Bug #13116
closedmodulo, divmod range problem: float_val % 1 may return 1.0
Description
mod, divmod range problem: float_val % 1 may return 1.0
x % y sometimes returns y (not in 0 <= result < y)
float_val = 0.0.prev_float
float_val % 1 #=> 1.0
Math.sin(2*Math::PI) % 10 #=> 10.0
I not sure this is a bug or a feature
but I think returning x.prev_float rather than x would be appropriate.
# when would this feature cause problem?:
require 'chunky_png'
image = ChunkyPNG::Image.new 100, 100
points = 100.times.map{|i|[10*Math.sin(Math::PI*i/10), i, 0xaabbccff]}
# render points to an image object(circular boundary)
points.each do |x, y, color|
image[(x % image.width).floor, (y % image.height).floor] = color
# => ChunkyPNG::OutOfBounds: Coordinates (100,20) out of bounds!
end
image.save 'out.png'
# to avoid error:
(x % w % w).floor
[(x % w).floor, w - 1].min
xx = (x % w).floor; xx -= 1 if xx == w
# but it's a little bit weird
# sample implementation in ruby:
class Float
alias_method :original_modulo, :%
alias_method :original_divmod, :divmod
def divmod x
div, mod = original_divmod x
if mod == x
mod = x > 0 ? mod.prev_float : mod.next_float
end
[div, mod]
end
def % x
mod = original_modulo x
return mod if mod != x
x > 0 ? mod.prev_float : mod.next_float
end
end
Updated by snood1205 (Eli Sadoff) almost 8 years ago
For what it's worth, this result fits in with with modulo
is documented to be. The documentation states
x.modulo(y) means x-y*(x/y).floor
I implemented this in C to check if the results held and they did. (Note that 0.0.prev_float
is -5.0e-324
).
#include <stdio.h>
#include <math.h>
double modulo(double, double);
int main()
{
double val = -5.0e-324;
printf("%.17g\n", modulo(val, 1));
}
double modulo(double x, double y)
{
return x - y * floor(x / y);
}
This returns 1
as the answer as well. The question more so becomes whether or not the ruby implementation of mod should be this or more in line with C's fmod
. I would say this is not a bug though because the implementation and the documentation match up.
Updated by nobu (Nobuyoshi Nakada) almost 8 years ago
- Status changed from Open to Rejected
1.0 - 0.0.prev_float
results in 1.0 because of the finite precision.