From: "Peña, Botp" <botp@...>
Date: 2007-08-30T11:14:44+09:00
Subject: Re: Bug in % (Float)?

From: Charlie Lehardy [mailto:charlie.lehardy@gmail.com] :
# irb(main):001:0> 1 % 0.1
# => 0.1
# 
# Shouldn't 1 % 0.1 be 0.0 and not 0.1? 

never trust floats in division :)  
ruby tries to be friendly at the expense of more surprises... 

irb(main):343:0> system "qri numeric.divmod | head -7"
--------------------------------------------------------- Numeric#divmod
     num.divmod( aNumeric ) -> anArray
------------------------------------------------------------------------
     Returns an array containing the quotient and modulus obtained by
     dividing num by aNumeric. If q, r = x.divmod(y), then

         q = floor(float(x)/float(y))
=> true

irb(main):345:0> 1.divmod 0.1
=> [9, 0.1]
irb(main):347:0> 9*0.1+0.1
=> 1.0

:(

you can use Rational instead (or BigDecimal, see below),

irb(main):358:0> x=Rational(1,1)
=> Rational(1, 1)
irb(main):360:0> y=Rational(1,10)
=> Rational(1, 10)
irb(main):361:0> r = x % y
=> Rational(0, 1)
irb(main):362:0> puts r
0
=> nil


# Also, look at the 
# following example:
# irb(main):009:0> a += (a % b) if ((a % b) > 0)
# => 1.59999999999999

again, do not put all your trust in floats or in machines for that matter :)

irb(main):344:0> 1.5.divmod 0.1
=> [14, 0.0999999999999999]

this time, let us use BigDecimal. 

irb(main):369:0> require 'bigdecimal'
=> true
irb(main):375:0* x=BigDecimal.new "1.5"
=> #<BigDecimal:b7adfacc,'0.15E1',8(8)>
irb(main):376:0> y=BigDecimal.new "0.1"
=> #<BigDecimal:b7adb904,'0.1E0',4(8)>
irb(main):377:0> r = x%y
=> #<BigDecimal:b7ad7dcc,'0.0',4(16)>
irb(main):378:0> puts r
0.0
=> nil
irb(main):379:0> q,r = x.divmod(y)
=> [#<BigDecimal:b7accaa8,'0.15E2',4(12)>, #<BigDecimal:b7accb0c,'0.0',4(16)>]
irb(main):380:0> puts q, r
0.15E2
0.0
=> nil

BigDecimal is powerful, Rational is less heavy. 
Your choice though.

kind regards -botp