Inconsistent rounding in Ruby?

Question

Does Ruby have a bug in its rounding? Why does it behave like this:

>> [1.14, 1.15, 1.16].map{|x| "%.1f" % x}
=> ["1.1", "1.1", "1.2"]
>> [1.4, 1.5, 1.6].map{|x| "%.0f" % x}
=> ["1", "2", "2"]

as in, why does 1.15 get rounded to 1.1, but 1.5 gets rounded to 2? At the very least, isn't this inconsistent? the behaviour is the same in ruby 1.9.1 and ruby 1.8.7.

Answer 1

You're using floating point numbers. Floating point numbers aren't precise. See http://en.wikipedia.org/wiki/IEEE_754-2008 for an introduction in the standard.

The short version is: NEVER use floats for anything where you need precision in any way!

Answer 2

Take a look at my answer to this question

http://stackoverflow.com/questions/1651671/why-does-perls-sprintf-not-round-floating-point-numbers-correctly/1651731#1651731

This may be the same thing

Answer 3

It's useful to recall and also quite ironic to contemplate, but floating point numbers only represent exactly: (a) a few fractions or (b) all integers.

So, to have an exact representation a fraction must be composed of (negative) powers of two. So, the following fractions are the only ones between .01 and .99 that are exactly represented:

0.25
0.50
0.75

In other words, FP is perfectly accurate when dealing with integers. Go figure.