mod of a negative number

Thread view:

Enable EMail Alerts

Start New Thread

gk - 05 Nov 2006 15:21 GMT

Int a = -5; Int b = -2;
System.out.println(a % b); // -1

how this is working ?

Reply to this Message

Lars Enderin - 05 Nov 2006 15:28 GMT

gk skrev:

> Int a = -5; Int b = -2;
> System.out.println(a % b); // -1
>
> how this is working ?

(How does this work?)
According to the definition of the % operator -1 is the rest after
taking out a multiple (-4) of -2 from -5.

Reply to this Message

gk - 05 Nov 2006 15:51 GMT

> gk skrev:
> > Int a = -5; Int b = -2;
[quoted text clipped - 5 lines]
> According to the definition of the % operator -1 is the rest after
> taking out a multiple (-4) of -2 from -5.

true.....but -2<-1 , so we can divide it further theoretically ...is
not it ? hmm...but if we keep on dividing its going to be an infinite
loop.

Reply to this Message

Patricia Shanahan - 05 Nov 2006 15:39 GMT

> Int a = -5; Int b = -2;
> System.out.println(a % b); // -1
>
> how this is working ?

Strictly speaking, there is no "mod" operator in Java. % is defined to
be "remainder". It is the same as modulo for positive operands.

For integer operands, % is designed to maintain the identity:
(a/b)*b+(a%b) is equal to a.

Java integer division rounds towards zero, so -5/-2 is 2.

( (-5)/(-2) ) * (-2) + (-1) is -5.

See the JLS,
http://java.sun.com/docs/books/jls/second_edition/html/expressions.doc.html#239829

Patricia

Reply to this Message

Richard F.L.R.Snashall - 05 Nov 2006 16:05 GMT

>> Int a = -5; Int b = -2;
> For integer operands, % is designed to maintain the identity:
[quoted text clipped - 3 lines]
>
> ( (-5)/(-2) ) * (-2) + (-1) is -5.

A nitpick question: Is the division ( (8)/(3) ) also 2 in Java?
If so, why is this be called "rounding"?

Reply to this Message

Lee Weiner - 05 Nov 2006 17:12 GMT

>>> Int a = -5; Int b = -2;
>> For integer operands, % is designed to maintain the identity:
[quoted text clipped - 6 lines]
>A nitpick question: Is the division ( (8)/(3) ) also 2 in Java?
>If so, why is this be called "rounding"?

It is not called rounding. It is called integer division.

Reply to this Message

Patricia Shanahan - 05 Nov 2006 17:46 GMT

>>> Int a = -5; Int b = -2;
>> For integer operands, % is designed to maintain the identity:
[quoted text clipped - 6 lines]
> A nitpick question: Is the division ( (8)/(3) ) also 2 in Java?
> If so, why is this be called "rounding"?

I usually use "rounding" in the sense in which it is used in e.g. the
IEEE 754 standard, to mean modifying the infinitely precise result of a
calculation to fit in the destination's format.

In that usage, it includes rounding directed rounding, such as rounding
towards zero, as well as the various flavors of round to nearest. Some
people use "truncation" when the rounding is towards either zero or
negative infinity.

Patricia

Reply to this Message

gk - 05 Nov 2006 20:13 GMT

> > Int a = -5; Int b = -2;
> > System.out.println(a % b); // -1
[quoted text clipped - 15 lines]
>
> Patricia

nice explanation!

Reply to this Message

mod of a negative number

Free Magazines