Newton's Method Plot of Errors

Hi! I am new to MATLAB and I am using this code

%to solve nonlinear equations using newton
%f(x)=(x+1)*(x-1/2)
%df(x)=2*x+1/2

%initial conditions 
x0=-1.2;
maxIter=10;
tolX=0.001;

%computation using newton
x=x0;
xold=x0;
for i=1:maxIter
    f=(x+1)*(x-1/2);
    df=2*x+1/2;
    x=x-f/df
    
    err=abs(x-xold);
    xold=x;
    if (err<tolX)
        break;
    end
end

to try and plot logarithms of the errors against each other. My though process is to write

>> prevErr=err(1:9);
>> currErr=err(2:10);
>> plot (log(prevErr),log(currErr),'--r')

but after the first line I am getting the notice "Index exceeds the number of array elements (1)." if anyone know how I can fix this please let me know!

Answer 1

[Image Analyst]

You need to index err:

err(i) = abs(x - xold);

Otherwise it's just a scalar, not a vector with 9 elements because you're overwriting it every time.