#include <stdbool.h>
bool Equality(double a, double b, double epsilon)
{
if (fabs(a-b) < epsilon) return true;
return false;
}
I tried this method to compare two doubles, but I always get problems since I don't know how to chose the epsilon
, actually I want to compare small numbers (6 6 digits after the decimal point) like 0.000001. I tried with some numbers, sometimes I get 0.000001 != 0.000001
and sometimes 0.000001 == 0.000002
Is there another method else than comparing with the epsilon?
My purpose is to compare two doubles (which represent the time in my case). The variable t which represents the time in milliseconds is a double. It is incremented by another function 0.000001 then 0.000002 etc. each time t changes, I want to check if it is equal to another variable of type double tt, in case tt == t, I have some instructions to execute..
Thanks for your help
To compare two floating point or double values, we have to consider the precision in to the comparison. For example, if two numbers are 3.1428 and 3.1415, then they are same up to the precision 0.01, but after that, like 0.001 they are not same.
Because comparing floats with == is problematic, it's unwise to use them as IDs; the names in your example code suggest that's what you are doing; long integers (longs) are preferred, and the de facto standard for IDs.
Description. It is very usual for the C programming language beginners to compare a floating point number using the "==" operator. Floating point numbers must not be compared with the "==" operator.
How To Compare Floats in Python. If abs(a - b) is smaller than some percentage of the larger of a or b , then a is considered sufficiently close to b to be "equal" to b . This percentage is called the relative tolerance. You can specify the relative tolerance with the rel_tol keyword argument of math.
Look here: http://floating-point-gui.de/errors/comparison/
Due to rounding errors, most floating-point numbers end up being slightly imprecise. As long as this imprecision stays small, it can usually be ignored. However, it also means that numbers expected to be equal (e.g. when calculating the same result through different correct methods) often differ slightly, and a simple equality test fails.
And, of course, What Every Computer Scientist Should Know About Floating-Point Arithmetic
First: there's no point in computing a boolean value (with the <
operator) and then wrapping that in another boolean. Just write it like this:
bool Equality(float a, float b, float epsilon)
{
return fabs(a - b) < epsilon;
}
Second, it's possible that your epsilon itself isn't well-represented as a float
, and thus doesn't look like what you expect. Try with a negative power of 2, such as 1/1048576 for instance.
Alternatively, you could compare two integers instead. Just multiply your two floats by the desired precision and cast them to integers. Be sure to round up/down correctly. Here is what it looks like:
BOOL floatcmp(float float1, float float2, unsigned int precision){
int int1, int2;
if (float1 > 0)
int1 = (int)(float1 * precision + .5);
else
int1 = (int)(float1 * precision - .5);
if (float2 > 0)
int2 = (int)(float2 * precision + .5);
else
int2 = (int)(float2 * precision - .5);
return (int1 == int2);
}
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