Why do I get this error: A(I): index out of bounds; value 2 out of bound 1 (line 13)

Question

Mohanad Arafae 2016-11-13

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回答： Roger Stafford 2016-11-14

function root=newtonmethod(f,fprime,x0,maxiter,tol)
%input:
% f string that names the function f(x).
% fprime string that names the derivative f’(x).
% x0 the initial point
% tol is the termination tolerances
% maxiter the maximum number of iteration
x(1) = x0;
n = 2;
while abs(f(x(n))) > tol & n < maxiter
      m = n+1;
      x(m) = x(n) - f(x(n))/fprime(x(n));    
      if x(n) - f(x(n))/fprime(x(n)) < 0
        root = x(n);
        break;
      end
end
root = x(n)

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Answer 1

Daniel kiracofe 2016-11-13

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Because the very first time the while statement's condition is checked, n=2, and x is a vector that has 1 element. You are asking to evaluate the 2nd element of an array that has only 1 element. You probably want to start with n=1 instead of n=2.

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Answer 2

Roger Stafford 2016-11-14

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Besides the error that Daniel gives you, another error is that ’n’ never changes within your while-loop, so the (corrected) loop would never stop unless you just happened to start with the right value.