Calculate the area of a circle

/*
time:20200415
where:gfdx
man:g-7.net
function: Experience built-in functions * / 
#include <iostream>
 using  namespace std;
inline float circle ( float r) // built-in function 
{
     return  3.14159 * r * r;
}
int main ()
{
    for ( int i = 1 ; i <= 10 ; i ++) // c ++ can define variables flexibly 
    {
        cout << "r=" << i <<'\t'<< "area=" << circle(i) << endl;
    }
    return 0;
}
Precautions:
Built-in functions must be completely defined before they are called. Built-in functions are usually written before the main function.