How to determine the last nonzero decimal digit in a float?

Question

I am writing a float printing and formatting library and want to avoid printing trailing zero digits.

For this I would like to accurately determine the last nonzero digit within the first N decimal places after the decimal point. I wonder whether there is a particular efficient way to do this.

Answer 1

This (non-trivial) problem has been fully solved. The idea is to print exactly enough digits so that if you converted the printed digits back to a floating-point number, you would get exactly the number you started with.

The relevant paper is "Printing Floating-Point Numbers Quickly and Accurately", by Robert Burger and R. Kent Dybvig. You can download it here.