For-loop in C++ using double breaking out one step early, boundary value not reached

Question

I have a simple C++ program compiled using gcc 4.2.4 on 32-bit Ubuntu 8.04. It has a for-loop in which a double variable is incremented from zero t

Answer 1

As others have said, not every real number is exactly representable as a floating point value, so you can expect a small, "random" rounding error in floating-point calculations. It is similar to what happens with normal decimal digits: 1/3 isn't exactly representable using three decimal digits (0.33), so (1/3)*3 would become 0.99 and not exactly 1.

It is possible to use some sort of "precision" in your comparisons, but I would recommend avoiding floating-point numbers for loops, and instead use integers.

For example, your loop

stepSize = 0.05;
for (double index = rangeMin; index <= rangeMax; index+= stepSize)
{
    cout << index << endl;
}

could be replaced by something along the lines of

stepSize = 0.05;
for (int index = 0; index < 21; ++index)
{
    double value = rangeMin + index * stepSize;
    cout << value << endl;
}