#include <iostream>
#include <cmath>
#include <string>
#define M_PI 3.14159265358979323846
using namespace std;

// 函数声明
double calculateArea(const string& shapeType = "rectangle", double dim1 = 1.0, double dim2 = 1.0, double dim3 = 1.0);

int main() {
    int shapeChoice;
    cout << "请输入要计算的图形类型(1-圆形，2-矩形，3-三角形): ";
    cin >> shapeChoice;

    
    switch (shapeChoice) {
    case 1: { 
        double radius;
        cout << "请输入圆形的半径：";
        cin >> radius;
        double area = calculateArea("circle", radius);
        cout << "该图形为圆形，其面积为：" << area << endl;
        break;
    }
    case 2: {
        double length, width;
        cout << "请输入矩形的长和宽：";
        cin >> length >> width;
        double area = calculateArea("rectangle", length, width);
        cout << "该图形为矩形，其面积为：" << area << endl;
        break;
    }
    case 3: { 
        double a, b, c;
        cout << "请输入三角形的三条边长：";
        cin >> a >> b >> c;
        double area = calculateArea("triangle", a, b, c);
        cout << "该图形为三角形，其面积为：" << area << endl;
        break;
    }
    default:
        cout << "无效的输入，请重新运行程序。" << endl;
        break;
    }

    return 0;
}

// 计算不同形状的面积
double calculateArea(const string& shapeType, double dim1, double dim2, double dim3) {
    if (shapeType == "circle") {
        // π * r²
        return M_PI * dim1 * dim1;
    }
    else if (shapeType == "rectangle") {
        // 长 * 宽
        return dim1 * dim2;
    }
    else if (shapeType == "triangle") {
        //海伦公式计算三角形面积
        double s = (dim1 + dim2 + dim3) / 2; // 半周长
        return sqrt(s * (s - dim1) * (s - dim2) * (s - dim3));
    }
    else {
        
        return 0;
    }
}