How can I get my while loop to run?

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B M 2019-7-13

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https://ww2.mathworks.cn/matlabcentral/answers/471579-how-can-i-get-my-while-loop-to-run

回答： Vimal Rathod 2019-7-17

I am trying to iterate with a time step of 0.0001. The point is for a given distance (x = 8.2 miles) I am to find two values of theta to allow for this. When i run the code the while loop won't run the iteration and I keep getting just the initial condition values. How do i get my loop to run the iteration with the initial conditions?

% Set Intitial Conditions
x(1) = 0;
z(1) = 0;
M(1) = 7;
K = 1.4;
R = 53.353;
A = 0.202;
m = 1.37;
g = 32.2;
[ T_atm , P_atm , ro ] = atmos_funEE(z(1));
C = sqrt(K*R*T_atm);
Vabs(1) = M(1)*C;
Vx(1) = Vabs(1)*cos(0);
Vz(1) = Vabs(1)*sin(0);
Cd(1) = hypervelcd(M(1));
Fd(1) = 0.5*ro*(Vabs(1))^2*Cd(1)*A;
Fdx = Fd(1)*cos(0);
Fdz = Fd(1)*sin(0);
ax(1) = -Fdx/m;
az(1) = -g-Fdz/m;
% Begin Iteration
I = 1;
dt = 0:0.0001
while z(I) >= 0
    x(I+1) = x(I) + Vx(I).*dt
    z(I+1) = z(I) + Vz(I).*dt
    Vx(I+1) = Vx(I) + ax(I).*dt
    Vz(I+1) = Vz(I) + az(I).*dt
    theta(I+1) = arctan(Vz(I+1), Vx(I+1));
    [ T_atm , P_atm , ro ] = atmos_funEE(z(I+1));
    C = sqrt(K*R*T_atm);
    Vabs(I+1) = M(I+1) * C;
    M(I+1) = Vabs(I+1) / C;
    Cd(I+1) = hypervelcd(M(I+1));
    Fd(I+1) = 0.5*ro*(Vabs(I+1))^2*Cd(I+1)*A;
    Fdx = Fd(I+1)*cos(theta(I+1));
    Fdz = Fd(I+1)*sin(theta(I+1));
    ax(I+1) = -Fdx/m;
    az(I+1) = -g-Fdz/m;
    I = I + 1;
end

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per isakson 2019-7-13

See TUTORIAL: how to ask a question (on Answers) and get a fast answer

It's much easier to help if the code of the question is possible to run.

dpb 2019-7-13

在 MATLAB Online 中打开

Problem here is

[ T_atm , P_atm , ro ] = atmos_funEE(z(1));

altho for debugging the integration issues a set of constants would suffice undoubtedly.

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Answer 1

Vimal Rathod 2019-7-17

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此回答的直接链接

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在 MATLAB Online 中打开

Hi,

I understand that your primary concern is to make the while loop run and your code to work. I see that you have used another form of code in the comment's section. If the latest code is the one in the comments which you have posted.

I = 1; 
dt = 0:0.0001:10 
while z >= 0 
ax(I+1) = -Fdx/m; 
az(I+1) = -g-Fdz/m; 

In this code the while loop is taking a vector 'z' and comparing it with zero which returns a vector. Instead of that try using the value ‘z(I)’ which provides a single output Boolean value for the while condition statement. The other error which you got is due to assigning a single value in a vector to a vector.

Instead of using the below code.

x(I+1) = x(I) + Vx(I)*dt

You could try using the code snippet below where I have used a specific value of dt

x(I+1) = x(I) + Vx(I)*dt(I)

You can refer to the while loop documentation and colon operator and indexing in the MATLAB documentation. Below are the links provided for the following.

While loop: https://www.mathworks.com/help/matlab/ref/while.html

Colon operator: https://www.mathworks.com/help/matlab/ref/colon.html

Indexing: https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html