I want to make a function in which it takes 2 Input argument (polynomial order n , polynomial coefficient vector ) and gives the output polynomial equation f(x). Also, In the function i take input values of x and my function gives output values of f

function poly(n,V)
  syms x
  f(x)=V(1)*(x^0);
  for i=n:-1:1
    f(x)=V(i+1)*(x^i)+f(x);
  end
  f(x)
  s=input('values of x :');
  f(s)
end

It doesn't give right answer. any other method then please suggest.

[madhan ravi]

what's the value of n and V?

[Vipul Kumar]

n is my order of polynomial. example- for x^2+2*x+3 , my n=2

[madhan ravi]

What is the exact answer you need and what is the value of V?

[Dimitris Kalogiros]

As far as it concerns your example, what is the corresponding V factor ? V=[3 2 1] ?

[Vipul Kumar]

n is a single integer which represents order of polynomial and V is a row vector or array which represent coefficient of polynomial. example- for x^2+2*x+3 ; n=2 and V=[3,2,1]

Answer 1

[dpb]

function [y fn]=poly(V,x)
  y=polyval(V,x);                          % return values for inputs
  n=length(V);                             % length of coeff vector determines order
  trms=[V;[n-1:-1:0]];                     % terms to write to functional
  trms=trms(:); trms=trms(1:end);          % don't write out x^0
  fmt=[repmat('%d*x.^%d+',1,n-1) ,'%d'];   % format string to build polynomial
  fn=sprintf(fmt,trms);
end

Retrurns the polynomial string as string; not sure what end use is to be for...if the thought is it is needed to do a polynomial evaluation given the coefficient vector, then there's no need for it at all (or the function), just use polyval directly.

If there's some other purpose (like for display) then the string should suffice altho might use the new string class or a cellstr instead of just char string array.

If it is wanted to make a functional, then modify slightly--

fmt=['@(x)' repmat('%d*x.^%d+',1,n-1) ,'%d'];
fn=str2func(sprintf(fmt,trms))
fn =
function_handle with value:
  @(x)1*x.^2+2*x.^1+3
fn(1:3)
ans =
   6    11    18

To compare...

polyval(1:3,1:3)  
ans =
   6    11    18

Answer 2

[Dimitris Kalogiros]

clear; clc; close all;

V_example=[3 4 5 6 7 ];
n_example=length(V_example)-1;

y=my_poly(n_example, V_example)

%... Function definition...
function y=my_poly(n,V)
  syms x
  f(x)=V(1)*(x^0);
  for i=n:-1:1
    f(x)=V(i+1)*(x^i)+f(x);
  end
  f(x)
  s=input('values of x :');
  y=f(s);
end