Re: Perl is ignoring fractional part sometimes

Hi jinnicky,

Although I'm no Tk expert, I investigated the AM's hint about dual-valued variables, and there indeed seems to be something buggy going on. I boiled the code down to the following:

use warnings;
use strict;
use Tk;
use Devel::Peek;

my $scale = '22.5 '; # note the space in the output as well
my $foo = 0 + $scale; # use in numeric context (set NV and IV)
Dump($scale);

my $mw = MainWindow->new;
$mw->Entry(-textvariable=>\$scale)->pack;
$mw->bind("<Key-Return>",\&calc);
calc();
MainLoop;

sub calc {
    print "str: <$scale> / num: <".(0+$scale)."> \n";
}
[download]

Output:

SV = PVNV(0x1e3f350) at 0x1e59a40
  REFCNT = 2
  FLAGS = (NOK,POK,IsCOW,pIOK,pNOK,pPOK)
  IV = 22
  NV = 22.5
  PV = 0x1e6f870 "22.5 "\0
  CUR = 5
  LEN = 10
  COW_REFCNT = 1
str: <22.5 > / num: <22>
[download]

It appears that binding $scale to the Tk::Entry widget is causing the variable to become dual-valued with the wrong initial values (I guess the PV and IV values?). This only seems to be happening once on init though, when I input something everything seems to work fine. I don't know enough about the internals of Tk to really suggest a solution (perhaps a bug report is in order), but some workaround(s). You apparently can initialize $scale with a numeric value to begin with, my $scale = 22.5; seems to work for me. Or, as you've already discovered, you can stringify $scale when reading it out.

Hope this helps,
-- Hauke D

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Re^2: Perl is ignoring fractional part sometimes by jinnicky (Sexton) on Jul 18, 2016 at 17:11 UTC
It looks like I'm going to have to stringify every value I get from a Tk::Entry box. OpenSUSE Leap 42.1 regressed perl to 5.18.2 from 5.20.1 in 13.2. It didn't work either. I'm currently upgrading Manjaro which has an up to date perl 5.24.0. I'll report on that once the update is done. Looks like I'm going to have to submit a bug on Tk. Strange that no one has noticed this already. Thanks for all your help	[reply]
Re^3: Perl is ignoring fractional part sometimes by jinnicky (Sexton) on Jul 19, 2016 at 17:14 UTC
Just for completeness, I tested it with python and tcl to see if the underlying Tk has a problem. Both worked without problem. I did test it on perl 5.24.0 with the following code: #!/usr/bin/perl -w use warnings; use strict; use Tk; use Devel::Peek; my $scale = 22.5; my $x = 14; my $foo = $x * $scale; # use in numeric context (set NV and IV) my $sb = 22.5 * 14; my $i=0; print "\n0 str: <$scale> / num: <".(0+$scale)."> product $foo should b +e $sb\n"; Dump($scale); my $mw = MainWindow->new; $mw->Entry(-textvariable=>\$scale)->pack; $mw->bind("<Key-Return>",\&calc); MainLoop; sub calc { $foo = $scale * $x; my $s2 = $scale.' '; my $foobar = $s2 * $x; $i++; print "\n$i str: <$scale> / num: <".(0+$scale)."> product $foo str +ingified $foobar should be $sb\n"; Dump($scale); } [download] This seemed to work originally. If you just hit enter a couple of times it fails with: 0 str: <22.5> / num: <22.5> product 315 should be 315 SV = PVNV(0x19572d0) at 0x1982d20 REFCNT = 2 FLAGS = (NOK,pIOK,pNOK) IV = 22 NV = 22.5 PV = 0x196fa80 "22.5"\0 CUR = 4 LEN = 32 1 str: <22.5> / num: <22> product 315 stringified 315 should be 315 SV = PVMG(0x1ed5f60) at 0x1982d20 REFCNT = 3 FLAGS = (GMG,SMG,IOK,NOK,POK,pIOK,pNOK,pPOK,UTF8) IV = 22 NV = 22.5 PV = 0x196fa80 "22.5"\0 [UTF8 "22.5"] CUR = 4 LEN = 32 MAGIC = 0x1fa6a30 MG_VIRTUAL = &PL_vtbl_uvar MG_TYPE = PERL_MAGIC_uvar(U) MG_LEN = 24 MG_PTR = 0x1f24b40 "\260(=\252\254\177\0\0\20==\252\254\177\0\0\0j +\372\1\0\0\0\0" 2 str: <22.5> / num: <22> product 308 stringified 315 should be 315 SV = PVMG(0x1ed5f60) at 0x1982d20 REFCNT = 3 FLAGS = (GMG,SMG,IOK,NOK,POK,pIOK,pNOK,pPOK,UTF8) IV = 22 NV = 22.5 PV = 0x196fa80 "22.5"\0 [UTF8 "22.5"] CUR = 4 LEN = 32 MAGIC = 0x1fa6a30 MG_VIRTUAL = &PL_vtbl_uvar MG_TYPE = PERL_MAGIC_uvar(U) MG_LEN = 24 MG_PTR = 0x1f24b40 "\260(=\252\254\177\0\0\20==\252\254\177\0\0\0j +\372\1\0\0\0\0" [download] As you can see in section 2, the product reverted to 308 going back to the IV value. The same thing happened on 5.20.1. The flags changed a little, but the results were the same. I've submitted a report on this to bug-Tk at rt.cpan.org	[reply] [d/l] [select]