Various errors whilst running a while loop.

2020 3 14
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更新时间：2021 8 20
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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.

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Walter Roberson
Walter Roberson 2020-3-14 
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
Sam Potter 2020-3-14
Thaks for the reply, I thought I had assigned values with the line 'a(1)=0.03535'; and 'v(1)=59.29'.
Walter Roberson
Walter Roberson 2020-3-14
Fd(1)=0.5*c*(p(1))*a(1)*(v(1))^2;
is before those lines
Sam Potter
Sam Potter 2020-3-14
编辑：Walter Roberson 2020-3-14
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
Walter Roberson 2020-3-14
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
Sam Potter 2020-3-14
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?

回答（1 个）

Sriram Tadavarty
Sriram Tadavarty 2020-3-14

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1 个评论
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Sam Potter
Sam Potter 2020-3-14
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.

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Sam Potter
2020-3-14

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