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ConvertDoubleToString slightly wrong and does not roundtrip #1

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opened 2021-06-01 20:51:21 +00:00 by benibela · 2 comments
benibela commented 2021-06-01 20:51:21 +00:00 (Migrated from github.com)
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The last digits of ConvertDoubleToString do not fit. Perhaps something is rounded wrong?

This round trip test

uses PasDblStrUtils;

var j: integer;
    s: single absolute j;
    d, d1: double;
    i: integer;
    stri: string;

begin
  for i := low(i) to high(i) do begin
    j := i;
    d := s;
    stri := ConvertDoubleToString(d);
    d1 := ConvertStringToDouble(stri);
    if d <> d1 then writeln('a: ', d, ' ', stri, ' ', d1);
end;

finds many broken numbers:

a: -1.3872854796815689E-043 -1.387285479681264e-43 -1.3872854796812640E-043
a: -2.7745709593631378E-043 -2.774570959362528e-43 -2.7745709593625280E-043
a: -4.9746095483531006E-043 -4.974609548352007e-43 -4.9746095483520069E-043
a: -5.5491419187262756E-043 -5.549141918725056e-43 -5.5491419187250561E-043
a: -6.5300508437536476E-043 -6.5300508437522124e-43 -6.5300508437522122E-043
a: -7.1045832141268225E-043 -7.104583214125261e-43 -7.1045832141252613E-043
a: -8.0854921391541945E-043 -8.085492139152418e-43 -8.0854921391524182E-043
a: -2.1958346935969884E-042 -2.1958346935946508e-42 -2.1958346935946506E-042
a: -2.3107411676716233E-042 -2.3107411676691635e-42 -2.3107411676691636E-042
a: -2.4256476417462583E-042 -2.4256476417436762e-42 -2.4256476417436763E-042
a: -4.9129524159228087E-042 -4.912952415917579e-42 -4.9129524159175788E-042
a: -5.0278588899974437E-042 -5.0278588899920914e-42 -5.0278588899920915E-042
a: -5.1427653640720787E-042 -5.142765364066604e-42 -5.1427653640666041E-042
a: -5.2576718381467137E-042 -5.257671838141117e-42 -5.2576718381411168E-042
a: -5.3725783122213486E-042 -5.3725783122156294e-42 -5.3725783122156294E-042
a: -5.4874847862959836E-042 -5.487484786290142e-42 -5.4874847862901421E-042
a: -5.6023912603706186E-042 -5.602391260364655e-42 -5.6023912603646548E-042
a: -5.7172977344452536E-042 -5.7172977344391674e-42 -5.7172977344391674E-042
a: -7.3960532947063845E-042 -7.396053294698511e-42 -7.3960532946985108E-042
a: -7.6258662428556545E-042 -7.625866242847537e-42 -7.6258662428475374E-042
a: -7.8556791910049245E-042 -7.855679190996562e-42 -7.8556791909965614E-042
a: -8.0854921391541945E-042 -8.085492139145587e-42 -8.0854921391455867E-042
a: -1.3470682137554467E-041 -1.3470682137540127e-41 -1.3470682137540126E-041
a: -1.3700495085703737E-041 -1.3700495085689152e-41 -1.3700495085689151E-041
a: -1.3930308033853007E-041 -1.3930308033838177e-41 -1.3930308033838177E-041
a: -1.4160120982002277E-041 -1.4160120981987203e-41 -1.4160120981987202E-041
a: -1.4275027456076912E-041 -1.4275027456061715e-41 -1.4275027456061715E-041
a: -1.4389933930151547E-041 -1.4389933930136228e-41 -1.4389933930136227E-041
a: -1.4504840404226182E-041 -1.450484040421074e-41 -1.4504840404210740E-041
a: -1.4619746878300817E-041 -1.4619746878285253e-41 -1.4619746878285253E-041
a: -1.4734653352375452E-041 -1.4734653352359766e-41 -1.4734653352359765E-041
a: -1.4849559826450087E-041 -1.484955982643428e-41 -1.4849559826434280E-041
