Thread: use of static cast

Well, the first question, is_prime is declared int, but it really should be declare "bool" in my opinion. It may be a case of the author using an older compiler that didn's support "bool" as a type.

The second question, I take, is why is n declared int, rather than double, and thus forcing the use of static_cast for the sqrt() call... Well, if n is not an integer, you can't really use the modulo operation (and even if you could, it would be quite a bit slower).

Of course, you don't really want to use sqrt() in a condition. It would be better to do something like

Code:

  int x = sqrt(static_cast<double>(n));
  while(i < x && is_prime) {
     ... 
  }

This way, the sqrt() isn't calculated each loop, and the loop ends when we find that a number isn't a prime.

--
Mats

Code:

while (i <= sqrt(static_cast<double>(n)))

That is pretty silly, really. For one thing, passing an int to a function that is expecting a double will result in an implicit cast, as long as you have the prototype for the function, which you do. (sqrt()'s prototype is in <math.h> or <cmath>.)

Code:

int x = sqrt(static_cast<double>(n));

That wouldn't do, either. The return value of sqrt() is a double, and that should be casted before being stuffed into an int. I'd use something like

Code:

int x = static_cast<int>(sqrt(n));

Besides, there's a much simpler way to do this! Why not just use this?

Code:

while(i*i <= n)

Sure, i*i might overflow for large numbers, but n is of the same type as i, so it doesn't matter.

In C++, sqrt() and the other <cmath> functions are overloaded to take and return any of the standard floating-point types (float, double, long double). I'm not sure what it does if passed an integer type.

http://www.cplusplus.com/reference/c...math/sqrt.html

> Besides, there's a much simpler way to do this! Why not just use this?

That's probably safer also, since one has to make sure that if n is an exact square, then i actually takes on the highest value and that rounding error doesn't prevent this. It's probably also faster than sqrt().