Generate random long number

Question

I know that to generate random long number, I do following steps in Java:

Random r = new Random();
return r.nextLong();

What will be equivalent of this code in C++? like this?

return (long)rand();

Answer 1

<cstdlib> provides int rand(). You might want to check out the man page. If long is bigger than int on your system, you can call rand() twice and put the first value in the high word.

#include <cstdlib>

long lrand()
{
    if (sizeof(int) < sizeof(long))
        return (static_cast<long>(rand()) << (sizeof(int) * 8)) |
               rand();

    return rand();
}

(it's very unlikely that long is neither the same as or double the size of int, so this is practical if not theoretically perfect)

Check your docs for rand() though. It's not a great generator, but good enough for most things. You'll want to call srand() to initialise the random-number generation system. Others have commented that Windows doesn't return sizeof(int) randomised bits, so you may need to tweak the above.