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Bug #13210

Updated by dexter3 (Alex V) over 5 years ago

~~~ ruby 
 0.1 - 0.0001 => 0.0999 
 0.1 - 0.001    => 0.099 
 0.1 **0.1 - 0.01     => 0.09000000000000001 0.09000000000000001** 

 0.01 - 0.0001 => 0.00999 
 0.01 - 0.001 => 0.0099 
 0.01 **0.01 - 0.01 =>    0.009000000000000001 0.009000000000000001** 
 ~~~ 

 There is something strange going on. 
 As you can see, the subtraction of A-B with B being ten times smaller as A returns a wrong result. But this is somehow not always the case: 

 ~~~ ruby 
 2.4.0 :001 > for i in 1..21 do p "#{0.1*i} - #{0.01*i} = #{0.1*i - 0.01*i}" end 
 "0.1 - 0.01 = 0.09000000000000001" 
 "0.2 - 0.02 = 0.18000000000000002" 
 "0.30000000000000004 - 0.03 = 0.27" 
 "0.4 - 0.04 = 0.36000000000000004" 
 "0.5 - 0.05 = 0.45" 
 "0.6000000000000001 - 0.06 = 0.54" 
 "0.7000000000000001 - 0.07 = 0.6300000000000001" 
 "0.8 - 0.08 = 0.7200000000000001" 
 "0.9 - 0.09 = 0.81" 
 "1.0 - 0.1 = 0.9" 
 "1.1 - 0.11 = 0.9900000000000001" 
 "1.2000000000000002 - 0.12 = 1.08" 
 "1.3 - 0.13 = 1.17" 
 "1.4000000000000001 - 0.14 = 1.2600000000000002" 
 "1.5 - 0.15 = 1.35" 
 "1.6 - 0.16 = 1.4400000000000002" 
 "1.7000000000000002 - 0.17 = 1.5300000000000002" 
 "1.8 - 0.18 = 1.62" 
 "1.9000000000000001 - 0.19 = 1.7100000000000002" 
 "2.0 - 0.2 = 1.8" 
 "2.1 - 0.21 = 1.8900000000000001" 
  => 1..21 
 ~~~ 

 Even 0.1*x seems sometimes to fail: 

 ~~~ ruby 
 2.4.0 :002 > for i in 1..21 do p "0.1*#{i} = #{0.1*i}" end 
 "0.1*1 = 0.1" 
 "0.1*2 = 0.2" 
 "0.1*3 = 0.30000000000000004" 
 "0.1*4 = 0.4" 
 "0.1*5 = 0.5" 
 "0.1*6 = 0.6000000000000001" 
 "0.1*7 = 0.7000000000000001" 
 "0.1*8 = 0.8" 
 "0.1*9 = 0.9" 
 "0.1*10 = 1.0" 
 "0.1*11 = 1.1" 
 "0.1*12 = 1.2000000000000002" 
 "0.1*13 = 1.3" 
 "0.1*14 = 1.4000000000000001" 
 "0.1*15 = 1.5" 
 "0.1*16 = 1.6" 
 "0.1*17 = 1.7000000000000002" 
 "0.1*18 = 1.8" 
 "0.1*19 = 1.9000000000000001" 
 "0.1*20 = 2.0" 
 "0.1*21 = 2.1" 
  => 1..21 
 ~~~ 

 And to be sure that this can lead to problems: 

 
 ~~~ ruby 
 2.4.0 :003 > 0.1 * 3 == 0.3 
  => false 
 ~~~ 

 I successfully reproduced the wrong results on another device. Both devices are running on Ubuntu. I also reproduced it with ruby 2.3.1. 

 The wrong results somehow have has a pattern, but I'm not sure about their origin. 

 I suggest ranking this bug as critical, as ruby should be able to calculate correctly. 

 BR Alex 


 Edit: typo

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