# for loop ode 45 changing parameter values

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gorilla3 2017년 12월 13일
댓글: gorilla3 2017년 12월 15일
I'm trying to run this code and use only the last value of the tspan for the ode45, however I always get 2 values for the final value of the tspan (when t=100)
This is the code:
ABP=linspace(40,170,131);
ti=0; tf=100;
%change in ABP at every loop
for i=1:1:length(ABP)
sol = ode45(@first,ti:1:tf, [0 0 0 0 0.205 62.516 12 13.201 0.2 1],[],ABP(i)); %(function, timespan, initial condition for xq,xc,xm1,xm,Ca,P1,V_sa,P2)
end
%%========= FUNCTION ==================
function dvdt = first(t,v,ABP)
%code
...ABP used in a set of differential eq...
if t==100
van= (Vsa+Vla)/(Vsab+Vla)
fileID=fopen('van.txt','a');
fprintf(fileID,' %4.3f\n',van);
fclose(fileID);
end
end
Hence, I should end up having a .txt file with 131 values of van, one for each value of ABP. Instead, I end up with double the amount. I also printed the ABP values in the txt file to check, and indeed there were two different (very similar) values of van for each value of ABP. Any suggestions on how to fix this?
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gorilla3 2017년 12월 15일
Instead of getting 131 values for "van" I get 262. i.e. I get 2 results for each value of ABP

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### 채택된 답변

David Goodmanson 2017년 12월 13일
편집: David Goodmanson 2017년 12월 13일
Hi gorilla3,
Here is the simplest possible example of what ode45 is doing, that also inputs an extra parameter. To find the times that ode45 employs, I used the low-rent method of uncommenting the commands in ‘first’ and letting the results run out in the command window. If you do that you will see that the last time occurs twice, although it of course only occurs once in the output sol.x.
I suppose that Vsa, Vsab, etc. depend on v, APB, and possibly t. I think you will probably have to take some code outside of ode45 (a copy if some of it was actually needed to help get dvdt) and do the calculation within the ABP loop, using sol.y(end) and (if time is needed) sol.x(end).
ABP = .1;
sol = ode45(@first,[0:100],,[],ABP);
plot(sol.x,sol.y,'o-')
function dvdt = first(t,v,ABP)
t
dvdt = -v*ABP
end
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gorilla3 2017년 12월 15일
Thank you very much for elaborating your reply. It's very helpful!

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### 추가 답변(1개)

Jan 2017년 12월 14일
Fo not include an output inside the function to be integrated. Remember that the integrator can call this function with different inputs to evaluate one step. The integrator can even reject steps.
Use the OutputFcn instead, which is called after a successful step:
options = odeset('OutputFcn', @yourOutput)
For completeness I repeat, that providing the parameters as input of ODE45 is deprecated for over 17 years now, see http://www.mathworks.com/matlabcentral/answers/1971> .

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