in reply to Re^4: Why am I losing accuracy?
in thread Why am I losing accuracy?

I don't mean to put pressure - I just leave this here for anyone.

both of these fail with an infinite loop printing i=10 on linux fedora 30, perl 5.28.2. 10 and 15 are the min numbers to do this.

use bignum (p=>1);

for(my $i = 1;$i->blt(15);$i->binc()){
    print "i=$i\n";
    
}

[download]
for(my $i = 1;$i<=10;$i++){
    print "i=$i\n";
}

[download]

this shows to me that it is not the loop or the overloaded increment but that when p=>1 it thinks that it's adding that value divided by 10 or something 

my $i = 10;
        print "1 i=$i\n";
$i++;
        print "2 i=$i\n";
$i = $i + 1;
        print "3 i=$i\n";
$i = $i + 2;
        print "4 i=$i\n";
$i = $i + 3;
        print "5 i=$i\n";
$i = $i + 4;
        print "6 i=$i\n";
$i = $i + 5;
        print "7 i=$i\n";
$i = $i + 6 ;
        print "8 i=$i\n";

[download]
1 i=10
2 i=10
3 i=10
4 i=10
5 i=10
6 i=10
7 i=20
8 i=30

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I have filed a bug to bigint

Replies are listed 'Best First'.
Re^6: Why am I losing accuracy?
by bliako (Abbot) on Jan 11, 2020 at 12:07 UTC

    The maintainer of bigint replied to filing a bug:

    Here is your problem: p=>1 specifies a precision of 1. See these for 
comparisation:

  $ perl -Mbignum=p,1 -wle '$i = 1; print $i + 123456788,"\n"'
  123456790

  $ perl -Mbignum=p,2 -wle '$i = 1; print $i + 123456788,"\n"'
  123456800

  $ perl -Mbignum=p,5 -wle '$i = 1; print $i + 123456788,"\n"'
  123500000

Looks to me it works as designed.

Here is the same with different accuracy values:

  $ perl -Mbignum=a,1 -wle '$i = 1; print $i + 123456788,"\n"'
  100000000

  $ perl -Mbignum=a,2 -wle '$i = 1; print $i + 123456788,"\n"'
  120000000

  $ perl -Mbignum=a,3 -wle '$i = 1; print $i + 123456788,"\n"'
  123000000

and with negative values of p:

  $ perl -Mbignum=p,-3 -wle '$i = 1; print $i + 123456789.1,"\n"'
  123456790.100

  $ perl -Mbignum=p,-6 -wle '$i = 1; print $i + 123456789.1,"\n"'
  123456790.100000

Best regards,

Tels

    [download]
[reply]
[d/l]

      I am thinking to reply with maybe bignum should be put under the ACME tree? but holding back thinking I am too extreme. I still can't digest that forloop behaviour. Fair enough it's a design choice but the unawary, common-sense Perl user is in for a big surprise when trying to ... increase precision.

[reply]
[d/l]
        Fair enough it's a design choice but the unawary, common-sense Perl user is in for a big surprise when trying to ... increase precision.

        I'm a little confused (about the whole subthread, actually, as well as this one) - isn't p=>1 explicitly asking for reduced precision?

[reply]
[d/l]

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