## 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

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

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

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'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?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