Boris Boris - 4 months ago 11
Python Question

plotting in pyhon using matplot?

I have been trying to simulate the first order differential equation using the fourth order rungge kutta method,but i am having problems in plotting it.

#simulation of ode using 4th order rk method dy/dx=-2y+1.3e^-x,y(0)=5,h=0.01 from sympy import*
import math
import numpy as np
import matplotlib.pyplot as plt
h=0.01;
ti=0;
x=0;
n=0;
y=5;
def f(x,y):
return 1.3*math.exp(-x)-2*y


while x < 10:

k1=f(x,5);
k2=f(x+h/2,y+(h/2)* k1);
k3=f(x+h/2,y+(h/2)* k2);
k4=f(x+h,y+h*k3);
y=y+h/6*(k1+2*(k2+k3)+k4);
x=x+h;
plt.plot(x,y);


i know that the problem is because of updation of the x,y values every time the lloop runs,but can somebody assist to plot all the values of (x,y)

Answer

As suggested in the comment, you can create two lists to store x and y values and plot it after the while loop:

import math
import numpy as np
import matplotlib.pyplot as plt
h=0.01;
ti=0;
x=0;
n=0;
y=5;
def f(x,y):
    return 1.3*math.exp(-x)-2*y

xs = [x]       # <<<
ys = [y]       # <<<
while x < 10:

    k1=f(x,5);
    k2=f(x+h/2,y+(h/2)* k1);
    k3=f(x+h/2,y+(h/2)* k2);
    k4=f(x+h,y+h*k3);
    y=y+h/6*(k1+2*(k2+k3)+k4);
    x=x+h;
    xs.append(x)    # <<<
    ys.append(y)    # <<<

plt.plot(xs,ys);

enter image description here