# Euler VS Runge kutta – Circle – Matlab code

In the context of this exercise, lets suppose having the two dependent following function:

With initial conditions

The equation of a circle look like the following

where the r is a constant.

If we add our equations at 0, the following will be true

We are now going to be using Euler methods and the Runge-Kutta to model a circle of a radius 1 using the above mentioned equations.

## Forward Euler method

Based on the equations

We can write the following based on our example.

Here is the Matlab code deriving from the above.

clc; clear all; h=0.2; % value of the step, you can vary this to see how it affect the graph N=51; x=zeros(1,N); y=zeros(1,N); x(1)=1; %initial value x(0) y(1)=0; %initial value y(0) %iteration with 50 step % Forward Euler method for i=1:N x(i+1)=x(i)+h*(y(i)); y(i+1)=y(i)-h*(x(i)); end angle=linspace(0,2*pi); % used to plot the unit circle plot(x,y,'r',sin(angle),cos(angle),'b')

And the following plot results from it.

## Backward Euler method

Just like the previous method we will start from the formula, then the derivation according to this exercises, the Matlab code and then the graph

Formula

Derivation

we need to arrange the above equation for it to look like the following* (substituting and putting n+1 elements on one side and n elements on the other side )*

Matlab code

clc; clear all; h=0.2; N=51; x=zeros(1,N); y=zeros(1,N); x(1)=1; %initial value x(0) y(1)=0; %initial value y(0) %iteration with 50 step % Forward Euler method for i=1:N x(i+1)=(x(i)+h*(y(i)))/(1+h^2); y(i+1)=(y(i)-h*(x(i)))/(1+h^2); end angle=linspace(0,2*pi); % used to plot the unit circle plot(x,y,'r',sin(angle),cos(angle),'b') legend('Euler', 'Unit circle')

Plot

## Semi-implicit Euler method

Derivation

Matlab code

clc; clear all; h=0.2; N=51; x=zeros(1,N); y=zeros(1,N); x(1)=1; %initial value x(0) y(1)=0; %initial value y(0) %iteration with 50 step % Forward Euler method for i=1:N x(i+1)=x(i)+h*(y(i)); y(i+1)=y(i)-h*(x(i+1)); end angle=linspace(0,2*pi); % used to plot the unit circle plot(x,y,'r',sin(angle),cos(angle),'b') legend('Euler', 'Unit circle')

Plot

## Runge Kutta method

Derivation

Working the same way with *y *

Matlab code

clc; clear all; h=0.2; N=51; x=zeros(1,N); y=zeros(1,N); x(1)=1; %initial value x(0) y(1)=0; %initial value y(0) %iteration with 50 step % Forward Euler method for i=1:N xk1=y(i); yk1=-x(i); xk2=y(i)+0.5*h*yk1; yk2=-(x(i)+0.5*h*xk1); x(i+1)=x(i)+h*xk2; y(i+1)=y(i)+h*yk2; end angle=linspace(0,2*pi); % used to plot the unit circle plot(x,y,'r',sin(angle),cos(angle),'b') legend('Range Kutta', 'Unit circle')

Plot

Can you please explain how you arrived at the various equations for the Runge-Kutta method?