Creating a polyval function

Hi everybody, I'm actually trying to create a function wich would work like a polyval function but without any S and MU. Just with p and x : polyval(p,x). http://www.mathworks.com/help/matlab/ref/polyval.html But I have absolutely no idea how to do it... I know what i would have to do on a paper but no idea in MATLAB. For example, i don't know how to write this in MATLAB : y = p1*x^n + p2*x^n–1 + … + p*n^x + p*n+1 So if someone could just show me a tip or a way to find it... Thx :)

[raviraja]

Same Problem Here... Need Answer

Answer 1

[Simon]

Hi!

But you can use polyval with just p and x!

[Patrick Allaglo]

Yeah i know i just wanted to show you. But i dont know how to make it...

[Simon]

Maybe the best point to start is to enter

 edit polyval

in your command window and take a look at the code.

Or you start from scratch, write a function that accepts a scalar/vector X and some polynomial coefficients P. Then of course you can directly calculate with

Y = P(1)*X^N + P(2)*X^(N-1) + ... + P(N)*X + P(N+1)

Maybe use a loop (for scalar X) like

Y = 0;
N = length(P) - 1;
e = N:-1:0;
for n = 1:N+1
Y = Y + P(n) * X^e(n);
end

I didn't try it, but it should get you started.

[Patrick Allaglo]

Well i just tried something and it works (with your loop suggestion :)) Thx

Answer 2

[Azzi Abdelmalek]

Example

x=0:10

If you want to calculate a polynomial p=4x^2+2*x+5

p=[4 2 5]
out=polyval(p,x)

[Patrick Allaglo]

No you don't understand, I'm trying to create a function which would work like polyval. I'm not trying to calculate a polynomial. :)

Answer 3

[ledinh lam]

function p = poly_val(c0,c,x)
if isempty(c)==true 
    p =c0;
elseif isscalar(c)==true
    p = c0+c*x;
else 
    w=c(:);
    n=length(c);
    a=x.^[1:n];
    p=c0+a*w;

end
end

if true
  % code
end

Answer 4

[Jos (10584)]

I do not really get your problem, but are you after something like this?

x = -5:5 
p = [3 2 4] 
y = p(1)
for k=2:numel(p),
  y = y + p(k) * x.^(k-1) % you can change this to another formula f(k,p(k),x)
end

Answer 5

[Patrick Allaglo]

Hey again :D I have a little question, how can i save data from a loop with a variable evolving between -5 to 5 for example, because when I try to save data by using variable_stored, it says that i have to use positive integer or logical...

[Simon]

Make a table like

tt = -5:5;

Access your variable with index -3 like

V(tt==-3)