Below is some code in C++. If you try something like -2%5 in python the result is positive 3 while many other languages like c++ C# (code) and flash give -2
Why do they give -2 and is one version more correct than the other?
#include <cstdio>
int main(){
printf("%d\n", 2%5);
printf("%d\n", -2%5);
printf("%d\n", -2%77);
printf("%d\n", 2%-77);
printf("%d\n", -2%-77);
}
Output:
2
-2
-2
2
-2
If r = a % n, then a = n * q + r for some q. That means that you have many choices for the value of r, depending on the value of q that gets chosen.
I'd recommend reading http://en.wikipedia.org/wiki/Modulo_operation, which says that most programming languages choose r with -n < r < n. That means that, unless r is zero, you have two choices for the value of r - one positive, one negative. Different programming languages make different decisions about whether to take the positive or negative one. You'll find a table on that page that summarizes what different languages do:
r with the same sign as n (which is what you see above).r with the same sign as a (and before the 2011 standard, it's implementation defined).If you want to be sure that you get the positive one in Python, use this:
r = a % n
if r < 0:
r += n
According to the C++ documentation:
For negative values, the result may vary depending on the library implementation.
Which seems odd. The Python documentation says only this:
The modulo operator always yields a result with the same sign as its second operand (or zero); the absolute value of the result is strictly smaller than the absolute value of the second operand.
It seems to me that the Python way is more logical, but that's just a gut feeling.
I think you should look at the below. In addition to using slightly different algorithms, operator precedence matters. Try it with brackets:
In [170]: 2%5
Out[170]: 2
In [171]: -2%5
Out[171]: 3
In [172]: (-2)%5
Out[172]: 3
In [173]: -(2%5)
Out[173]: -2
