Question

I edited a C program for my assignment, previously there wasn’t typecasting and the iteration stopped at i=1, now with the typecasting it stops at i=6.

Any ideas why? Thanks in advance!

int main(void)
{

    int i = 0;
    double d = 0.0;

    while ( (i == (int) (d * 10)) && (i < 10) )
    {
        i = i + 1;
        d = (double) (d + 0.1);
        printf("%d %lf\n", i, d);
    }

    printf("%d %lf\n", i, d);

    getch();

    return 0;
}

Answer 1

As has been stated many times before, the reason this doesn’t work is that binary floating point numbers cannot represent all decimal floating point binary numbers, it just isn’t possible. To read more, check out this really great article:

What Every Programmer Should Know About Floating-Point Arithmetic

Now, on the more practical side of things, when using floating point and comparing it to another number, you should almost always round the value or use an epsilon value, like this:

if (ABS(doubleValue - intValue) < 0.00001) // 0.00001 is a margin-of-error for floating point arithmetic
    // the two numbers are even (or close to it)