#include <iostream>  
#include <cmath>  
#include <string>  
double calculateArea(const std::string& shapeType = "rectangle", double param1 = 1.0, double param2 = 1.0, double param3 = 1.0);   
double calculateCircleArea(double radius);   
double calculateRectangleArea(double length, double width);  

double calculateTriangleArea(double a, double b, double c);  
  
int main() {  
    int choice;  
    std::cout << "请输入要计算的图形类型(1-圆形，2-矩形，3-三角形): ";  
    std::cin >> choice;  
  
    std::string shapeType;  
    double param1, param2, param3;  
  
    switch (choice) {  
        case 1:  
            shapeType = "circle";  
            std::cout << "请输入圆形的半径：";  
            std::cin >> param1;  
            param2 = 1.0;  
            param3 = 1.0;  
            break;  
        case 2:  
            shapeType = "rectangle";  
            std::cout << "请输入矩形的长和宽：";  
            std::cin >> param1 >> param2;  
            param3 = 1.0; 
            break;  
        case 3:  
            shapeType = "triangle";  
            std::cout << "请输入三角形的边长：";  
            std::cin >> param1 >> param2 >> param3;  
            break;  
        default:  
            std::cerr << "无效的选择！" << std::endl;  
            return 1;  
    }  
  
    double area = calculateArea(shapeType, param1, param2, param3);  
    std::cout << "该图形为" << shapeType << "，其面积为：" << area << std::endl;  
  
    return 0;  
}   
double calculateArea(const std::string& shapeType, double param1, double param2, double param3) {  
    if (shapeType == "circle") {  
        return calculateCircleArea(param1);  
    } else if (shapeType == "rectangle") {  
        return calculateRectangleArea(param1, param2);  
    } else if (shapeType == "triangle") {  
        return calculateTriangleArea(param1, param2, param3);  
    } else {  
        std::cerr << "未知的形状类型！" << std::endl;  
        return -1.0;  
    }  
}  
 
double calculateCircleArea(double radius) {  
    return 3.14159265358979323846 * radius * radius;  
}    
double calculateRectangleArea(double length, double width) {  
    return length * width;  
}   
double calculateTriangleArea(double a, double b, double c) {  
    double s = (a + b + c) / 2; 
    return std::sqrt(s * (s - a) * (s - b) * (s - c));  
}