# Help! me set the subscript indices properly so the function can perform while loop iterations

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Paul Silano on 1 Apr 2013
Help! me set the subscript indices properly so the function can perform the req'd iterations. MATLAB errors: Subscript indices must either be real positive Error in ==> connection_analysis_1 at 32
j=0; % rotation angle - assumed THETA1(1)=1; % rotation angle - calculated % THETA2(j)=10; r(1)=0;
while (1.00<r(j) r(j)<0.99)
M=1;
T(j)=1/(2*THETA1(j))*(P - ((2*M)/L));
if T(j)>=Tu
DELTA(j)=DELTAu;
else
DELTA(j)=-4E-10*T(j)^4-4E-08*T(j)^3+0.0003*T(j)^2-0.0255*T(j)+0.4375;
end
THETA2(j)=sqrt(2*(T(j)/E*A + DELTA(j)/L) + (T(j)/E*A + DELTA(j)/L)^2);
r(j)=THETA1(j)/THETA2(j);
if j==1
THETA(j+1)=(THETA1(j)+THETA2(j))/2;
else if r(j)>1 && r(j-1)<1 || r(j)<1 && r(j-1)>1
THETA1(j+1)=(THETA2(j-1)+THETA2(j))/2;
else
THETA1(j+1)=(THETA1(j)+THETA2(j2))/2;
end
end
j=j+1;
end

Brian B on 1 Apr 2013
Edited: Brian B on 1 Apr 2013
Indices start at 1, so
j=0;
...
while (1.00<r(j) r(j)<0.99)
is referencing a value that does not exist (the 0th value of r). Another problem with this code is that you need to use a logical operator to combine the conditions in the while statement. For example, if you wanted 1.00<r(j) OR r(j)<0.99 then use:
while (1.00<r(j) || r(j)<0.99)
Brian B on 1 Apr 2013
My suggestion would be to set a breakpoint and step through the program to see where it begins doing things you didn't expect. Try writing down the same steps it should follow on a piece of paper and compare the values at each step. If breakpoints are new, you might find this helpful: http://www.mathworks.com/help/matlab/matlab_prog/debugging-process-and-features.html#brqxeeu-184.