Trying to find the roots of polynomial with a script file

function [y] = Func_6_23(x)

y = ((x^5) - (12*(x^4)) - (9*(x^3)) + (392(x^2)) - (372*x) - 2160);
end

I'm trying to execute this script file to find roots and I keep getting this error,

Error in roots (line 26) when I don't even have a line 26.
if ~all(isfinite(c))

[Walter Roberson]

Did you happen to name your script roots ? If you did then you might have accidentally activated MATLAB's roots() function instead.

[Go Detroit Lions]

No I named my script file Func_6_23. Also gives me error: Undefined function 'isfinite' for input arguments of type 'function_handle'. My professor called it from the command line with the following text: roots(@Func_6_23)

[Walter Roberson]

Any .m whose first executable word is "function" or "classdef" is not a script. You defined a function, not a script .

The MATLAB function roots() is not defined for function handles.

Perhaps it is intended that you define the function roots() that has to find roots of a polynomial ?

[Go Detroit Lions]

The assignment stated to:

Use MATLAB's roots function, write a script M-file to find the roots of the polynomial. State the commands of the script file you used.

I found an error in the file i wrote, (392(x^2))-->(392*(x^2)) but still gives the same error, so I'll try to fix it later. Thanks for your help.

[Walter Roberson]

hint: the roots of 10*x^3 - 7*x^2 - 11*x + 4 can be found by roots([10, 7, 11, 4])

Answer 1

[madhan ravi]

y=[1 -12 -9 392 -372 -2160]
roots(y)

Note: Always use coefficients to find roots in Matlab.

[madhan ravi]

If it worked , accept the answer

Answer 2

[Image Analyst]

"My professor called it from the command line with the following text: roots(@Func_6_23)"

It seems like your professor wants a function from you that determines (returns) a vector with the polynomial coefficients, which you can then pass in to the built-in roots() function. It might go something like:

function [y] = Func_6_23(inputPolynomial)
% For example inputPolynomial is a string "((x^5) - (12*(x^4)) - (9*(x^3)) + (392(x^2)) - (372*x) - 2160);"
% and we're supposed to return y as [1, 12, 9, 392, 372, -2160]
inputPolynomial(inputPolynomial== ' ') = [];  % Remove any spaces.
% Now loop over orders, say the first 20 or whatever and find 'x^n'
for k = 1 : 20
    thisString = sprintf('x^%d', k); % E.g. 'x^5'
    index = strfind(inputPolynomial, thisString); % Find x^5 in the whole polynomial string.
    % etc. to find coefficients of thisString.
    y(k) = .....
end
% Here, you should have y which is a vector containing all the coefficients,
% which you can then pass in to roots().