Why am I losing accuracy?

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Why am I losing accuracy? by haukex (Archbishop) on Jan 05, 2020 at 19:34 UTC
See the Math::BigInt docs in regards to its `div_scale` parameter: "This is the accuracy used when neither accuracy nor precision is set explicitly. It is used when a computation might otherwise attempt to return an infinite number of digits." The default value is 40, which is what you're seeing there. You can either pass an accuracy to the `bdiv` method explicitly, for example: `$digest = $digest->bdiv(64, 1000)->as_int;`, or perhaps do this instead: `use Math::BigRat; Math::BigRat->new($digest, 64)->as_int;`, both of these result in your desired answer. (There are probably one or two other methods but these are the first I've thought of.)	[reply] [d/l] [select]
Re^2: Why am I losing accuracy? by ikegami (Patriarch) on Jan 06, 2020 at 09:10 UTC
Could also use `$digest->brsft(6);` (since 64 is 2⁶)	[reply] [d/l]
Re: Why am I losing accuracy? by bliako (Monsignor) on Jan 07, 2020 at 12:33 UTC
Why not keep it simple and use Math::BigInt or Math::BigFloat only for those variables who need it? (and not for ALL, e.g. loop variable $i)? ~~In fact, ikegami points that all you are doing is right shifts. So, probably you need BigInt (or a simple `substr`!).~~ Sorry this is not what was suggested, I misunderstood that the default base for `brsft()` is `2`, so it is not a right shift on base 10, i.e. `$digest /= 64` bw, bliako	[reply] [d/l] [select]
Re^2: Why am I losing accuracy? by Anonymous Monk on Jan 08, 2020 at 12:32 UTC
Finally got this working (thanks, all) with just the following line of code: `use bigint (a => 80);` [download] The final code actually does division by numbers other than 64, so I can't just use shift. :) I am curious about how to assign Math::BigInt or Math::BigFloat (or bignum or bigint) only for the variables that need it. Can you enlighten me on how to do that? Thanks, again.	[reply] [d/l]
Re^3: Why am I losing accuracy? by haukex (Archbishop) on Jan 08, 2020 at 14:16 UTC
I am curious about how to assign Math::BigInt or Math::BigFloat (or bignum or bigint) only for the variables that need it. Can you enlighten me on how to do that? In that case don't `use bignum;`, instead `use Math::BigInt;` and/or `use Math::BigFloat;`, and then create the objects explicitly, as in `my $i = Math::BigInt->new(1);`. If you're done with the calculations and don't want them to be objects anymore, you can stringify them, e.g. `$i = "$1";` - don't use them as numbers or you'll loose the accuracy.	[reply] [d/l] [select]
Re^3: Why am I losing accuracy? by bliako (Monsignor) on Jan 08, 2020 at 13:06 UTC
Is the following returning correct results? # for https://perlmonks.org/?node_id=11110997 # by bliako, 08/01/2020 use strict; use warnings; use Math::BigInt; use Digest::SHA qw(sha256_hex); for(my $i=1;$i<=10;$i++){ # the sha as a hex string (no '0x' prepended) my $hexstr = sha256_hex($i); # to create a bigint from a hex string we need to prepend '0x' to +it my $digest = Math::BigInt->new('0x'.$hexstr); # specify accuracy and precision $digest->accuracy(80); $digest->precision(1); # operate print "$i $digest\n"; $digest /= 64; print "$i $digest\n"; } [download] 1 48635463943209834798109814161294753926839975257569795305637098542720 +658922320 1 75992912411265366872046584627023053010687461339952805165057966473001 +0295660 2 96094161643976066833367867971426158458230048495430276217795328666133 +331159860 2 15014712756871260442713729370535337259098445077410980659030520104083 +33299370 3 35293215426786447154857697798367884701614677727176325092965345248689 +205321680 3 55145649104353823679465152809949819846272933948713007957758351951076 +8833150 4 33984360982413536682390860969296307922929415152052354251133793603654 +468157320 4 53100564035021151066235720264525481129577211175081803517396552505710 +1064960 5 10818240655469999731486825095773005325907358940236581977057259273658 +4172823450 5 16903501024171874580448164212145320821730248344119659339151967615091 +27700370 6 10492023800686533720501340709024820017001830686534338836405100876796 +5015414400 6 16393787188572708938283344857851281276565360447709904431882970119994 +53365850 7 54734117258892461880478870895348100103198302433611928089023761078319 +507514450 7 85522058217019471688248235773981406411247347552518637639099626684874 +2304910 8 20075373234943686845167158285967784892467090849631486320124245130906 +619831460 8 31367770679599510695573684821824663894479829452549197375194133017041 +5934870 9 11463511883541201508393500803711559652562721115888819482889049505863 +143503290 9 17911737318033127356864845005799311957129251743576280442014139852911 +1617240 10 3359281535163583886622750554468341756416232122807515421831543601247 +6449103320 10 5248877398693099822848047741356783994400362691886742846611786876949 +44517240 [download] Still, I would like to find out what's going on in that loop and gets stuck to 10 !!!! bw, bliako	[reply] [d/l] [select]
