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## mod/div (was: reduce metaoperator on an empty list)

From:
=?ISO-8859-1?Q?=22TSa_=28Thomas_Sandla=DF=29=22?=
Date:
May 23, 2005 15:49
Subject:
mod/div (was: reduce metaoperator on an empty list)
Message ID:
42925DEA.4030304@orthogon.com
```mark.a.biggar@comcast.net wrote:
> There are actuall two usefull definition for %.  The first which Ada calls 'mod' always returns a value 0<=X<N and yes it has no working value that is an identity.  The other which Ada calls 'rem' defined as follows:
>
> Signed integer division and remainder are defined by the relation:
>
> A = (A/B)*B + (A rem B)
>
>    where (A rem B) has the sign of A and an absolute value less than the absolute value of B. Signed integer division satisfies the identity:
>
> (-A)/B = -(A/B) = A/(-B)
>
> It does have a right side identity of +INF.

This is the truncating div-dominant definition of modulo.
The eulerian definition is mod-dominant and nicely handles
non-integer values. E.g.

3.2 == 1.5 * 2 + 0.2 -+-->  3.2 / 1.5 == 2 + 0.2 / 1.5 == 2 + 1/15
|               == 2 + 0.1333...
+-->  3.2 % 1.5 == 0.2

Note that -3.2 == -4 + 0.8 == -4.5 + 1.3 == ...

-3.2 / 1.5 == -3 + 1.3 / 1.5 == -3 + 0.8666... == -2.1333
-3.2 % 1.5 ==  1.3

With integers:

8  /   3  ==  (2 + 2/3) ==  2
8  / (-3) == -(2 + 2/3) == -2
(-8) /   3  == -(3 - 1/3) == -3  # this might surprise some people ;)
(-8) / (-3) ==  (3 - 1/3) ==  3

8  % (-3) ==   8  % 3 == 2
(-8) % (-3) == (-8) % 3 == 1  # this as well, but it's just -3 * 3 + 1

Real valued division can be considered as % 0, that is infinite precision.
While integer arithmetic is % 1. I.e. int \$x == \$x - \$x % 1.

floor \$x == \$x - \$x % 1    # -1.2 - (-1.2) % 1 == -1.2 - 0.8 == -2
ceil  \$x == 1 + floor \$x
round \$x == floor( \$x + 0.5 )
trunc \$x == \$x < 0 ?? ceil \$x :: floor \$x

To @Larry: how are mod and div defined in Perl6?
--
TSa (Thomas Sandlaß)

```