Various errors whilst running a while loop.

h(1)=100000;   %initial Height
t(1)=0;   %Initial Time
dt=0.005;
p(1)=(((h(1))/71)+1.4);   %Air Density
g(1)=(40*10^7)/(6371+h(1))^2;   %initial gravity
A=5;   %Area
c=0.7; %Drag Coefficient
m=850; %Mass
Fd(1)=0.5*c*(p(1))*a(1)*(v(1))^2;
v(1)=59.29;   %Initial velocity
a(1)=0.03535; %Initial Acceleration
i=1; %Loop Counter

while h(end)>=0
    t(i+1)=t(i)+dt;
    h(i+1)=100000-((v(i+1)*t(i+1))+(0.5*(a(i))*(t(i+1))^2));
    p(i+1)=(((h(i+1))/71)+1.4);
    Fd(i+1)=0.5*c*(p(i+1))*A*(v(i+1))^2;
    g(i+1)=(40*10^7)/(6371+h(i+1))^2;
    a(i+1)=g(i+1)-(Fd(i+1)/850);
    v(i+1)=v(1)+(a(i+1)*t(i+1));
end

With this code I want to be able to plot graphs and get values for a and v. When I run it an error comes up either saying 'undefined function or variable a'. or 'undefined function or variable v'. One time I managed to stop these errors but I still got an error saying 'Index exceed number of array elements'. Any help would be appreciated.

[Walter Roberson]

Fd(1)=0.5*c*(p(1))*a(1)*(v(1))^2;

That does need a and v but you have not assigned anything to them. You assigned to A but not a

[Sam Potter]

Thaks for the reply, I thought I had assigned values with the line 'a(1)=0.03535'; and 'v(1)=59.29'.

[Walter Roberson]

Fd(1)=0.5*c*(p(1))*a(1)*(v(1))^2;

is before those lines

[Sam Potter]

Hi, I made it so v and a is before Fd.

h(1)=100000;   %initial Height
t(1)=0;   %Initial Time
dt=0.005;
p(1)=(((h(1))/71)+1.4);   %Air Density
g(1)=(40*10^7)/(6371+h(1))^2;   %initial gravity
A=5;   %Area
c=0.7; %Drag Coefficient
m=850; %Mass
v(1)=59.29;  %Initial velocity
a(1)=0.03535; %Initial Acceleration
Fd(1)=0.5*c*(p(1))*a(1)*(v(1))^2;
i=1; %Loop Counter

while h(end)>=0
    t(i+1)=t(i)+dt;
    h(i+1)=100000-((v(i+1)*t(i+1))+(0.5*(a(i))*(t(i+1))^2));
    p(i+1)=(((h(i+1))/71)+1.4);
    Fd(i+1)=0.5*c*(p(i+1))*A*(v(i+1))^2;
    g(i+1)=(40*10^7)/(6371+h(i+1))^2;
    a(i+1)=g(i+1)-(Fd(i+1)/850);
    v(i+1)=v(1)+(a(i+1)*t(i+1));
end

However, I still get the error: 'Index exceeds number of array elements (1)'.

[Walter Roberson]

    h(i+1)=100000-((v(i+1)*t(i+1))+(0.5*(a(i))*(t(i+1))^2));

When i is 1, that tries to use v(1+1) and t(1+1) and a(1) . You assigned to t(1+1) in the previous line of code, so that exists, and a(1) exists . But v(2) does not exist yet, and will not exist until the end of the loop (and it relies upon the updated a(2) so you cannot just move it to earlier in the code.)

[Sam Potter]

This is my current code now:

>> clear

>> h(1)=100000; %initial Height

t(1)=0; %Initial Time

dt=0.005;

p(1)=(((h(1))/71)+1.4); %Air Density

g(1)=(40*10^7)/(6371+h(1))^2; %initial gravity

A=5; %Area

c=0.7; %Drag Coefficient

m=850; %Mass

Fd(1)=0.5*c*(p(1))*a(1)*(v(1))^2;

v(1)=59.29; %Initial velocity

a(1)=0.03535; %Initial Acceleration

i=1; %Loop Counter

while h(end)>=0

t(i+1)=t(i)+dt;

h(i+1)=100000-((59.29*t(i+1))+(0.5*(a(i))*(t(i+1))^2));

p(i+1)=(((h(i+1))/71)+1.4);

Fd(i+1)=0.5*c*(p(i+1))*A*(v(i))^2;

g(i+1)=(40*10^7)/(6371+h(i+1))^2;

a(i+1)=g(i+1)-(Fd(i+1)/850);

v(i+1)=v(1)+(a(i+1)*t(i+1));

end

I have tried to make it so that everything in the loop is defined previously. However, for some reason my code doesnt save some of the initial variables (like u(1) and v(1) into the workspace?

Answer 1

[Sriram Tadavarty]

Hi Sam,

This is the same as asked before here.

https://www.mathworks.com/matlabcentral/answers/510426-running-a-kinematics-while-loop-containing-an-if-loop-i-get-the-error-index-exceeds-number-of-arra#answer_419769

Hope this helps.

Regards,

Sriram

[Sam Potter]

Hi, because the code was different I thought it warrented a diiferent thread, apologies if not. I have replied to you in the other thread with my issue.