## digitalmars.D.learn - Floating-point Modulus math.fmod

• Fendercaster (43/43) Nov 06 2016 I'm not quite sure if this is the right forum to ask this
• Ilya Yaroshenko (3/11) Nov 07 2016 Python uses 64-bit doubles. You may want to try with `double` and
• Fendercaster (4/16) Nov 07 2016 I'll give it a shot. Since the "real" type usually considers
Fendercaster <wk5436dmij sihkx.anonbox.net> writes:
I'm not quite sure if this is the right forum to ask this
question:
I've been trying to implement the "floating-point modulus"
function from the math library. Equivalently that's what I've
tried in Python too. Problem is - the results are different and
not even close. Please have a look at both snippets. Maybe
someone can help me out:

[code = D]
import std.math;
import std.stdio;

immutable real MODULUS = 1e8;

real floatModPow(real base, ulong exp, real mod)
{
real r = 1.0;

while (exp) {
if (exp & 1) r = fmod(r * base, mod);
exp >>= 1;
base = fmod(base*base, mod);
}

return r;
}

void main()
{
floatModPow(3.866, 987654321UL, MODULUS).writeln;
}
[/code]

In this case the result is 6.44588e+07.

[code = Python]
import math

MOD = 1e8

def fpmod(b, e, m):
r = 1.0
while e:
if (e & 1):
r = math.fmod(r * b, m)
e >>= 1
b = math.fmod(b * b, m)
return r

print (fpmod(3.866, 987654321, MOD))
[/code]

In this case the result is 82031250.0.

AFAIK the D.fmod and Python.fmod functions serve the same
purpose. What am I missing?
Nov 06 2016
Ilya Yaroshenko <ilyayaroshenko gmail.com> writes:
On Sunday, 6 November 2016 at 21:45:28 UTC, Fendercaster wrote:
I'm not quite sure if this is the right forum to ask this
question:
I've been trying to implement the "floating-point modulus"
function from the math library. Equivalently that's what I've
tried in Python too. Problem is - the results are different and
not even close. Please have a look at both snippets. Maybe
someone can help me out:

[...]
Python uses 64-bit doubles. You may want to try with `double` and `core.stdc.tgmath` -- Ilya
Nov 07 2016
Fendercaster <kn5ngliatc bykck.anonbox.net> writes:
On Monday, 7 November 2016 at 09:21:11 UTC, Ilya Yaroshenko wrote:
On Sunday, 6 November 2016 at 21:45:28 UTC, Fendercaster wrote:
I'm not quite sure if this is the right forum to ask this
question:
I've been trying to implement the "floating-point modulus"
function from the math library. Equivalently that's what I've
tried in Python too. Problem is - the results are different
and not even close. Please have a look at both snippets. Maybe
someone can help me out:

[...]
Python uses 64-bit doubles. You may want to try with `double` and `core.stdc.tgmath` -- Ilya
I'll give it a shot. Since the "real" type usually considers 80-bit operands, is the D variant more precise? And which of those two results is correct, if any?
Nov 07 2016