What I thought was a trivial addition in standard C code compiled by GCC has confused me somewhat.
If I have a double called A and also a double called B, and A = a very small exponential say 1e-20 and B is a larger value for example 1e-5 – why does my double C which equals the summation A+B take on the dominant value B? I was hoping that when I specify to print to 25 decimal places I would get 1.00000000000000100000e-5.
Instead what I get is just 1.00000000000000000000e-5. Do I have to use long double or something else?
Very confused, and an easy question for most to answer I’m sure! Thanks for any guidance in advance.
Yes, there is not enough precision in the double mantissa. 2^53 (the precision of the double mantissa) is only slightly larger than 10^15 (the ratio between 10^20 and 10^5) so binary expansion and round off can easily squash small bits at the end.
http://en.wikipedia.org/wiki/Double-precision_floating-point_format
Google is your friend etc.