function root = newton(x0,error_bd,max_iterate,index_f)
%
% function newton(x0,error_bd,max_iterate,index_f)
%
% This is Newton's method for solving an equation f(x) = 0.
%
% The functions f(x) and deriv_f(x) are given below.
% The parameter error_bd is used in the error test for the 
% accuracy of each iterate.  The parameter max_iterate
% is an upper limit on the number of iterates to be 
% computed.  An initial guess x0 must also be given.
%
% For the given function f(x), an example of a calling sequence 
% might be the following:
%    root = newton(1,1.0E-12,10,1)
% The parameter index_f specifies the function to be used.
%
% The program prints the iteration values
%      iterate_number, x, f(x), deriv_f(x), error
% The value of x is the most current initial guess, called
% previous_iterate here, and it is updated with each iteration. 
% The value of error is 
%    error = newly_computed_iterate - previous_iterate
% and it is an estimated error for previous_iterate.
% Tap the carriage return to continue with the iteration. 

format short e
error = 1;
it_count = 0;
while abs(error) > error_bd & it_count <= max_iterate
    fx = f(x0,index_f);
    dfx = deriv_f(x0,index_f);
    if dfx == 0
        disp('The derivative is zero.  Stop')
        return
    end
    x1 = x0 - fx/dfx;
    error = x1 - x0;
%   Internal print of newton method. Tap the carriage
%   return key to continue the computation.
    iteration = [it_count x0 fx dfx error]
    pause
    x0 = x1;
    it_count = it_count + 1;
end

if it_count > max_iterate
    disp('The number of iterates calculated exceeded') 
    disp('max_iterate.  An accurate root was not')
    disp('calculated.')
else
    format long
    root = x1
    format short e
    error
    format short
    it_count
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function value = f(x,index)

% function to define equation for rootfinding problem.

switch index
case 1        
    value = x.^6 - x - 1;
case 2
    value = x - exp(-x);
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function value = deriv_f(x,index)

% Derivative of function defining equation for rootfinding 
% problem.

switch index
case 1
    value = 6*x.^5 - 1;
case 2
    value = 1 + exp(-x);
end