGN as the divisor, so I 
> think this is a bug. That's what every other modulus operator 
> that I have ever seen does. Would you mind doing

I would agree that a positive modulus is preferable.  However, the
negative result is also mathematically correct.
 
> foodb=> SELECT version();
> 
> (3) MySQL just rounds large numbers to the highest value that 
> the type will support, and apparently, no arbitrary precision 
> types are listed on this page:
> 
> http://dev.mysql.com/doc/mysql/en/Numberic_type_overview.html
> 
> You can't expect to get a modulus from an out-of-range number.

Should it not (therefore) throw an error of some sort?

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Subject: Re: Arbitrary precision modulo operation

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