Runge Kutta (RK4) 2nd order DE in C++ ERROR CODE

问题

I am receiving an error that I have never recognized before. For some background as to what this code is, I am writing a algorithm with Runge-Kutta to solve a second order differential equation (the angle of a pendulum in relation to time).

Even as I am typing this I already know there are probably many errors in this. This is part of my final project in my first ever coding course. Any help I can get, in the simplest language possible, could really help!

Here is the ERROR code

second_order.cpp: In function ‘double dxdt(double, double)’:
second_order.cpp:16:17: error: invalid operands of types ‘double’ and ‘time_t(time_t*)throw () {aka long int(long int*)throw ()}’ to binary ‘operator/’
    return theta/time;
                 ^

second_order.cpp: In function ‘int main()’:
second_order.cpp:51:22: error: cannot convert ‘time_t (*)(time_t*)throw () {aka long int (*)(long int*)throw ()}’ to ‘double’ for argument ‘1’ to ‘double dxdt(double, double)’
  kx1=dt*dxdt(time,x,v);
                      ^

second_order.cpp:52:28: error: cannot convert ‘time_t (*)(time_t*)throw () {aka long int (*)(long int*)throw ()}’ to ‘double’ for argument ‘1’ to ‘double dvdt(double, double, double, double)’
  kv1=dt*dvdt(time,x,v,coeff);
                            ^

second_order.cpp:53:22: error: invalid operands of types ‘time_t(time_t*)throw () {aka long int(long int*)throw ()}’ and ‘double’ to binary ‘operator+’
  kx2=dt*dxdt(time+dt/2,x+kx1/2,v+kv1/2);
                      ^

second_order.cpp:54:22: error: invalid operands of types ‘time_t(time_t*)throw () {aka long int(long int*)throw ()}’ and ‘double’ to binary ‘operator+’
  kv2=dt*dvdt(time+dt/2,x+kx1/2,v+kv1/2);
                      ^

second_order.cpp:55:22: error: invalid operands of types ‘time_t(time_t*)throw () {aka long int(long int*)throw ()}’ and ‘double’ to binary ‘operator+’
  kx3=dt*dxdt(time+dt/2,x+kx2/2,v+kv2/2);
                      ^

second_order.cpp:56:22: error: invalid operands of types ‘time_t(time_t*)throw () {aka long int(long int*)throw ()}’ and ‘double’ to binary ‘operator+’
  kv3=dt*dvdt(time+dt/2,x+kx2/2,v+kv2/2);
                      ^

second_order.cpp:57:19: error: invalid operands of types ‘time_t(time_t*)throw () {aka long int(long int*)throw ()}’ and ‘double’ to binary ‘operator+’
  kx4=dt*dxdt(time+dt,x+kx3,v+kv3);
                   ^

second_order.cpp:58:19: error: invalid operands of types ‘time_t(time_t*)throw () {aka long int(long int*)throw ()}’ and ‘double’ to binary ‘operator+’
  kv4=dt*dvdt(time+dt,x+kx3,v+kv3);
                   ^

make: *** [second_order.o] Error 1

Here is my code

#include <iostream>
#include <cmath>

//dvdt=-(g/L)*sin(theta)
//v=dxdt

double theta1;
double h;
double L;
double g=9.8;
double coeff=-1*(g/L);
double timepassed;

double dxdt( double timepassed, double theta )
    {
        return theta/time;
    }
double v = dxdt(theta1, timepassed);

double x = theta1;

double dvdt( double theta, double coeff, double x, double v)
    {
        return coeff*sin(x);
    }
double d2xdt2 = dvdt(timepassed, theta1, v, coeff);

int main(){
   double theta;
   double theta1;
   double h;
   double L;
   double timepassed;
   double time1;

std::cout << "Please input initial angle (in decimal radians), length of the pendulum and the time desired. Click ENTER key after each value."<<"\n";
std::cin >> theta1;
std::cin >> L;  
std::cin >> timepassed;

double g=9.8;
double coeff=-1*(g/L);

double kx1,kv1;
double kx2, kv2;
double kx3, kv3;
double kx4, kv4;
double dt;
double x = theta1;

kx1=dt*dxdt(time,x,v);
kv1=dt*dvdt(time,x,v,coeff);
kx2=dt*dxdt(time+dt/2,x+kx1/2,v+kv1/2);
kv2=dt*dvdt(time+dt/2,x+kx1/2,v+kv1/2);
kx3=dt*dxdt(time+dt/2,x+kx2/2,v+kv2/2);
kv3=dt*dvdt(time+dt/2,x+kx2/2,v+kv2/2);
kx4=dt*dxdt(time+dt,x+kx3,v+kv3);
kv4=dt*dvdt(time+dt,x+kx3,v+kv3);
x = x + (1.0/6.0)*(kx1 + 2*kx2 + 2*kx3 + kx4);
v = v + (1.0/6.0)*(kx1 + 2*kv2 + 2*kv3 + kv4);
}

回答1:

There are several problems with your code:

You cannot use the variable time because it is not declared. Use timepassed instead.
You can call function only within other functions. You cannot initialize a variable with a call to a run-time function if you are not in a function. Move these lines similar to double v = dxdt(theta1, timepassed); inside the main.
Do not initialize variable at global scope (it is allowed but I would discourage it because of ordering issues).
Initialize variable before using it otherwise you get UB.
You have variable declared at different scope with the same name i.e. L and h for instance. When you use L in the main the L declared at global scope will be shadowed.

来源：https://stackoverflow.com/questions/58934550/runge-kutta-rk4-2nd-order-de-in-c-error-code

标签

c++

differential-equations