Re^4: Why am I losing accuracy? by syphilis (Archbishop) on Jan 08, 2020 at 13:32 UTC
Re^5: Why am I losing accuracy? by bliako (Monsignor) on Jan 08, 2020 at 22:33 UTC
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Re^4: Why am I losing accuracy? by Anonymous Monk on Jan 08, 2020 at 22:33 UTC
Re^5: Why am I losing accuracy? by bliako (Monsignor) on Jan 08, 2020 at 22:59 UTC
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Re^5: Why am I losing accuracy? by syphilis (Archbishop) on Jan 08, 2020 at 23:15 UTC
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Re: Why am I losing accuracy? by ForgotPasswordAgain (Priest) on Jan 05, 2020 at 19:31 UTC
I think you need to set the precision to 1. bignum Options section	[reply]
Re^2: Why am I losing accuracy? by Anonymous Monk on Jan 06, 2020 at 01:14 UTC
I am definitely into things I don't understand here. :( #!/usr/bin/perl -w use strict; use bignum; use Digest::SHA qw(sha256_hex); # a small expansion on the previous code block for (my $i = 1; $i <= 9; $i++) { my $digest = hex(sha256_hex($i)); print "$i $digest\n"; $digest /= 64; print "$i $digest\n"; } 1 48635463943209834798109814161294753926839975257569795305637098542720 +658922315 1 75992912411265366872046584627023053010690000000000000000000000000000 +0000000 2 96094161643976066833367867971426158458230048495430276217795328666133 +331159861 2 15014712756871260442713729370535337259100000000000000000000000000000 +00000000 3 35293215426786447154857697798367884701614677727176325092965345248689 +205321678 3 55145649104353823679465152809949819846270000000000000000000000000000 +0000000 4 33984360982413536682390860969296307922929415152052354251133793603654 +468157322 4 53100564035021151066235720264525481129580000000000000000000000000000 +0000000 5 10818240655469999731486825095773005325907358940236581977057259273658 +4172823453 5 16903501024171874580448164212145320821730000000000000000000000000000 +00000000 6 10492023800686533720501340709024820017001830686534338836405100876796 +5015414403 6 16393787188572708938283344857851281276570000000000000000000000000000 +00000000 7 54734117258892461880478870895348100103198302433611928089023761078319 +507514449 7 85522058217019471688248235773981406411250000000000000000000000000000 +0000000 8 20075373234943686845167158285967784892467090849631486320124245130906 +619831459 8 31367770679599510695573684821824663894480000000000000000000000000000 +0000000 9 11463511883541201508393500803711559652562721115888819482889049505863 +143503287 9 17911737318033127356864845005799311957130000000000000000000000000000 +0000000 [download] I start getting incorrect mathematical calculations here. #!/usr/bin/perl -w use strict; # changed only this line use bignum (p => 1); use Digest::SHA qw(sha256_hex); for (my $i = 1; $i <= 9; $i++) { my $digest = hex(sha256_hex($i)); print "$i $digest\n"; $digest /= 64; print "$i $digest\n"; } 1 48635463943209834798109814161294753926839975257569795305637098542720 +658922315 1 81059106572016391330183023602157923211399958762616325509395164237867 +7648705 2 96094161643976066833367867971426158458230048495430276217795328666133 +331159861 2 16015693607329344472227977995237693076371674749238379369632554777688 +88852664 3 35293215426786447154857697798367884701614677727176325092965345248689 +205321678 3 58822025711310745258096162997279807836024462878627208488275575414482 +0088695 4 33984360982413536682390860969296307922929415152052354251133793603654 +468157322 4 56640601637355894470651434948827179871549025253420590418556322672757 +4469289 5 10818240655469999731486825095773005325907358940236581977057259273658 +4172823453 5 18030401092449999552478041826288342209845598233727636628428765456097 +36213724 6 10492023800686533720501340709024820017001830686534338836405100876796 +5015414403 6 17486706334477556200835567848374700028336384477557231394008501461327 +50256907 7 54734117258892461880478870895348100103198302433611928089023761078319 +507514449 7 91223528764820769800798118158913500171997170722686546815039601797199 +1791907 8 20075373234943686845167158285967784892467090849631486320124245130906 +619831459 8 33458955391572811408611930476612974820778484749385810533540408551511 +0330524 9 11463511883541201508393500803711559652562721115888819482889049505863 +143503287 9 19105853139235335847322501339519266087604535193148032471481749176438 +5725055 [download] And here, things have really gone off of the rails. #!