Why does an int pass a value when defined outside the main() function, but not inside for a for loop?

Question

this works
#include <iostream>
using namespace std;
int a;
int main()
{
for (a; a < 10; a++) {
cout << a << endl;
}

return 0;
}

but this doesnt
#include <iostream>
using namespace std;

int main()
{
int a;
for (a; a < 10; a++) {
cout << a << endl;
}

return 0;
}

Answer 1

The loop condition should be writen: (; a < 10; a++) (try removing the initial "a")

Answer 2

When inside it's "local" and only the parent can access it.

Answer 3

yes but shouldn't all of the data that is local to the same function be able to be accessed in side that function?