C ++程序已停止工作-解常微分方程 [英] C++ program has stopped working- Solving ordinary differential equations

问题描述

我正在编写一个C ++程序，以查找大学分配的一阶微分方程的解.程序启动，然后输入要执行的迭代次数，我收到错误消息"Euler的method.exe已停止工作".这是我的代码:

I'm writing a C++ program to find solutions for first order differential equations for a college assignment. The program starts up and then once I enter the number of iterations to do I get the error message "Euler's method.exe has stopped working". This is my code:

#include <functional>
#include <vector>

using namespace std;

 
double f_r(double x, double r) {  
  return r;
  } 

double f_s(double x, double s) {
  return -x/s;  
  }
 

double eulerstep(const function<double(double,double)>& f, double xsub0, double ysub0, double h) {
  double ysub1 = ysub0+ h * f(xsub0,ysub0);
  return ysub1;
  }


double euler(const function<double(double,double)>& f, double xsub0, double ysub0, double h, int n) {
  vector<double> xsub;
  vector<double> ysub;
  xsub[0] = xsub0;
  ysub[0] = ysub0;
  n = ysub.size();
  for (int i=1; i<n; i++){
  	xsub[i+1] = xsub[i] + h;
  	ysub[i+1] = ysub[i] + h * f(xsub[i],ysub[i]);
  	cout << xsub[i] << " , " << ysub[i] << endl;
  }
  return ysub[n];
  }
  
int main() {
  int nsteps = 0;
  cout << "Number of steps?" << endl;
  cin >> nsteps;
  double h = 1.0 / nsteps;

  double r = euler(f_r,0,1,h,nsteps);
  

  double s = euler(f_s,0,1,h,nsteps);
 
  return 0;
}

我相当确定问题出在我如何定义向量的问题上，但是我是新手，因此无法看到哪里出了问题.如果有人能指出我的方法中的错误，我将非常感谢

I'm fairly sure the problem lies in how I've defined my vectors but I'm new to using them so can't see where I've gone wrong. I'd be extremely grateful if anyone could point out the mistakes in my method

谢谢

推荐答案

这是一个更新的功能.您必须设置大小，而不要询问空白向量.

Here's an updated function. You have to set the sizes, not ask an empty vector for its size.

double euler(const function<double(double, double)>& f, double xsub0, double ysub0, double h, int n)
{
    vector<double> xsub;
    vector<double> ysub;
    xsub.resize(n+1); // so we can access [n], it must be size n+1
    ysub.resize(n+1);
    xsub[0] = xsub0;
    ysub[0] = ysub0;
    for (int i = 1; i<n; i++) {
        xsub[i + 1] = xsub[i] + h;
        ysub[i + 1] = ysub[i] + h * f(xsub[i], ysub[i]);
        cout << xsub[i] << " , " << ysub[i] << endl;
    }
    return ysub[n];
}

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