1

大家好，当我使用这段代码（不包括makefile）时，我应该在同一组轴上绘制两个正弦输出，但由于某种原因，尽管输出数据是准确的，但图表并不准确。面对它甚至不是平滑的曲线，它有孔和尖。有人知道为什么吗？我为时间轴编写的数组是否与此有关？（注意编码初学者）

#include <iostream> 
#include <cmath>
#include <stdio.h>
#include "cpgplot.h"

//d2xdt2 = -(g/L)*sin(x)
//dwdt = -d2xdt2
//coeff = -(g/L)
//dxdt= w

float dxdt(float w)
        {
            return w;
        }

float dwdt(float x, float coeff)
        {
            return coeff*sin(x);
        }

int main(){

// Standard Variables   
    float wARR[5];  
    float xARR[5]; 
    float tARR[5];    
    float w;       //w = angular velocity
    float x;     //x = theta used in second order equation
    float xo;          //xo = theta initial 
    float wo = 0;       //wo = initial angular velocity 
    float dt = 1;       //h = step value

    float g=9.8;        //g = gravity
    float L;        //L = length of pendulum
    float to = 0;   
    float t;   
    float time;

    float kx1, kw1;
    float kx2, kw2;
    float kx3, kw3;
    float kx4, kw4;

// Input Printing
std::cout << "\n";
std::cout << "Click ENTER key after each value. Please input: \n The initial angle (in decimal radians): "<<"\n";
    std::cin >> xo;
std::cout << "The length of the pendulum (in meters)."<<"\n";
    std::cin >> L;  
    std::cout << "\n";

// Specific Variable Declarations
    float coeff=-(g/L);

//Checkpoint values 
    std::cout << "CHECKPOINT VALUES: ";
    std::cout << "\n";
    std::cout << "Confirming your initial angle is: " << xo << " radian(s)"; 
    std::cout << "\n";
    std::cout << "Confirming the length is: " << L << " meters(s)"; 
    std::cout << "\n";
    std::cout << "Confirming the gravity constant is: " << g << " meters/sec"; 
    std::cout << "\n";
    std::cout << "Coeff (-g/L): " << coeff;
    std::cout << "\n";
    std::cout << "\n";

    std::cout << "Insert the time (in seconds) you want to learn some cool stuff about your pendulum: "<<"\n";
    std::cin >> time;
    std::cout << "\n";

//Array info and Runge Kutta
    xARR[0] = xo;
    wARR[0] =  0;
    tARR[0] = 0;
    time=time+1;

for (int i = 0; i < time ; i++){
    x = xARR[i]; 
    w = wARR[i]; 
    t = tARR[i];

    kx1=dt*dxdt(w);
    kw1=dt*dwdt(x,coeff);
    kx2=dt*dxdt(w+kw1*0.5);
    kw2=dt*dwdt(x+kx1*0.5, coeff);
    kx3=dt*dxdt(w+kw2*0.5);
    kw3=dt*dwdt(x+kx2*0.5, coeff);
    kx4=dt*dxdt(w+kw3);
    kw4=dt*dwdt(x+kx3, coeff);

    std::cout << "\n";  
    std::cout << "RUNGE KUTTA VALUES: at " << i << " second(s)" << "\n";
    std::cout << "kx1:  " << kx1 << "\n";
    std::cout << "kx2:  " << kx2 << "\n";
    std::cout << "kx3:  " << kx3 << "\n";
    std::cout << "kx4:  " << kx4 << "\n";
    std::cout << "kw1:  " << kw1 << "\n";
    std::cout << "kw2:  " << kw2 << "\n";
    std::cout << "kw3:  " << kw3 << "\n";
    std::cout << "kw4:  " << kw4 << "\n";

    xARR[i+1] = xo + (1.0/6.0)*(kx1 + 2*kx2 + 2*kx3 + kx4); 
    wARR[i+1] = wo + (1.0/6.0)*(kw1 + 2*kw2 + 2*kw3 + kw4); 

    std::cout << "FINAL VALUES: at " << i << " second(s)" << "\n";
    std::cout << "The angle is: " << x << " radian(s)"; 
    std::cout << "\n";
    std::cout << "The velocity is: " << w << " radians/s"; 
    std::cout << "\n";
    std::cout << "\n";
    std::cout << "-----------------------------------------------------\n";
    std::cout << "\n";
}
// Graphing with arrays 
//To see the time dependence, let's suppose you have two arrays: one for w and one for x. Then the values x[i] and w[i] entry describe the position and velocity of your pendulum at time t_i.

// Open a plot window
    if (!cpgopen("/XWINDOW")) return 1;

// Set-up plot axes
// M_PI is defined in cmath
    cpgenv(0,time,-1,1,0,1);

// Label axes
    cpglab("time", "angle", "RED = Angle      BLUE = Velocity");

    cpgsci(2); //RED
    cpgline(10,tARR, xARR);
    cpgsci(4); //BLUE
    cpgline(10,tARR, wARR);
    cpgclos();
    }
4

0 回答 0