I saw this post on ruby-talk: raise "false" if ((625.91 + 900.00 + 22.00) != 1547.91) And yes, of course it raises the exception in ruby or in C. Guy Decoux (as always) answered quickly:

 svg% ruby -e 'p  "%.24f" % (625.91 + 900.00 + 22.00)'
 "1547.909999999999854480847716"
 svg%

 svg% ruby -e 'p  "%.24f" % 1547.91'
 "1547.910000000000081854523160"
 svg%

Dave Thomas explained: It’s about 40 years old, and unlikely to be fixed. Floating point numbers are not represented exactly inside computers, and so floating point comparisons are routinely deprecated in books on programming. Certain values cannot ever be expressed in floating point representation. If you want exact, fractional, math, you should probably use the ‘rational’ library and investigate ‘mathn’. This is the classic article to read link What Every Computer Scientist Should Know About Floating-Point Arithmetic.

Michael Neumann added: In Ruby you can use BigDecimal:

 require 'bigdecimal'
 BigDecimal.new("625.91") + 900 + 22 == BigDecimal.new("1547.91") # => true