Floating Point Precision Error
common-lisp, floating-accuracy, lisp, lispworks
Solution
Floats are not exact
Floats represent a subset of mathematical reals with certain precision, they are not exact numbers. Round off errors are inevitable.
The ANSI Common Lisp standard provides for 4(!) levels of float precision: short, single, double, long (all implementations provide at least 2). Neither is exact, but ORACLE is probably using `double`, so if you stick with doubles, you should be fine.
Theory
Please read What Every Computer Scientist Should Know About Floating-Point Arithmetic.
Use integers and ratios if you want exact numbers
If you want to do exact computations, you should be using `integer`s (e.g., representing currency as a number of cents, not dollars) or `ratio`s.
Problem
I am having problem with the LISP expression below. There is floating precision error while doing sum for floating point numbers. ``` CL-USER> (+ -380 -158.27 -35.52) Actual: -573.79004 Expected: -573.79000 ``` Please suggest me how can I achieve the expected result in LISP (I am using Lispworks).