MATLAB Answers

Plot using while loop

13 views (last 30 days)
Mohamad Mourad
Mohamad Mourad on 25 Jan 2020
Edited: Allen on 25 Jan 2020
Please how can I avoid these errors? The graph it doesn't appear (The script is bellow the graph )
close all
clear all
pi=8000;
ri=28;
rp=45;
rp2=35;
ro=80;
r=[ri:0.01:ro];
n=1;
while(r(n)<=rp2)
pa=0;
pb=pi;
Sr(n)= ((pa*ri^2 - pb*rp^2)/(rp^2 - ri^2)) + (((ri^2*rp^2)/r(n)^2)*((pa-pb)/(rp^2-ri^2)));
n=n+1;
end
%n=n-1;
while(r(n)>rp2 & r(n)<=rp)
pa=pi;
pb=0;
Sr(n)= ((pa*rp^2 - pb*ro^2)/(ro^2 - rp^2)) + (((rp^2*ro^2)/r(n)^2)*((pa-pb)/(ro^2-rp^2)));
n=n+1;
end
while (r(n)>rp & r(n)<=ro)
pa=pi;
pb=0;
Sr(n)= ((pa*ri^2 - pb*rp^2)/(rp^2 - ri^2)) + (((ri^2*rp^2)/r(n)^2)*((pa-pb)/(rp^2-ri^2)));
n=n+1;
end
plot(r',Sr','k')
xlabel('r')
ylabel('Sr')

  1 Comment

Mohammad Sami
Mohammad Sami on 25 Jan 2020
The way your code is written, it is possible for the value of n to exceed the length of the variable r.
At n = 5201, your last while loop does not terminate (80 > 45 & 80 <= 80), hence n gets incremented to 5202. However r only has 5201 elements. In the next iteration it gives you the error on index exceed number of array elements.
Also similarly for the plot, you need to make sure both r ad Sr will contain the same number of elements at the end of your code

Sign in to comment.

Answers (2)

KALYAN ACHARJYA
KALYAN ACHARJYA on 25 Jan 2020
pi=8000;
ri=28;
rp=45;
rp2=35;
ro=80;
r=[ri:0.01:ro];
n=1;
while(r(n)<=rp2)
pa=0;
pb=pi;
Sr(n)= ((pa*ri^2 - pb*rp^2)/(rp^2 - ri^2)) + (((ri^2*rp^2)/r(n)^2)*((pa-pb)/(rp^2-ri^2)));
n=n+1;
end
%n=n-1;
n=1;
while r(n)>rp2 & r(n)<=rp
pa=pi;
pb=0;
Sr(n)= ((pa*rp^2 - pb*ro^2)/(ro^2 - rp^2)) + (((rp^2*ro^2)/r(n)^2)*((pa-pb)/(ro^2-rp^2)));
n=n+1;
end
while (r(n)>rp & r(n)<=ro)
pa=pi;
pb=0;
Sr(n)= ((pa*ri^2 - pb*rp^2)/(rp^2 - ri^2)) + (((ri^2*rp^2)/r(n)^2)*((pa-pb)/(rp^2-ri^2)));
n=n+1;
end
plot(r(1:length(Sr))',Sr','k')
xlabel('r')
ylabel('Sr')

  0 Comments

Sign in to comment.


Allen
Allen on 25 Jan 2020
Edited: Allen on 25 Jan 2020
Sami is correct that you are running n to a higher value than the total number of elements in r. I better way to approach this problem is to eliminate the loops. Using logical indexing is a cleaner and much more efficient way of doing this. The following code is a direct replacement to yours but with logical indexing.
close all
clear
Pi = 8000; % Change pi to Pi to avoid confusion with pi = 3.1415;
ri = 28;
rp = 45;
rp2 = 35;
ro = 80;
r = ri:0.01:ro;
Sr = NaN(size(r));
pa = 0;
pb = Pi;
idx = r<=rp2; % Logical index for all values of r that meet criteria.
Sr(idx) = (pa*ri^2-pb*rp^2)/(rp^2-ri^2)+ri^2*rp^2./r(idx).^2.*((pa-pb)/(rp^2-ri^2));
pa = Pi;
pb = 0;
idx = r>rp2 & r<=rp; % Logical index for all values of r that meet criteria.
Sr(idx) = (pa*rp^2-pb*ro^2)/(ro^2-rp^2)+ro^2*rp^2./r(idx).^2.*((pa-pb)/(ro^2-rp^2));
pa = Pi;
pb = 0;
idx = r>rp & r<=ro; % Logical index for all values of r that meet criteria.
Sr(idx) = (pa*ri^2-pb*rp^2)/(rp^2-ri^2)+ri^2*rp^2./r(idx).^2.*((pa-pb)/(rp^2-ri^2));
plot(r,Sr,'k')
xlabel('r')
ylabel('Sr')
Since the equation for Sr is similar, but with a change to the variables you could also make your code much more concise by incorporating an anonymous function for forming Sr.
close all
clear
Pi = 8000;
ri = 28;
rp = 45;
rp2 = 35;
ro = 80;
r = ri:0.01:ro;
Sr = NaN(size(r));
% FuncName = @(A,B,R1,R2,R) (A*R1^2-B*R2^2)/(R2^2-R1^2)+R1^2*R2^2./R.^2.*((A-B)/(R2^2-R1^2));
FuncName = @(A,B,R1,R2,R) (A*R1^2-B*R2^2+(A-B)*(R1*R2./R).^2)./(R2^2-R1^2); % More concise arrangement
Sr(r<=rp2) = FuncName(0,Pi,ri,rp,r(r<=rp2));
Sr(r>rp2 & r<=rp) = FuncName(Pi,0,rp,ro,r(r>rp2 & r<=rp));
Sr(r>rp & r<=ro) = FuncName(Pi,0,ri,rp,r(r>rp & r<=ro));
plot(r,Sr)
xlabel('r')
ylabel('Sr')

  0 Comments

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by