How do I use modulus for float/double?

I'm creating an RPN calculator for a school project and having trouble with the modulus operator. Since we're using the double data type, modulus won't work on floating-point numbers. For example, 0.5 % 0.3 should return 0.2, but I'm getting a division by zero exception.

The instruction says to use fmod(). I've looked everywhere for fmod(), including javadoc, but I can't find it. I'm starting to think it's a method I'm going to have to create?

Edit: Hmmm, strange. I just plugged in those numbers again and it seems to be working fine… but just in case. Do I need to watch out for using the mod operator in Java when using floating types? I know something like this can't be done in C++ (I think).

You probably had a typo when you first ran it.

evaluating 0.5 % 0.3 returns '0.2' (A double) as expected.

Mindprod has a good overview of how modulus works in Java.

Unlike C, Java allows using the % for both integer and floating point and (unlike C89 and C++) it is well-defined for all inputs (including negatives):

From JLS §15.17.3:

> The result of a floating-point > remainder operation is determined by > the rules of IEEE arithmetic: > > * If either operand is NaN, the result is NaN. > * If the result is not NaN, the sign of the result equals the sign of > the dividend. > * If the dividend is an infinity, or the divisor is a zero, or both, the > result is NaN. > * If the dividend is finite and the divisor is an infinity, the result > equals the dividend. > * If the dividend is a zero and the divisor is finite, the result > equals the dividend. > * In the remaining cases, where neither an infinity, nor a zero, nor > NaN is involved, the floating-point > remainder r from the division of a > dividend n by a divisor d is defined > by the mathematical relation r=n-(d·q) > where q is an integer that is negative > only if n/d is negative and positive > only if n/d is positive, and whose > magnitude is as large as possible > without exceeding the magnitude of the > true mathematical quotient of n and d.

So for your example, 0.5/0.3 = 1.6... . q has the same sign (positive) as 0.5 (the dividend), and the magnitude is 1 (integer with largest magnitude not exceeding magnitude of 1.6...), and r = 0.5 - (0.3 * 1) = 0.2

I thought the regular modulus operator would work for this in Java, but it can't be hard to code. Just divide the numerator by the denominator, and take the integer portion of the result. Multiply that by the denominator, and subtract the result from the numerator.

x = n/d
xint = Integer portion of x
result = n - d*xint

fmod is the standard C function for handling floating-point modulus; I imagine your source was saying that Java handles floating-point modulus the same as C's fmod function. In Java you can use the % operator on doubles the same as on integers:

int x = 5 % 3; // x = 2
double y = .5 % .3; // y = .2