Hi, my while loop stops before meeting its designated conditions and gives me a result. Why is this?

Question

Abdul Akif Jabir 2018-11-30

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关闭： MATLAB Answer Bot 2021-8-20

while error(end) <= 0.01
    
    [t,z] = ivpSolver_Assignment_2(t0,z0,dt,tend,theta(n));
    w(n) = z(1,end);
    error(n) = w(n) - 2.1; 
    theta(n+1) = (-error(n-1)/((error(n)-error(n-1))/(theta(n)-theta(n-1)))) + theta(n-1);
    n = n+1;
end

t and z are vectors

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Answer 1

Georgios Pyrgiotakis 2018-11-30

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编辑：Georgios Pyrgiotakis 2018-11-30

If you have not defined error (i.e. error=ones(1,x) where x is the length of the matrice) the loop will stop imedaitely since W(end) by default is null or zero. instead write:

error=[1];
while error(end) <= 0.01
    
    [t,z] = ivpSolver_Assignment_2(t0,z0,dt,tend,theta(n));
    w = z(1,end);
    error=[error, w - 2.1;]
    theta(n+1) = (-error(n-1)/((error(n)-error(n-1))/(theta(n)-theta(n-1)))) + theta(n-1);
end

If you want to also keep the values for w as well, you need to also define w and append values to it.

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Abdul Akif Jabir 2018-12-2

Hi, I had already initalised vectors for error and w, is this an appropriate way?

w = [];
error = [];
theta = [];

Answer 2

Georgios Pyrgiotakis 2018-12-3

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This is correct in the general context, but since your while depends on it the error null value will terminate the loop before even starts. You need to initiate them with values that will work for the calculations later.

theta = [x]; % You need to find a value that can be used for the next theta calculation
w= [];
error=[1];
while error(end) <= 0.01
    
    [t,z] = ivpSolver_Assignment_2(t0,z0,dt,tend,theta(n));
    w=[w,z(1,end)];
    error=[error, w(end) - 2.1];   
    theta_tmp = (-error(end-1)/((error(end)-error(end-1))/(theta(end)-theta(end-1)))) + theta(end-1);
    theta=[theta, theta_tmp]
end