ID: 9288
Updated by: hholzgra
Reported By: [EMAIL PROTECTED]
Old-Status: Bogus
Status: Closed
Bug Type: Math related
Assigned To:
Comments:
short answer:
never use floats or doubles for financial data!
longer answer:
the internal number format in modern computers
is binary (base 2) and not decimal (base 10) for performance
and complexity reasons
while it is possible to convert decimal numbers into binaries
and back this does not hold true for fractions
something like 0.3 (decimal) would be a periodic binary
fraction like 10/3 is 0.33333333333333333333...
in decimal
this leads to loss of precision when calculation with
decimal fractions as you have when storing currency values
solution:
if you just summ up values then you should store values in
the smalest unit your currency has (pennies?) instead of
what you are used to (Pounds?) to totally avoid fractions
if you cannot avoid fractions (like when dealing with
percentage calculations or currency conversions) you
should just be aware of the (usually very small) internal
conversion differences
(0.000000000000000027755575615629 in your example)
or use the bcmath extension, although for monetary
values you should go perfectly fine with using round(...,2)
on your final results
Previous Comments:
---------------------------------------------------------------------------
[2001-02-15 14:51:28] [EMAIL PROTECTED]
This is very normal in the world of floating point math. All numbers you write and
have a fraction are only very good approximations of a number.
---------------------------------------------------------------------------
[2001-02-15 14:29:53] [EMAIL PROTECTED]
Please, I believe I have found a serious bug. I can reproduce it 100%.
Essentially I have a loop (I think this might be significant?) that checks through
some rows in a database and if the value of a row (an amount of money) is less than
the total ($total2pay) then the ID for that row is inserted into an array to be
cancelled, and the total is reduced by the amount of that row.
The loop works fine, unless there are a number of items which add up exactly to the
total. Then, when subtracting the final item (so we get x - x) instead of 0 we get
something like 2.7755575615629E-17.
Below is the problem script and underneath that is the output I get. The script has
been expanded out to use more variables to make it clearer and prints some debugging
stuff. It's not incredibly efficient on SQL requests, but that's irrelevant.
I will gladly give any more information or debug output on request if it's helpful.
NB I do NOT have the Zend Optimiser running for this.
The database structure of table 'debts' is:
id int(10), owed_to varchar(20), owed_by varchar(20), amount decimal (10,2)
// Let there be initially 3 rows in the db which match the query below, // with amount
= "0.71", "0.20" and "0.10" respectively
$res = db_query("SELECT * FROM debts WHERE owed_by='$who_is_paying' AND
owed_to='$payee' ORDER BY amount DESC");
// Let $total2pay = 1.01
do {
while ($row = mysql_fetch_array($res)) {
if ($row[amount] <= $total2pay) { // cancel item
$cancel[] = $row[id];
print "oldtotal:$total2pay,type ".gettype($total2pay)."<br>";
$new = $row[amount];
print "amount2delete:$new, type ".gettype($new)."<br>";
// BELOW LINE IS THE PROBLEM ONE
$fish= $total2pay - $new;// deduct it from the total owed
print "newtotal:$fish,type ".gettype($fish)."<br>";
$total2pay = $fish;
}
}
// delete the marked items from the database
if ($cancel) {
reset($cancel);
while ($item = each($cancel)) {
db_query("DELETE FROM debts WHERE id='$item[value]'");
}
}
// retrieve the fresh, updated item list from the databas
mysql_free_result($res);
$res = db_query("SELECT * FROM debts WHERE owed_by='$who_is_paying' AND
owed_to='$payee' ORDER BY amount DESC");
$rows = db_num_rows($res);
} while (($total2pay > 0) && ($rows > 0) && $cancel);
The output from this is:
oldtotal:1.01,type string
amount2delete:0.71, type string
newtotal:0.3,type double
oldtotal:0.3,type double
amount2delete:0.20, type string
newtotal:0.1,type double
oldtotal:0.1,type double
amount2delete:0.10, type string
newtotal:2.7755575615629E-17,type double
I have played a bit to try to fix it, and found that if I set $new explicitly to type
double just before the problem line, the output is exactly the same, i.e. the output
is:
oldtotal:1.01,type double
amount2delete:0.71, type double
newtotal:0.3,type double
oldtotal:0.3,type double
amount2delete:0.2, type double
newtotal:0.1,type double
oldtotal:0.1,type double
amount2delete:0.1, type double
newtotal:2.7755575615629E-17,type double
Another thing: if there is only one row in the database and this adds up to the total,
it works fine:
oldtotal:1.01,type string
amount2delete:1.01, type double
newtotal:0,type double
I really hope someone can help me get to the bottom of this!
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