relations/identities

[aarklon@gmail.com]

Hi all,

the following are the relations involving modulus operator that i have
found working

1) a = a %b + (a/b) * b ; for integral values of a and b

2) w % n = w & (n-1);

can any one give more examples for relations like this ...????

[Richard Tobin]

Re: relations/identities

In article <f10506df-970d-4404-bd0f-541a79dd83ae@b4 0g2000prf.googl egroups.com>,
<aarklon@gmail. comwrote:

>2) w % n = w & (n-1);

You need a constraint on n here. 5 % 3 = 2 but 5 & 2 = 0.

-- Richard
--
:wq

[aarklon@gmail.com]

Re: relations/identities

On Dec 7, 1:42 pm, rich...@cogsci. ed.ac.uk (Richard Tobin) wrote:

In article <f10506df-970d-4404-bd0f-541a79dd8...@b4 0g2000prf.googl egroups.com>,
>
<aark...@gmail. comwrote:

2) w % n = w & (n-1);

>
You need a constraint on n here. 5 % 3 = 2 but 5 & 2 = 0.
>
-- Richard
--
:wq

sorry i made a mistake

it should be rather w % (n-1) = w & n;

now
5 % 2 = 1
5 & 3 = 1

[Peter Nilsson]

Re: relations/identities

aark...@gmail.c om wrote:

rich...@cogsci. ed.ac.uk (Richard Tobin) wrote:

<aark...@gmail. comwrote:

2) w % n = w & (n-1);

You need a constraint on n here.

And w.

5 % 3 = 2 but 5 & 2 = 0.

>
sorry i made a mistake

Your correction was a much bigger mistake.

it should be rather w % (n-1) = w & n;

Even on its own terms this makes no sense. [Try w == n.]

--
Peter

[aarklon@gmail.com]

Re: relations/identities

On Dec 8, 2:30 am, Peter Nilsson <ai...@acay.com .auwrote:

aark...@gmail.c om wrote:

rich...@cogsci. ed.ac.uk (Richard Tobin) wrote:

<aark...@gmail. comwrote:
2) w % n = w & (n-1);

>

You need a constraint on n here.

>
And w.
>

5 % 3 = 2 but 5 & 2 = 0.

>

sorry i made a mistake

>
Your correction was a much bigger mistake.
>

it should be rather w % (n-1) = w & n;

>
Even on its own terms this makes no sense. [Try w == n.]

let us add a constraint n < w and n >= 2

[Richard Heathfield]

Re: relations/identities

aarklon@gmail.c om said:

On Dec 8, 2:30 am, Peter Nilsson <ai...@acay.com .auwrote:

>aark...@gmail. com wrote:

rich...@cogsci. ed.ac.uk (Richard Tobin) wrote:
<aark...@gmail. comwrote:
2) w % n = w & (n-1);

>>

You need a constraint on n here.

>>
>And w.
>>

5 % 3 = 2 but 5 & 2 = 0.

>>

sorry i made a mistake

>>
>Your correction was a much bigger mistake.
>>

it should be rather w % (n-1) = w & n;

>>
>Even on its own terms this makes no sense. [Try w == n.]

>
let us add a constraint n < w and n >= 2

Okay. Let n = 79, and let w = 83. w % n is 4, but w & (n - 1) is 66 if my
bit-twiddling is right. Last I checked, 4 != 66.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[James Kuyper]

Re: relations/identities

aarklon@gmail.c om wrote:

Hi all,
>
the following are the relations involving modulus operator that i have
found working

....

2) w % n = w & (n-1);
>
can any one give more examples for relations like this ...????

The second relationship is true only when n is a power of 2.

[aarklon@gmail.com]

Re: relations/identities

On Dec 8, 7:57 am, Richard Heathfield <r...@see.sig.i nvalidwrote:

aark...@gmail.c om said:
>
>
>

On Dec 8, 2:30 am, Peter Nilsson <ai...@acay.com .auwrote:

aark...@gmail.c om wrote:
rich...@cogsci. ed.ac.uk (Richard Tobin) wrote:
<aark...@gmail. comwrote:
2) w % n = w & (n-1);

>

You need a constraint on n here.

>

And w.

>

5 % 3 = 2 but 5 & 2 = 0.

>

sorry i made a mistake

>

Your correction was a much bigger mistake.

>

it should be rather w % (n-1) = w & n;

>

Even on its own terms this makes no sense. [Try w == n.]

>

let us add a constraint n < w and n >= 2

>
Okay. Let n = 79, and let w = 83. w % n is 4, but w & (n - 1) is 66 if my
bit-twiddling is right. Last I checked, 4 != 66.

I have seen this result in one book, but i don't remember it
correctly.I think it is in your book "C unleashed"

[ptkmartin@gmail.com]

Re: relations/identities

On Dec 8, 10:00 am, aark...@gmail.c om wrote:

w % n = w & (n-1);
I have seen this result in one book, but i don't remember it correctly.

You remembered it wrong.

The formula above is guaranteed only if n is a power of 2.
That is, n must be 1, or 2, or 4, or 8, or 16, or 32...