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Re: floating point comparison

  • To: mathgroup at smc.vnet.net
  • Subject: [mg95263] Re: floating point comparison
  • From: dh <dh at metrohm.com>
  • Date: Tue, 13 Jan 2009 07:03:16 -0500 (EST)
  • References: <gkgp8r$3vg$1@smc.vnet.net>


Hi Bert,

due to finite precision, it is not a good idea to accurately compare 

machine numbers. Therefore, some "grey zone" is usually defined. In Mathematica , 

from the manual:

"Approximate numbers are considered unequal if they differ beyond their 

last two decimal digit"

Now, what are the last 2 digits? Consider:

1.0 != 1.00000000000001

this gives False, both nu numbers are in agreement for digit 1..14, they 

differ in the 15. digit. Now consider:

1.0 - 1.00000000000001 != 0

this gives True because the numbers 1.0 - 1.00000000000001 and 0 already 

differ in the first digit.

hope this helps, Daniel



Bert Aerts (rm X) wrote:

> In the code shown below, the result of iterFixedPointList[Cos,0.0] in 94

> iterations is equal to FixedPointList[Cos,0.0]. The while loop comparison

> happens as newV - oldV != 0.0. However if the comparison is written as

> newV != oldV, then the there are only 82 iterations and the result is not

> the final one. Why are these 2 comparisons not equal?

> 

> Remark: machine precision numbers are best shown in 16 digits precision:

> Global Mathematica definition in Edit / Preferences : Appearance /

> Numbers / Formatting / Displayed Precision = 16 in Mathematica 7.0.0 x86_64

> on Linux

> 

> Clear[iterFixedPointList];

> iterFixedPointList[function_, initValue_] := 

>  Module[{i, oldV, newV, listV},

>   i = 1;

>   oldV = initValue;

>   newV = function[initValue];

>   listV = {oldV, newV};

>   While[newV - oldV != 0.0,

>    i++;

>    oldV = newV;

>    newV = function[newV];

>    listV = Append[listV, newV];

>    (*

>     Appending newV here already before it is tested

>    makes that the last 2 values in listV will be equal

>    *)

>    ];

>   Print["Number of iterations is ", i];

>   listV

>   ]

> 

> 

> 




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