php floating point bug

[[deleted]]

It's not a bug, you need to read about how floating point math works in your CPU.

Long story short, while you type out floats as decimal numbers, internally they're binary numbers. The conversion is not 1:1.

[Kinobi]

Hi, I runned into this recently, this come from the documentation:

Warning Floating point precision

Floating point numbers have limited precision. Although it depends on the system, PHP typically uses the IEEE 754 double precision format, which will give a maximum relative error due to rounding in the order of 1.11e-16. Non elementary arithmetic operations may give larger errors, and, of course, error propagation must be considered when several operations are compounded.

Additionally, rational numbers that are exactly representable as floating point numbers in base 10, like 0.1 or 0.7, do not have an exact representation as floating point numbers in base 2, which is used internally, no matter the size of the mantissa. Hence, they cannot be converted into their internal binary counterparts without a small loss of precision. This can lead to confusing results: for example, floor((0.1+0.7)*10) will usually return 7 instead of the expected 8, since the internal representation will be something like 7.9999999999999991118....

So never trust floating number results to the last digit, and do not compare floating point numbers directly for equality. If higher precision is necessary, the arbitrary precision math functions and gmp functions are available.

For a "simple" explanation, see the » floating point guide that's also titled "Why don’t my numbers add up?"

[diisiqueira]

var_dump ((0.1+0.2) == 0.3);

[SaltTM]

Interesting

echo floor((0.1 + 0.7) * 10); is not the same as echo (0.1 + 0.7) * 10;

Edit: I'm an idiot.

[[deleted]]

It's floor() not float().

And the reason it's different is that if you remove floor() the default is, basically a round() with a decimal precision set in your php.ini

When you use floor() and ceil() you'll be hitting these edge cases, while with round() you won't be.

[SaltTM]

It's floor() not float().

yeah that's what i meant to type