PHP unexpected result of float to int type cast

Question

I'trying to convert a float to an int value in php:

var_dump((int)(39.3 * 100.0)); //Returns 3929 but should be 3930!
var_dump((int)(39.2 * 100.0)); //Returns 3920

I can use ceil to make it work but can somebody explain this to me?

var_dump((int)ceil(39.3 * 100.0)); //Returns 3930

Answer 1

This is because numbers that have a finite representation in base 10 may or may not have an exact representation in the floating point representation PHP uses.

See

>php -r "echo var_dump(sprintf('%.40F', 39.3 * 100.0));"
string(45) "3929.9999999999995452526491135358810424804688"

Since int always rounds the number down, a small error in the representation makes the cast round it one number down that you would otherwise expect.

Consider using round instead.

Answer 2

This might be late but the proper way to do it is as follows:

(int) bcmul("39.3", "100.0"); // 3930
(int) bcmul("39.2", "100.0"); // 3920

and for dealing with digits / calculation for floats, or anything that involves money transaction, you should never use directly multiplication/division + casting.

please refer to this as well:

http://php.net/manual/en/book.bc.php

notice: the casting in my answer is just to convert String to Int, (you don't need to do it)