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'Index exceeds matrix dimensions' error

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Soho
Soho on 12 Jul 2017
Commented: Moe_2015 on 12 Jul 2017
Hi, I'm trying to write an iteration program that runs a while loop until the variable 'theta' is almost the same as the output x (within a certain accuracy). I keep getting this error, but when I try to change the code to run, it only runs for the first loop. Can anyone help? My code is below (i've been using 0.01 as thetaInitial because i know this is pretty close to the correct answer)
function x=iteration(thetaInitial)
theta(1)=thetaInitial;
x(1)=0.0248;
error=1e-8;
Re=10000;
i=1;
while abs(x(i)-theta(i))>error
x(i)=(((6.25*log(2))+1.5)*log(Re*(theta(i)^0.5))+0.09)^-1;
i=i+1;
end
x=x(1:i+1);
plot(x)
[x(end) i]

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Accepted Answer

Moe_2015
Moe_2015 on 12 Jul 2017
theta(i) is not defined after the first index. You set theta(1) but then never do anything with theta again. So when you say theta(i) it is exceeding the matrix dimension (which is 1). Also, you should move i = i+1 as the first line inside the while loop. Also outside the while loop x =x(1:i+1) will fail because you do not reach an x with length (i+1)... your x will be of length i

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Moe_2015
Moe_2015 on 12 Jul 2017
So something like this (note: set theta(i) to whatever it should be):
function x=iteration(thetaInitial)
theta(1)=thetaInitial;
x(1)=0.0248;
error=1e-8;
Re=10000;
i=1;
while abs(x(i)-theta(i))>error
i=i+1;
theta(i) = 0; %set this to whatever it is supposed to be
x(i)=(((6.25*log(2))+1.5)*log(Re*(theta(i)^0.5))+0.09)^-1;
end
x=x(1:i); % this line isn't really necessary but if you want it this is what it should be
plot(x)
[x(end) i]

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