/usr/bin/perl -w use strict; use bignum (p => 1); use Digest::SHA qw(sha256_hex); # here, I change 9 to 10 for (my $i = 1; $i <= 10; $i++) { my $digest = hex(sha256_hex($i)); print "$i $digest\n"; $digest /= 64; print "$i $digest\n"; } 1 48635463943209834798109814161294753926839975257569795305637098542720 +658922315 1 81059106572016391330183023602157923211399958762616325509395164237867 +7648705 2 96094161643976066833367867971426158458230048495430276217795328666133 +331159861 2 16015693607329344472227977995237693076371674749238379369632554777688 +88852664 3 35293215426786447154857697798367884701614677727176325092965345248689 +205321678 3 58822025711310745258096162997279807836024462878627208488275575414482 +0088695 4 33984360982413536682390860969296307922929415152052354251133793603654 +468157322 4 56640601637355894470651434948827179871549025253420590418556322672757 +4469289 5 10818240655469999731486825095773005325907358940236581977057259273658 +4172823453 5 18030401092449999552478041826288342209845598233727636628428765456097 +36213724 6 10492023800686533720501340709024820017001830686534338836405100876796 +5015414403 6 17486706334477556200835567848374700028336384477557231394008501461327 +50256907 7 54734117258892461880478870895348100103198302433611928089023761078319 +507514449 7 91223528764820769800798118158913500171997170722686546815039601797199 +1791907 8 20075373234943686845167158285967784892467090849631486320124245130906 +619831459 8 33458955391572811408611930476612974820778484749385810533540408551511 +0330524 9 11463511883541201508393500803711559652562721115888819482889049505863 +143503287 9 19105853139235335847322501339519266087604535193148032471481749176438 +5725055 10 3359281535163583886622750554468341756416232122807515421831543601247 +6449103317 10 5598802558605973144371250924113902927360386871345859036385906002079 +40818389 10 3359281535163583886622750554468341756416232122807515421831543601247 +6449103317 10 5598802558605973144371250924113902927360386871345859036385906002079 +40818389 10 3359281535163583886622750554468341756416232122807515421831543601247 +6449103317 10 5598802558605973144371250924113902927360386871345859036385906002079 +40818389 [repeats indefinitely] [download]	[reply] [d/l] [select]
Re^3: Why am I losing accuracy? by syphilis (Archbishop) on Jan 06, 2020 at 07:05 UTC
I am definitely into things I don't understand here I, too, don't know why it repeats indefinitely, but I can confirm that I see the same behaviour. If you want something that DWIMs nicely, and works quickly and reliably, I would suggest using either Math::GMPz or Math::GMP. Either will do, though they both require the GMP C library: `#!/usr/bin/perl -w use strict; use Math::GMPz; # use Math::GMP; use Digest::SHA qw(sha256_hex); for (my $i = 1; $i <= 10; $i++) { my $digest = Math::GMPz->new(sha256_hex($i), 16); # my $digest = Math::GMP-> new(sha256_hex($i), 16); print "$i $digest\n"; $digest /= 64; print "$i $digest\n"; }` [download] Output is: 1 48635463943209834798109814161294753926839975257569795305637098542720 +658922315 1 75992912411265366872046584627023053010687461339952805165057966473001 +0295661 2 96094161643976066833367867971426158458230048495430276217795328666133 +331159861 2 15014712756871260442713729370535337259098445077410980659030520104083 +33299372 3 35293215426786447154857697798367884701614677727176325092965345248689 +205321678 3 55145649104353823679465152809949819846272933948713007957758351951076 +8833151 4 33984360982413536682390860969296307922929415152052354251133793603654 +468157322 4 53100564035021151066235720264525481129577211175081803517396552505710 +1064958 5 10818240655469999731486825095773005325907358940236581977057259273658 +4172823453 5 16903501024171874580448164212145320821730248344119659339151967615091 +27700366 6 10492023800686533720501340709024820017001830686534338836405100876796 +5015414403 6 16393787188572708938283344857851281276565360447709904431882970119994 +53365850 7 54734117258892461880478870895348100103198302433611928089023761078319 +507514449 7 85522058217019471688248235773981406411247347552518637639099626684874 +2304913 8 20075373234943686845167158285967784892467090849631486320124245130906 +619831459 8 31367770679599510695573684821824663894479829452549197375194133017041 +5934866 9 11463511883541201508393500803711559652562721115888819482889049505863 +143503287 9 17911737318033127356864845005799311957129251743576280442014139852911 +1617238 10 3359281535163583886622750554468341756416232122807515421831543601247 +6449103317 10 5248877398693099822848047741356783994400362691886742846611786876949 +44517239 [download] Cheers, Rob	[reply] [d/l] [select]


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