optimization

in the following flow control:
for($i=10; $i < ($num/2); $i++){

You can limit the number of check by doing this:
for($i=10; $i < sqrt($num); $i++){

Thanks Jacq! Spot on!

If anyone has any optimisation recommendations then they are most welcome to suggest them.

Hi Ahmad,

I was using your class for ProjectEuler.net's Problem #7 where one has to find a specific prime number (the 10,001st prime) and I discovered that the class has a slight issue starting with the number 121 which results in a false positive.

The issue happens within the loop because the sqrt(121) = 11 (and therefore the number shouldn't be prime since the square root results in a whole number).

There are two ways to resolve the issue, one is to change the ending condition in the for loop to use <= rather than < (that way 11 would be checked properly). Another thing to do is add in a check to see if the sqrt($num) is a whole number (if it isn't some decimal number then we know the number is not prime).

In my case I just went ahead and implemented both:
$squareRoot = sqrt($num);

// If the square root ends up being a whole number
// then that number cannot be prime:
if ($squareRoot == (int) $squareRoot)
{
return false;
}

for($i = 10; $i <= $squareRoot; $i = $i + 1)

And that should do the trick :-).

-Omar

I should really do more checks and pay more attention... Didn't see this post until recently. Code fixed now. Thank you.