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Most effective way for float and double comparison
In my program some double variables take values of the form 1.00000001. Equality check of these variables with 1 obviously fails.
I wanted to know how can I reduce the precision of double type variables in c, so that equality with integers works.
You should almost never check floating point values for exact equality, expecially when they comes from comutation. You should check the absolute value of the difference from the compare value being less than a certain epsilon. The "precision" of the double is given by the internal number representation and you can't change it. How to exactly choose epsilon can be difficult, there is some comment to that answer discussing that, read it, but you eventually come with the pratical epsilon based equality.
abs(a-x) < epsilon is a good idea only for numbers close to zero (both a and x) otherwise relative epsilon or ulps. And you sometimes have to compare doubles using ==There's no portable way, no.
With the GNU C library, you can use this API to change the rounding mode.
But in general, it's better to express it with code, so that your expectations become clear and portable:
#define EQUALITY_EPSILON 1e-3 /* Or whatever. */
if(fabs(x - y) <= EQUALITY_EPSILON)
{
}
You should avoid checking for equality in floating-point comparisons. Instead, use a precision value epsilon like so:
if (fabs(a - b) < epsilon)
{
// treat a and b as equal
}
The choice of epsilon is probably complicated, but my knowledge doesn't go that far.
Do a Google search for "What Every Programmer Should Know About Floating-Point Arithmetic". It's a well-known article that covers all this stuff.
It doesn't make a lot of sense to 'reduce the precision of double type variables'. You should probably think instead of using functions such as ceil, floor, and round to make an integer out of your double in a way that you have good control over and then use that integer in your comparisons.
in C99
#include <stdio.h>
#include <math.h>
int main(){
double d = 1.00000001;
int i = (int)round(d);
printf("%d\n", i);
return 0;
}
1.5?1.5equality should return false. Let us say, anything after fifth decimal place should be ignored.