What's wrong with this simple C++ code?

Question

Why will this code give an error?

#include <iostream>
using namespace std;
int main() {
for (x=5;x<12;x++) {
cout <<x;
}
}

Is it because the variable x was not declared?
Any other reasons?

Answer 1

Yes, undefined variable <x> is the cause.

Answer 2

use it :
for(int x = 5;x<12;x++)

Answer 3

The datatype of x not defined any where.