[M3devel] integer division/mod questions?

[Jay K]

I think I missed a sign.




-2147483648 - 2147483647  * -2
 actually -2147483648 + 2 * 2147483647

  actually 2147483646

which agrees.

so 




-2147483648 div 2147483647 = -2 
-2147483648 mod 2147483647 = 2147483646 

-2 * 2147483647 + 2147483646  = -2147483648

I'll make sure m3_mod works this way if it doesn't already.
Presumably we are stuck with these rules.

 - Jay

From: jay.krell at cornell.edu
To: m3devel at elegosoft.com
Subject: integer division/mod questions?
Date: Sun, 17 Jan 2010 10:44:53 +0000








-2147483648 div 2147483647 ?
-2147483648 mod 2147483647 ?

quotient = -1, remainer = -1 seems reasonable.
2147483647 * -1 + -1 == -2147483648


However, Modula-3 specifies div as rounding down, so

-2147483648 div 2147483647  == -2

and then

http://www.modula3.com/cm3/doc/reference/arithmetic.html

The value x DIV y is the floor of 
the quotient of x and y; that is, the maximum integer 
not exceeding the real number z such that z * y = x.
For integers x and y, the value of x MOD y is 
defined to be x - y * (x DIV y).


This means that for positive y, the value of x MOD y
lies in the interval [0 .. y-1], regardless of 
the sign of x.  For negative y, the value of
x MOD y lies in the interval [y+1 .. 0], regardless 
of the sign of x.


-2147483648 - 2147483647  * -2

-2147483648 +2  (due to overflow)


-2147483646

which contradicts the second part.

Maybe I'm confused? I should work this all through again?

 - Jay
 		 	   		  
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