While Loop code not working?

Question

Bob 2015-2-17

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评论： Bob 2015-2-18

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Question: I have to create a function m file called myfirstzero(f,a) which takes two inputs:

f: A function handle which you may assume will represent a polynomial.

a: A real number.

Does: Uses a while loop to find the smallest n such f(n)(a) = 0. Note that this means the nth derivative at x = a and note that n = 0 is fair game, where the 0th derivative of a function is just the function itself.

Returns: This value of n.

This is my code:

function a = myfirstzero(f,a)
syms x;
n = 0;
d = abs(subs(f(x),a));
    while (d > 0);
        d = subs(diff(f(x),n),a);
        n = n+1;
    end
    a = n;
end

Testing: When I test the code with results =myfirstzero(@(x) x^3+2,0) I get a= 2 when the correct answer should actually be 1.

Can someone help me with fine tuning my code. It works with some of my test data but not all of it. Below is all the test data I have to test this code:

a = myfirstzero(@(x) 2*x^3-3*x^2-12*x+6,2)----> works
a =1
a = myfirstzero(@(x) x^3,0)----->works
a =0
a = myfirstzero(@(x) x^3+2,0)----> not work
a =1
a = myfirstzero(@(x) x^6-5*x^5-2*x^4-x^3+x^2-x+10,3)---->not work
a =7
a = myfirstzero(@(x) x^5-x^3,2)----->not work
a =6
a = myfirstzero(@(x) x^4-10*x^3+24*x^2,1)----->not work
a =2