php - why does floor round down a integer?

Question

I am confused as to why:

echo log10(238328) / log10(62);

results in 3

but

echo floor(log10(238328) / log10(62));

results in 2

I know floor rounds down but I thought it was only for decimal numbers.

How can I get an answer of 3 out of the latter statment whilst still normally rounding down?

Answer 1

PHP uses double-precision floating point numbers. Neither of the results of the two logarithms can be represented exactly, so the result of dividing them is not exact. The result you get is close to, but slightly less than 3. This gets rounded to 3 when being formatted by echo. floor, however returns 2.

You can avoid the inexact division by taking advantage of the fact that log(x, b) / log(y, b) is equivalent to log(x, y) (for any base b). This gives you the the expression log(238328, 62) instead, which has a floating point result of exactly 3 (the correct result since 238328 is pow(62, 3)).

Answer 2

It's due to the way floating point numbers are polished in PHP.

See the PHP Manual's Floating Point Numbers entry for more info

A workaround is to floor(round($value, 15));. Doing this will ensure that your number is polished quite accurately.