Thread: Simple mod function won't work

Substitute
quotient = num/den;

in
return (num - quotient * den); //this math should produce the remainder

and you get
return (num - num/den * den); //this math should produce the remainder

otherwise known as num - num
or put another way, 0

Go through an example...
7.0 / 2.0 equals 3.5 (not 3)
7.0 - 3.5 * 2.0 = 0
The trick is that wou want the answer from that first line to give 3.0 and not 3.5 because then...
7.0 - 3.0 * 2.0 = 1.0
So, do you know what kind of function throws away the fractional part of the result?

I tried to experiment by doing the following, which truncates the quotient before continuing with the operation:

Code:

int main()
{
	double num = 10.3;
	double den = 3.3;
	double mod;
	int trun;
	trun = num/den;
	mod = num - trun*den;
}

The mod becomes 0.40000000000000213. Is this math even correct?

For some reason, my variables are showing up in my Visual Studio Autos window like this:

num 10.300000000000001
den 3.2999999999999998

I'm not sure why.
------------------------------------------------------------------------------

Using the same variable values, but instead using the fmod function for the calculation gives me a slightly different result, which is the following code block:

Code:

#include <math.h>
int main()
{
	double num = 10.3;
	double den = 3.3;
	double mod;
	mod = fmod(num,den);
}

The mod result here is 0.40000000000000124, which is slightly different.

I understand the basics of dividing decimals, but I dont know how to get the remainder on paper, so I don't understand how the mod becomes .4 to begin with.

If someone wants to explain to me how to find the remainder of an operation where decimals are divided, I won't mind.

I really want to understand how the fmod function works, though.

Originally Posted by billybob2020

The mod becomes 0.40000000000000213. Is this math even correct?

Work it out on paper.

Originally Posted by billybob2020

For some reason, my variables are showing up in my Visual Studio Autos window like this:
...
Using the same variable values, but instead using the fmod function for the calculation gives me a slightly different result

If you want accuracy, don't use floating point numbers.


Quzah.