floating_point_equality.c

/*

    floating_point_equality.c

    https://www.learncpp.com/cpp-tutorial/relational-operators-and-floating-point-comparisons/


    You can find all my C++ programs at Dragan Milicev's pastebin:

    https://pastebin.com/u/dmilicev

*/

#include <stdio.h>

// returns largest value from given two values x and y
double double_max( double x, double y)
{
    if(x>y)
        return x;

    return y;
}

// returns absolute value of x
double double_abs( double x)
{
//printf("\n In double_abs x = %.16lf \n", x);

    if(x<0)
        return (-x);

    return x;
}

// return true if the difference between a and b is
// within epsilon percent of the larger of a and b
int isApproximatelyEqualAbs(double a, double b, double absEpsilon)
{
    return ( double_abs(a - b) <= ( double_max(double_abs(a), double_abs(b)) * absEpsilon) );
}

int isApproximatelyEqualAbsRel(double a, double b, double absEpsilon, double relEpsilon)
{
    // Check if the numbers are really close, needed when comparing numbers near zero.
    double diff = double_abs(a-b);

    printf("\n diff = %.8lf \n", diff);

    if (diff <= absEpsilon)
    {
        printf("\n 1, absEpsilon \n");
        return 1;
    }

    // Otherwise fall back to Knuth's algorithm
    printf("\n 2, relEpsilon \n");
    return( diff <= ( double_max(double_abs(a),double_abs(b)) * relEpsilon ) );
}


int main()
{
    double a = 123.456789;
    double b = 123.456779;

    printf("\n   a = %lf \n", a);
    printf("\n   b = %lf \n", b);
    printf("\n 1e-8 = %.8lf \n", 1e-8);

    // compare a and b
    printf("\n isApproximatelyEqualAbs(a, b, 1e-8) = %d \n",
            isApproximatelyEqualAbs(a, b, 1e-8) );

    // compare a and b
    printf("\n isApproximatelyEqualAbsRel(a, b, 1e-8, 1e-8) = %d \n",
            isApproximatelyEqualAbsRel(a, b, 1e-8, 1e-8) );


    return 0;
}