4

I get strange rounding errors from MachinePrecision values from Export. From this:

InputForm[Range[99.9997, 100.0003, 0.0001]]

I get: 

{99.9997, 99.99980000000001, 99.99990000000001, 100., 100.0001, 100.0002, 
100.0003}

Or (and more importantly) the same happens when I want to write the values in a file (Range and Table produce the same result):

Export["test.txt", Table[x, {x, 99.9997, 100.0003, 0.0001}]]

The file test.txt contains:

99.9997
99.99980000000001
99.99990000000001
100.
100.0001
100.0002
100.00030000000001

If I try to fix this using 

Export["test.txt", SetPrecision[99.99980000000001, 6]]

I get:

99.9998000000000075715433922596275806427`6.

and finally this:

Export["test.txt", N[99.99980000000001, 4]]

gives:

99.99980000000001

It sounds like a simple task but I can't figure out an elegant solution (that is not starting to build a string for example). Can you reproduce this behaviour? I'm using Mathematica 10.0.2.0. How can I get only numbers around 100 with 4 digit precision?

Kab
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  • Range[999997, 1000003, 1]/10000 // N - How it is seen depends on your settings of displayed precision. See your preferences/documentation. – ciao Feb 22 '17 at 09:59
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    A common question, e.g. (5580), (14511), (65298), (66477), (91420) – Mr.Wizard Feb 22 '17 at 10:04
  • Try converting to strings before exporting: ExportString[ToString /@ Table[x, {x, 99.9997, 100.0003, 0.0001}], "Text"]. Use NumberForm before ToString for more control. – Mr.Wizard Feb 22 '17 at 10:05
  • @ciao I understand the display part (at least a bit ;-)), but here its more important what ends up in the text file because I use it to feed another program. Thanks anyway. – Kab Feb 22 '17 at 11:43
  • @Mr.Wizard Thanks for the links. ToString@NumberForm[...] does the job (see answer below). I'm not sure why I would profit from ExportString, though. It works without it. – Kab Feb 22 '17 at 11:47

1 Answers1

3

Thanks to Mr.Wizard for bringing me on the right track. Here is the shortest workaround I found using NumberForm:

Export["test.txt",Table[ToString@NumberForm[x, 7], {x, 99.9997, 100.0003, 0.0001}]];

This gives:

99.9997
99.9998
99.9999
100.
100.0001
100.0002
100.0003

Or a more general solution:

min = 99.9997; max = 100.0003; step = 0.0001;
Export["test.txt", Table[ToString@NumberForm[x, StringLength@ToString[step, InputForm] + 1] , {x, min, max, step}]];

wich gives the same but determines the precision of step automatically.

Kab
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  • Yes, this is what I was suggesting. +1 for writing it up yourself. ExportString was only as an example to keep from having to actually write a file. I do that by default without thinking. Sorry for the confusion. – Mr.Wizard Feb 22 '17 at 11:49