PHP - weird multiplication

Question

I have the following code:

$convertedCredits = round($userBalance['points'] / $convertionPoints) * $convertionPrice;
echo 'points: ' . $userBalance['points'] . ' - to: ' . $convertedCredits;

Example data (I know there is no point in dividing by 9 and multiply by 9 but that data is dynamic):

(6000/9) * 9

My echo results in the following:

points: 6000 - to: 60030

Why does that result in 60030?

Read the warning [here](http://fr2.php.net/manual/en/language.types.float.php). — Blackhole, Jun 15 '14 at 18:56
The `round()` gives you a wrong result if the result of the division is not an integer — kero, Jun 15 '14 at 18:57
Use the [BC](http://php.net/bc) or [GMP](http://php.net/gmp) extension. — Sverri M. Olsen, Jun 15 '14 at 18:58
The result of dividing 6000/9 is 666,(6). Since your precision in round is 0 then it rounds it to 667 and `667 * 9 = 6003` — Zefiryn, Jun 15 '14 at 18:58
@Zefiryn yes exactly. however the result is 60030 (see the last 0). BTW I tried first without the round — Christophe, Jun 15 '14 at 19:05
@OllieJones yes it should be 6003 instead of 60030. however it multiplies with 9 and not 90 but that does match with the result I have. I'll look into that — Christophe, Jun 15 '14 at 19:08
@M.chaudhry I want to know why my result is 60030 instead of the expected 6003 — Christophe, Jun 15 '14 at 19:09
Are you sure the trailing `0` comes from this specific `echo` and not something further down the page? — DCoder, Jun 15 '14 at 19:14
http://stackoverflow.com/questions/2100490/floating-point-inaccuracy-examples — Félix Adriyel Gagnon-Grenier, Jun 15 '14 at 19:20
got to say that dCoder and Ollie jones are right you need to debugg your whole code to check — M.chaudhry, Jun 15 '14 at 19:25

O. Jones · Accepted Answer · 2014-06-15T19:16:51.900

5

Your php code says:

round(6000 / 9) * 9

So php computes 600 / 9, yielding 666.66667.

Then it uses round() on that figure, and gets 667.

Then it multiples the result by 9, yielding 6003.

It's behaving exactly as designed.

Try changing your echo line to this and see what happens. I bet you get

echo '(points: ' . $userBalance['points'] . ' - to: ' . $convertedCredits . ')  ';

I bet the extra zero comes from somewhere else in your program and you get

(points: 6000 - to: 6003)  0

Either that or your convertionPrice value is 90.

edited Jun 15 '14 at 19:16

answered Jun 15 '14 at 19:00

O. Jones

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I first tried without the round and had the same result. and no it does not behave as expected. Your result is correct however my result gives me 60030 (sixty thousand instead of six thousand) – Christophe Jun 15 '14 at 19:06
oooh yes. It's a response from an ajax call. ... god I feel so stupid. In my defense I was watching the World cup while working :p no more WK during programming :p – Christophe Jun 15 '14 at 20:14

PHP - weird multiplication

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