What does this mean?

[ragecoder]

Okay to start out I need some help with this small program. I started recently learning C++, and this is one of the examples in the book what I want to know about it is on line 33 where it has the word "double" inside (). I have tried to find it on the forms but to no avail, due in no small part to my lack of xp. I think that it's changing the type but I can't be sure.
thanks.

 #include <iostream>
#include <math.h>

int prime(int n);

int main()
{
    int i;
    //set up infinate loop. break if user enters 0.
    //otherwise, evaluate n from prime-ness.
    while(1)
    {
        std::cout << "\nEnter a number (0 to exit) and press ENTER: ";
        std::cin >> i;

        if(i == 0)
        {
            break; // exit
        }
        if(prime(i))    //call prime
        {
            std::cout << std::endl << i << " is prime.";
        }
        else
        {
            std::cout << std::endl << i << " is not prime.";
        }
    }
}

int prime(int n)
{
int i;
    for(i = 2; i <= sqrt((double)n); i++)
    {
        if(n % i  == 0)
            return false;
    }
    return true;
}

[Catfish4]

I think that it's changing the type but I can't be sure.

You are correct. It is called "casting".

In your example, that is an old style cast, inherited from the C language.
As a C++ programmer you should favor using the new style casts of C++, for clarity:

for (i = 2; i <= sqrt(static_cast<double> (n)); i++)
{
    // ...
}

http://www.cplusplus.com/doc/tutorial/typecasting/

[ragecoder]

Thanks so much I think the book i'm learning from is a bit old.

[Catfish4]

Here are some links that you may want to bookmark for future reading:

http://www.cplusplus.com/doc/tutorial/
http://www.cplusplus.com/reference/
http://en.cppreference.com/w/
http://www.icce.rug.nl/documents/cplusplus/
http://www.parashift.com/c++-faq/
http://c-faq.com/

The last link talks about the C language, but do not underestimate its relevance.