• 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?

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

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?