Dear Monks,

I am at my wits' end over the values "-3.5527136788005e-015" and "-1.4210854715202e-014" when 0 is expected in each case. I'm on Windows Perl 5.14.

Essentially, what I am doing is to subtract a total amount (a session value) from each item's amount. Sometimes I get the expected 0 but sometimes I get strange values even though last amount to be subtracted from the total amount is equal to that total amount:

foreach my $key (sort keys %$cart_info) {
    my $qty = $cart_info->{$key}->{qty};
    my $amount = $cart_info->{$key}->{amount};
    $session->param('CART')->{total_amount} -= $amount;
    $session->param('CART')->{total_qty} -= $qty;; 
}

# First sample
Before: session_total_amt: 585.86
After: key: 116, amount: 112.09, session_total_amt: 473.77
Before: session_total_amt: 473.77
After: key: 117, amount: 69.75, session_total_amt: 404.02
Before: session_total_amt: 404.02
After: key: 118, amount: 113.57, session_total_amt: 290.45
Before: session_total_amt: 290.45
After: key: 123, amount: 113.57, session_total_amt: 176.88
Before: session_total_amt: 176.88
After: key: 124, amount: 69.75, session_total_amt: 107.13
Before: session_total_amt: 107.13
After: key: 125, amount: 80.89, session_total_amt: 26.24
Before: session_total_amt: 26.24
After: key: 50, amount: 26.24, session_total_amt: -3.5527136788005e-01
+5

# Notice that both the session_total_amt and the amount to be subtract
+ed are 26.24. How did I end up with -3.5527136788005e-015?

# Second sample
Before: session_total_amt: 319.02
After: key: 116, amount: 112.09, session_total_amt: 206.93
Before: session_total_amt: 206.93
After: key: 117, amount: 69.75, session_total_amt: 137.18
Before: session_total_amt: 137.18
After: key: 118, amount: 113.57, session_total_amt: 23.61
Before: session_total_amt: 23.61
After: key: 56, amount: 23.61, session_total_amt: -1.4210854715202e-01
+4

# Same thing here. The session_total_amt and the amount to be subtract
+ed are both 23.61.

[download]

What am I missing here? (scratch head)?

In reply to Getting stranger values in subtraction by Anonymous Monk

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