Explicit Euler integration. Plot x(t) and y(t)

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Anastasia Kyriakou 2020-2-21

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https://ww2.mathworks.cn/matlabcentral/answers/506831-explicit-euler-integration-plot-x-t-and-y-t

评论： Giuseppe Inghilterra 2020-2-21

在 MATLAB Online 中打开

I have this exercise:

dx/dt = −x(1 − y), t0 = 0, x(t0) = 0.5,

dy/dt = y(1 − x), t0 = 0, y(t0) = 2

These equations are also known as Lotka-Volterra or predator-prey equations modeling evolution of species as a function of time t. In the equations above variable x stands for the number of predators, and y is the number of prey.

Let [0, 40] be the interval of integration.

Please implement explicit Euler integration scheme with ∆ = 0.001, ∆ = 0.002, and

∆ = 0.005 . Plot the values of x(t), y(t) for t ∈ [0, 40]

I have written this but i am getting an error. (I have divided dy/dt with dx/dt to find dy/dx)

Could someone help me ?

t0=0; %this is the left boundary of the integration interval [0,40]
t1=40 %this is the right boundary of the integration interval [0,40]
y(t0)=2; %this is the initial value of y at t0
x(t0)=0.5; %this is the initial value of x at t0
Delta=0.001; %definition of Delta
x=x0:Delta:x1; %setting up a grid of points in [0,2]
y=y0*ones(1,length(x)); %creating an array for y
if (length(x)>1)
for i=1:length(x)-1
y(i+1)=(y(i)+Delta*(y(i)*(1-x)/((-x)*(1-y(i)))));
end;
plot(x,y); % Finished! Let’s plot the estimates y_k of the solution y(x_k) now
else
disp('Please change the value of Delta');
end;

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Anastasia Kyriakou 2020-2-21

Error in Untitled (line 3)

y(t0)=2; %this is the initial value of y at t0

Giuseppe Inghilterra 2020-2-21

Hi, t0 is equal to 0, thus you are trying to evaluate y(0) that returns an error in Matlab. Matlab is one-based indexing, this means that 0 is not a correct index, the minimum one that you can use is 1. y(1) = 2.

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