float value condition

Question

I worked out the following code and got a bizarre output. Can anyone explain how it works?

main()
{
    float a=0.8;
    float b=0.25;
    if(a==0.8)
        printf("HELLO")
    if(b==0.25)
        printf("WORLD")
}

And the output that I got is surprisingly

WORLD

thanks in advance

Answer 1

This is because 0.25 is a power of two (i.e. 2^-2), while 0.8 is not. Only exact sums of powers of two can be represented exactly; all other numbers, including 0.8, are represented as an approximation, which has a different precision between float and double. The 0.8 in a==0.8 is a double, while a is a float. Their representations are different, and so are their values.