Re: [PHP] Dumb Question - Casting

On Thu, Feb 18, 2010 at 16:47, Chuck <[email protected]> wrote:
> Sorry, been doing heavy perl and haven't written any PHP in 3 years so a tad
> rusty.
>
> Can someone explain why the second expression in this code snippet evaluates
> to 7 and not 8?
>
> $a = (int) (0.1 +0.7);
>
> echo "$a\n";
>
> $x = (int) ((0.1 + 0.7) * 10);
>
> echo "$x\n";
>
> $y = (int) (8);
>
> echo "$y\n";
>

The reason why you get 7 instead of 8 is because you are using
floating point arithmetic. 0.1 (i.e. the fraction 1/10) does not have
a finite representation in base 2 (like you cannot finitely represent
1/3 in base 10). So the number 0.1 is represented in the computer as a
number that is strictly less than 0.1 so when you do 0.1+0.7=x then
you have x<0.8 in the computer (think 7.9999999...). When you cast to
int you just truncate the number, i.e. you chop off the fractional
part leaving you with 7.

-- 
Daniel Egeberg