How do I fix this error with function handle?

Hello,

I've been trying to work through this problem:

Write function 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.

And my attempt is:

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

However, when i put in the input like myfirstzero(@(x) x^3, 2), the function will run fine until it works to d = 6, which I want the function to stop, but it keeps running because d ~= 0, then it errors.

How do I fix this?

Thank you so much, Sea

[Areeba Shahid]

END: FUNCTION dydt = second_order(t,y,k);

Answer 1

[Star Strider]

while (d > 0) && (d <= 6)

[Star Strider]

Sea’s ‘Answer’ moved here:

That doesn't solve the problem :/

[Star Strider]

My error. Sorry. I intended:

while (d > 0) && (n < 6)

That should work.

[Sea]

It still doesn't work because as I put in x^3 for function handle, we only need to do derivative 4 times for x^3 to be zero right? so as x^3 reach 6(x^0) then this line:

f = matlabFunction(d)

makes

f = @()6.0

and thats where the error comes in because the next line wont work.

Sorry if I explained that a little unclear earlier.

Thanks though! :) Please help me solve this problem =3=

[Star Strider]

This took some diving into some functions I don’t usually use, especially in this context, but this works:

function f = myfirstzero(f,a)
syms x
n = 0;
d = subs(f(x), a)
while (d > 0) && (n < 6) 
    d = diff(f(x))
    n = n + 1
    if d ~= 0
        f = matlabFunction(d)
        fv = func2str(f)
        ep = strfind(fv,'()')
        if ~isempty(ep)
            break
        end
        d = f(a)
    end
end

The trick here is to convert ‘f’ into a string and then search for parentheses with nothing in them. If that’s ‘true’, the break out of the iteration and return ‘d’ as the output. I tested it to with x^8 and it seemed to perform as I believe you want it to. I don’t know what (if anything) you want to return as output, so I leave that to you. I opted to return ‘f’.

[Sea]

Works great! Thank you so much! I will take a look it detail and learn the new commands and functions you used here so I can solve this kind of problem in the future.

Thank you so much for your time!

[Star Strider]

My pleasure!

( The sincerest expression of appreciation here MATLAB Answers is to Accept the Answer that most closely solves your problem. )