Re: modulo question

[Kelly John Rose <mllists () ptbcanadian net>]

From a purely mathematical point of view that is right.

a = b (mod c) or a%c = b  <==> c divides (a-b) 

ie. -1 = 1 (mod 2) since 2 divides ((-1) - 1) = -2
and -1 = -1 (mod 2) since 2 divides ((-1) - (-1)) = 0

In a very twisted vein
-1 = 153 (mod 2) since 2 divides ((-1) - 153) = -154

But these very likely, these functions in python and C++ are defined to 
give the answer that lies within (c, -c)

so the results you are given are both correct results. The problem is 
that apparently C++ and Python define their modulos slightly 
differently. Now, I do not know the details of their implementations.

.....Kelly John Rose.....

martin wrote:

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> Hello list,
>
> I have slightly academic question about modulo. I sent this message here 
> because I thing there are many of gurus which working with modulo operator.
>
> I have tried in python and in C++ following:
>            python    C++   
> 1%2      1           1
> - -1%2     1          -1
> 1%-2    -1           1
> - -1%-2   -1          -1
>
> What is right?
> Do you know some applications which using modulo with negative numbers?
> Have you some references to definition (mathematical def. and properties) of 
> modulo?
>
> Thanks in advance,
> Martin
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