What is the Modulo function for negative decimals Objective-C? - objective-c

Question

I need to calculate modulos with decimals that can be negative as well
for example: fmod( -5.2, 3 );
while mod() works with integers, and fmod() (or fmodf()) works well with decimals, fmod() returns wrong results with negative decimals:
ex:
double modulo = fmod (5.2, 3);
NSLog (#"==> %f", modulo);
==> 2.2 // This is correct !!
double modulo = fmod (-5.2, 3);
NSLog (#"==> %f", modulo);
==> -2.2 // This is wrong !! should be 0.8
Is there another mod() in the library or should i write my own decimal negative mod function ?
something like :
if (res = fmod(x,m) < 0) {
res+=m;
}
Thx !

Answer 1

-2.2 is correct and is also -5.2 mod 3. The fmod function is a C function (and therefore also Objective C), so you can find more detail about it by typing man fmod into terminal. When doing fmod it will preserve the sign of the value that you are moding. So to get the mod you want, you will need to check the sign (of either the result, or the value you are passing in) and if it is negative you will need to add the modulo base, in this case 3.
This is the definition of the fmod function:
double
fmod(double x, double y);
Specifically, the functions return the value x-i*y, for some integer i such that, if y is non-zero, the result has the same sign as x and magnitude less than the magnitude of y.
from the OS X man page.

Answer 2

For your purposes, you can do something like this:
#include <math.h>
float f_mod(float a, float n) {
return a - n * floor(a / n);
}
Of course, be careful to check n>0.
f_mod(-5.2f, 2.0f) = 0.8
f_mod(5.2f, 2.0f) = 2.2

Answer 3

Thank you so i ended up writing a wrapper... What i was hopping i could avoid. This works great for me, and, in my opinion, represents the correct mathematical definition of the modulo (not the C implementation). I am sure this function can be optimized,but for clarity i leave it this way:
//--
//-- Modulo
//--
double calcModulo ( double x, double m) {
double res = INFINITY;
if (m==0)
return res ;
double posMod, negMod, posM, posX;
posM = m < 0 ? -m:m;
posX = x < 0 ? -x:x;
posMod = fmod (posX, posM);
negMod = fmod (-posX,posM) + posM;
// pick up the correct res
if ( x >= 0 ){
if (m > 0) {
res = posMod;
} else {
res = -negMod;
}
}else{
if (m > 0) {
res= negMod;
} else{
res= -posMod;
}
}
return res;
}