PHP 4.0 Bug #9288 Updated: Math operations give wrong result under certain type-conversion circumstances

ID: 9288 Updated by: hholzgra Reported By: tim@timj.co.uk 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] derick@php.net 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] tim@timj.co.uk 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! --------------------------------------------------------------------------- ATTENTION! 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