Question

I need to find maximum and minimum of 8 float values I get. I did as follows. But float comparisons are going awry as warned by any good C book!
How do I compute the max and min in a accurate way.

main()
{
    float mx,mx1,mx2,mx3,mx4,mn,mn1,mn2,mn3,mn4,tm1,tm2;

    mx1 = mymax(2.1,2.01); //this returns 2.09999 instead of 2.1 because a is passed as 2.09999.
    mx2 = mymax(-3.5,7.000001);
    mx3 = mymax(7,5);
    mx4 = mymax(7.0000011,0); //this returns incorrectly- 7.000001
    tm1 = mymax(mx1,mx2);
    tm2 = mymax(mx3,mx4);
    mx = mymax(tm1,tm2);

    mn1 = mymin(2.1,2.01);
    mn2 = mymin(-3.5,7.000001);
    mn3 = mymin(7,5);
    mn4 = mymin(7.0000011,0);
    tm1 = mymin(mx1,mx2);
    tm2 = mymin(mx3,mx4);
    mn = mymin(tm1,tm2);


    printf("Max is %f, Min is %f \n",mx,mn);

    getch();
}

float mymax(float a,float b)
{
    if(a >= b)
    {
        return a;
    }
    else
    {
        return b;
    }
}

float mymin(float a,float b)
{
    if(a <= b)
    {
        return a;
    }
    else
    {
        return b;
    }
}

How can I do exact comparisons of these floats? This is all C code.

thank you.

-AD.

Answer 1

You are doing exact comparison of these floats. The problem (with your example code at least) is that float simply does not have enough digits of precision to represent the values of your literals sufficiently. 7.000001 and 7.0000011 simply are so close together that the mantissa of a 32 bit float cannot represent them differently.

But the example seems artificial. What is the real problem you’re trying to solve? What values will you actually be working with? Or is this just an academic exercise?

The best solution depends on the answer to that. If your actual values just require somewhat more more precision than float can provide, use double. If you need exact representation of decimal digits, use a decimal type library. If you want to improve your understanding of how floating point values work, read The Floating-Point Guide.