How can I replace my values in an iteration?

This is a code I am using to run the modified secant open method. However, I don't understand why my values are not replacing the old ones each time I run the iteration.

function find_newX = modifiedsecant(initialGuess, pertubation, tolerance, func)

%%variables used as input input arguments:
%%initialGuess = initial guess for x root user inputs
%%pertubation  = small pertubation fraction
%%tolerance = amount of error allowed to stop iterations
%%func = name of the function 

%%the first step is to find the value of the func at the initial guess
%%func_initialGuess = func(initialGuess);

%%the second step is to find the value of the func at the (intial guess + perturbation*initial guess)
%%xp = initialGuess * perturbation
%%xp = initialGuess*pertubation;
%%func_initialGuess_and_xp = func(initialGuess + xp);

%%interative formula for modified secant method
%%this equation will be reiterated until the required tolerance is met
%%find_newX = (xp*func_initialGuess)/(func_initialGuess_and_xp - func_initialGuess);

%%fourth step is to calculate the relative approximate error to compare to the tolerance
%%assume first that ea = 100%
n = 0;

%%creating a while loop such it runs until the tolerance is reached
while n < 10
    func_initialGuess = func(initialGuess); %%these are the formulas made in the previous comments put in the while loop
    xp = initialGuess*pertubation;
    func_initialGuess_and_xp = func(initialGuess + xp);

      find_newX = (initialGuess - (xp*func_initialGuess))/(func_initialGuess_and_xp - func_initialGuess);

      ea = abs((find_newX - initialGuess)/find_newX); %%calculates the new ea

      find_newX = initialGuess; %%this is setting the find_newX value to the intialGuess variable

      n = n+1;
      display(initialGuess);
      display(find_newX);
  end

When I display the values for initialGuess, it will display the original value I assigned rather than replacing it with the find_newX value. I don't understand why.

Answer 1

[Geoff Hayes]

Maryam - perhaps the problem is with the first line of the while loop

 func_initialGuess = func(initialGuess);

Every iteration of the loop uses the same initialGuess that was passed as a parameter into this function. And the line of code

 find_newX = initialGuess;

will always ensure that the two are identical and overwrites the previous initialization of find_newX

 find_newX = (initialGuess - (xp*func_initialGuess))/(func_initialGuess_and_xp - func_initialGuess);

I think that you need to reset the initialGuess on each iteration to the "new" estimate. So instead of doing

 find_newX = initialGuess;

do

 initialGuess = find_newX;

This way, the initialGuess or new estimate is used on subsequent iterations.

As an aside, you may want to consider renaming this variable to something else as it is no longer an initial guess after the first iteration has completed.