c语言利用函数完成以下函数功能

如题，这个分段函数怎么用函数写出来呀
y=x+3 x>0

y=0 x=0;

y = -x x<0

源码和运行效果，供参考

#include<iostream>
using namespace std;
int main()
{
    int x = 0;
    cout << "输入x的值：" << endl;
    cin >> x;
    while (x != 999)
    {
        if (x > 0)
        {
            int y = 0;
            cout << "检测到x大于0，则y=x+3" << endl;
            y = x + 3;
            cout << "y = " << y<<endl;
        }
        else if (x == 0)
        {
            int y = 0;
            cout << "检测到x等于0，则y=0" << endl;
            cout << "y = " << y<<endl;
        }
        else {
            int y = 0;
            cout << "检测到x小于0，则y=-x" << endl;
            y = -x;
            cout << "y = " << y<<endl;
        }
        cout << "输入x=999退出函数赋值" << endl;
        cout << "再次为x赋值为："; cin >> x;
    }
}