PHP 7.0 - rounding and/or casting float to int gives incorrect values

Question

Consider dockerized Laravel (5.7) app running on nginx/php 7.0.33.

I'm experiencing weird rounding (?) error:

Can anyone explain me how floor($x) is 7256 instead of 7257?

Bonus:

Bonus 2: the same test in PHP 7.4 / 8 gives good results:

https://stackoverflow.com/questions/588004/is-floating-point-math-broken?r=SearchResults&s=1|1028.8158 seems consistent https://3v4l.org/upR3f — AbraCadaver, Jan 11 '21 at 16:55

score 2 · Accepted Answer · answered Jan 11 '21 at 17:02

The best answer you'll find to your question is that computers don't handle floats all that well. Internally, the number 72.57 is actually figured to 72.569999999999999999999, which in most cases will calculate okay, but will cause you to run into exactly what you ran into, where if you multiply by 100 (7256.999999999999) then use FLOOR, you get 7256.

There are entire articles on the problems computers have with handling floats, but the best solution when accuracy is important is to avoid the use of numbers to the right of the decimal point, where possible.

This article talks about the issue in Python, but it applies to all languages: https://www.geeksforgeeks.org/floating-point-error-in-python/#:~:text=It's%20a%20problem%20caused%20when,leads%20to%20small%20roundoff%20errors.

score 0 · Answer 2 · answered Jan 11 '21 at 16:56

0

Floor rounds down to the base of it, ceil would raise the value as per the names of the functions. round determines either-or (iirc)

so in your example, 99.9 floored would be 99 and ceil would be 100

answered Jan 11 '21 at 16:56

MysticSeagull

206
2
10

1

But here the value before flooring is `7257.0` – Gacek Jan 11 '21 at 16:59
1

I would imagine the decimal has something to do with it, but that's a good question then, didn't notice that fully my bad – MysticSeagull Jan 11 '21 at 17:01

PHP 7.0 - rounding and/or casting float to int gives incorrect values

2 Answers2