The last digits of ConvertDoubleToString do not fit. Perhaps something is rounded wrong? This round trip test ``` uses PasDblStrUtils; var j: integer; s: single absolute j; d, d1: double; i: integer; stri: string; begin for i := low(i) to high(i) do begin j := i; d := s; stri := ConvertDoubleToString(d); d1 := ConvertStringToDouble(stri); if d <> d1 then writeln('a: ', d, ' ', stri, ' ', d1); end; ``` finds many broken numbers: ``` a: -1.3872854796815689E-043 -1.387285479681264e-43 -1.3872854796812640E-043 a: -2.7745709593631378E-043 -2.774570959362528e-43 -2.7745709593625280E-043 a: -4.9746095483531006E-043 -4.974609548352007e-43 -4.9746095483520069E-043 a: -5.5491419187262756E-043 -5.549141918725056e-43 -5.5491419187250561E-043 a: -6.5300508437536476E-043 -6.5300508437522124e-43 -6.5300508437522122E-043 a: -7.1045832141268225E-043 -7.104583214125261e-43 -7.1045832141252613E-043 a: -8.0854921391541945E-043 -8.085492139152418e-43 -8.0854921391524182E-043 a: -2.1958346935969884E-042 -2.1958346935946508e-42 -2.1958346935946506E-042 a: -2.3107411676716233E-042 -2.3107411676691635e-42 -2.3107411676691636E-042 a: -2.4256476417462583E-042 -2.4256476417436762e-42 -2.4256476417436763E-042 a: -4.9129524159228087E-042 -4.912952415917579e-42 -4.9129524159175788E-042 a: -5.0278588899974437E-042 -5.0278588899920914e-42 -5.0278588899920915E-042 a: -5.1427653640720787E-042 -5.142765364066604e-42 -5.1427653640666041E-042 a: -5.2576718381467137E-042 -5.257671838141117e-42 -5.2576718381411168E-042 a: -5.3725783122213486E-042 -5.3725783122156294e-42 -5.3725783122156294E-042 a: -5.4874847862959836E-042 -5.487484786290142e-42 -5.4874847862901421E-042 a: -5.6023912603706186E-042 -5.602391260364655e-42 -5.6023912603646548E-042 a: -5.7172977344452536E-042 -5.7172977344391674e-42 -5.7172977344391674E-042 a: -7.3960532947063845E-042 -7.396053294698511e-42 -7.3960532946985108E-042 a: -7.6258662428556545E-042 -7.625866242847537e-42 -7.6258662428475374E-042 a: -7.8556791910049245E-042 -7.855679190996562e-42 -7.8556791909965614E-042 a: -8.0854921391541945E-042 -8.085492139145587e-42 -8.0854921391455867E-042 a: -1.3470682137554467E-041 -1.3470682137540127e-41 -1.3470682137540126E-041 a: -1.3700495085703737E-041 -1.3700495085689152e-41 -1.3700495085689151E-041 a: -1.3930308033853007E-041 -1.3930308033838177e-41 -1.3930308033838177E-041 a: -1.4160120982002277E-041 -1.4160120981987203e-41 -1.4160120981987202E-041 a: -1.4275027456076912E-041 -1.4275027456061715e-41 -1.4275027456061715E-041 a: -1.4389933930151547E-041 -1.4389933930136228e-41 -1.4389933930136227E-041 a: -1.4504840404226182E-041 -1.450484040421074e-41 -1.4504840404210740E-041 a: -1.4619746878300817E-041 -1.4619746878285253e-41 -1.4619746878285253E-041 a: -1.4734653352375452E-041 -1.4734653352359766e-41 -1.4734653352359765E-041 a: -1.4849559826450087E-041 -1.484955982643428e-41 -1.4849559826434280E-041 ```
BeRo1985 commented 2021-06-02 21:22:30 +00:00 (Migrated from github.com)
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Please recheck with the lastest version, where I've added Ryu-style algorithm variants.

Please recheck with the lastest version, where I've added Ryu-style algorithm variants.
benibela commented 2021-06-04 17:11:20 +00:00 (Migrated from github.com)
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Now it is losing zeros:

    WriteLn(ConvertDoubleToString(10));

=> 1

Now it is losing zeros: WriteLn(ConvertDoubleToString(10)); => 1
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