How do I use an If-statement when using RK4?
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Tanmoy Bari
el 24 de Mzo. de 2018
Editada: Walter Roberson
el 27 de Mzo. de 2018
Below is a RK4 program for which I solve VM.
Here's my problem - in my assignment, there is a certain condition for which the values for VM(2) will be reduced by 70% and VM(3) by 20% each and every year, i.e VM(2)=VM(2)*0.3 and VM(3)=VM(3)*0.8 which my code below does not include since I do not know how to include it.
I do not know how to enter this into to the RK4 while loop and then plot the graf from t to t_end. I am assuming that I need to use an if-statement but not sure how to do it. Does anyone know how to enter my condition in my RK4-program and then how to plot the new values from t:t_end?
Grateful for any help that I can get!
a1=16;
a2=1.8*10^-5;
a3=0.011;
b1=2.0;
b2=0.085;
%Calculating my starting values for VM
v_start=(b1.^(10/6)*a1/a3)/(a2/a3*b1.^(10/6)+b2.^(10/6));
s_start=a1/a3-a2/a3*v_slut;
%T= nr of years
t_end=12; t=3;
VM=[v_start, s_start 2]; T=t; VS=VM;
h=1/365;
while t<t_end-h/2
f1=fvs2(t,VM);
f2=fvs2(t+h/2, VM+h*f1/2);
f3=fvs2(t+h/2, VM+h*f2/2);
f4=fvs2(t+h, VM+h*f3);
VM=VM+h/6*(f1+2*f2+2*f3+f4); t=t+h; T=[T; t]; VS=[VS;VM];
end,
subplot(3,1,1), plot(T, VS(:,1))
subplot(3,1,2),plot(T,VS(:,2))
subplot(3,1,3),plot(T,VS(:,3))
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Abraham Boayue
el 25 de Mzo. de 2018
You actually did not mention the condition. Can you say exactly what the condition is? We know what will happen, but we need to know the condition as well.
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Más respuestas (4)
Abraham Boayue
el 25 de Mzo. de 2018
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/180455/image.png)
Is this what you want to do? I am having a bit of a difficulty understanding what you want to do. I set v_sutt = 1, since there was no value assigned to it.
V1 = 0.3*VS(:,2);
V2 = 0.8*VS(:,3);
subplot(3,1,1)
plot(T,VS(:,1),'linewidth',2,'color','b');
a = title('Something Else');
set(a,'fontsize',14);
a = xlabel('T[ 3 12]');
set(a,'fontsize',14);
a = ylabel('values');
set(a,'fontsize',14);
grid
subplot(3,1,2)
plot(T,V1,'linewidth',2,'color','r');
a = title('30% of Pests population');
set(a,'fontsize',14);
a = xlabel('T[ 3 12]');
set(a,'fontsize',14);
a = ylabel('values');
set(a,'fontsize',14);
grid
subplot(3,1,3)
plot(T,V2,'linewidth',2,'color','g');
a = title('80% of Predictor population');
set(a,'fontsize',14);
a = xlabel('T[ 3 12]');
set(a,'fontsize',14);
a = ylabel('values');
set(a,'fontsize',14);
grid
Abraham Boayue
el 27 de Mzo. de 2018
Editada: Walter Roberson
el 27 de Mzo. de 2018
Hi Tanmoy, sorry that I haven't been able to get back to you since our last communication.
In the following line of your code,
%Calculating my starting values for VM
v_start=(b1.^(10/6)*a1/a3)/(a2/a3*b1.^(10/6)+b2.^(10/6));
s_start=a1/a3-a2/a3*v_slut;
What is the value for v_slut?
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