This weird thing just happened in my code. All of a sudden, a simple addition with reasonably uncomplicated values (4253.95 + 0.9) result in something that looks like a float approximation. Like this:

4253.95 + 0.9 = 4254.84999999999

If I try to reproduce it, it comes out correct with 4254.85, so that's no help, except I can see that the internals of the vars are different (see below)

I can work around/force the correct hehaviour if I stringify the numerical values before doing the addition.

I thought Devel::Peek might discover something strange about this, but I'm not sure how to interpret the output of this piece of code: 

    my $self = shift;
    my ($amount) = @_;
    my $amountOld = $self->amountAccount;
    my $amountNew = $amountOld + $amount;
    $self->amountAccount($amountNew);
    if(length($amountOld) < 8 && length($amountNew) >= 8) {
        use Devel::Peek;    
        print("\n\n"), Dump($amountOld), Dump($amount), Dump($amountNe
+w), print("Went bad here: ($amountOld + $amount = $amountNew)\n\n");
        $amount = 0.9;
        $amountOld = 4253.95;
        $amountNew = $amountOld + $amount;
        print("\nNew values\n"), Dump($amountOld), Dump($amount), Dump
+($amountNew), print("Correct output: ($amountOld + $amount = $amountN
+ew)\n\n");
    }

[download]

Output: 

SV = PVNV(0x2c8ef8c) at 0x23b4958
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,POK,pNOK,pPOK)
  IV = 4256
  NV = 4253.95
  PV = 0x2d179a4 "4253.95"\0
  CUR = 7
  LEN = 35
SV = PVNV(0x2c8ef44) at 0x23b4928
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,pIOK,pNOK)
  IV = 0
  NV = 0.9
  PV = 0
SV = PVNV(0x2c8efa4) at 0x23b4988
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,POK,pNOK,pPOK)
  IV = 4309
  NV = 4254.84999999999
  PV = 0x2d17d9c "4254.84999999999"\0
  CUR = 16
  LEN = 35
Went bad here: (4253.95 + 0.9 = 4254.84999999999)


New values
SV = PVNV(0x2c8ef8c) at 0x23b4958
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,pNOK)
  IV = 4256
  NV = 4253.95
  PV = 0x2d179a4 "4253.95"\0
  CUR = 7
  LEN = 35
SV = PVNV(0x2c8ef44) at 0x23b4928
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,pIOK,pNOK)
  IV = 0
  NV = 0.9
  PV = 0x2dfa064 "0.9"\0
  CUR = 3
  LEN = 35
SV = PVNV(0x2c8efa4) at 0x23b4988
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,pNOK)
  IV = 4309
  NV = 4254.85
  PV = 0x2d17d9c "4254.84999999999"\0
  CUR = 16
  LEN = 35
Correct output: (4253.95 + 0.9 = 4254.85)

[download]

As you can see, the flags are different, but I'm not sure what that means. Is it something that is off in the first set?

Is there something else I can look at to understand what's going on here?

OS: w2k 

perl -v

This is perl, v5.8.3 built for MSWin32-x86-multi-thread
(with 8 registered patches, see perl -V for more detail)

[download]

Any ideas?

/J

In reply to 4253.95 + 0.9 = 4254.84999999999 (need help to interpret internals) by jplindstrom

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