二重積分定義如下（來源百度百科）

以z=xy在區域0<=y<=1,0<=x<=y內的二重積分為例，經計算理論值為：0.125，下面用C語言實現二重積分的離散計算

#include <iostream> using namespace std; double fun_dbjf(double step,double low_y, double high_y, double(*p_y_1)(double), double(*p_y_2)(double), double(*p_f_x_y)(double, double));//計算f(x,y)的二次積分 double fun_jf_fxy(double step, double low_x, double high_x, double y, double(*p_x)(double, double));//計算函數f(x,y0)的積分 double fun_fxy(double x, double y);//函數f(x,y) double fun_y_l(double x);//下限積分 double fun_y_h(double x);//上限積分 int main(int argc, char*argv[]) {cout << "二次積分值為：" << fun_dbjf(0.00001, 0, 1, fun_y_l, fun_y_h, fun_fxy) << endl;cout << "理論值為:" << 1.0 / 8.0;} double fun_dbjf(double step,double low_y, double high_y, double(*p_y_l)(double), double(*p_y_h)(double), double(*p_f_x_y)(double, double)) {double sum = 0;for (double t = low_y; t < high_y; t += step){sum += fun_jf_fxy(step, (*p_y_l)(t), (*p_y_h)(t), t, p_f_x_y)*step;}return sum; } double fun_jf_fxy(double step, double low_x, double high_x, double y,double(*p_x)(double,double)) {double sum = 0;for (double t = low_x; t < high_x; t += step){sum += (*p_x)(t, y)*step;}return sum; } double fun_fxy(double x, double y) {return x*y; } double fun_y_l(double x) {return 0;//f(x)=0 } double fun_y_h(double x) {return x;//f(x)=x }

運行結果：

運行時間較長，但結果與理論值十分接近

總結

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