15.5 Comparing floating-point numbers
Because floating-point representations are inexact, it is rarely a
good idea to test exact inequality. The general recommendation is
that rather than testing
x == y, an approximate test may be used
given an absolute or relative tolerance.
Given a positive absolute tolerance of
x can be compared
y using the conditional
abs(x - y) <= epsilon.
Absolute tolerances work when the scale of
y and the
relevant comparison is known.
Given a positive relative tolerance of
epsilon, a typical
2 * abs(x - y) / (abs(x) + abs(y)) <= epsilon.