MATLAB: How to evaluate a polynomial p at each point in y

polyfitpolynomialpolyval

How to evaluate a polynomial p at each point in y? I know polyval(p,x) makes the same but for each point in x, but I would like to know if there exist something similar for y.

Thanks!

Best Answer

You can use fzero(). For example

p = [1 0 0]; % polynomial y = x.^2
y = 4; % value of y
x_sol = fzero(@(x) polyval(p, x)-y, rand()); % corresponding value of x

Result

>> x_sol
x_sol =
     2

Related Solutions

MATLAB: How to find maximum of a multivariable function using max()

You can use fmincon function to find maximum value

z = @(x,y) (y+cos(y))./(x.^2);
sol = fmincon(@(x) -z(x(1),x(2)), 10*rand(2,1), [], [], [], [], [1;1], [15;15]);
x_sol = sol(1);
y_sol = sol(2);

MATLAB: Find the least positive root of the equation y=cos(sqrt(abs(polyval(p,x))))-sin(x)/2 with the use of fzero

Similarly to your earlier Question:

p = polyfit(x,y,7);                                     % Fit Polynomial
P = @(x,p) polyval(p,x);
f = @(x,p) cos(sqrt(abs(P(x,p)))) - sin(x)/2;           % Function
f_root = fzero(@(x) f(x,p), 1);                         % Find Single Root
xv0 = linspace(0, 5, 100);                              % Vector Of Initial Estimates For Multiple Roots
for k1 = 1:length(xv0)                                  % Find Multiple Roots
    f_root_v(k1) = fzero(@(x) f(x,p), xv0(k1));         % Find Vector Of Roots
end
roundn = @(x,n) round(x .* 10.^n)./10.^n;               % Round ‘x’ To ‘n’ Digits, Emulates Latest ‘round’ Function
function_roots = unique(roundn(f_root_v, 4));           % Find Unique Roots

EDIT — To find the least positive root, add this line:

LeastPositiveRoot = min(function_roots(function_roots > 0))

Best Answer

Related Solutions

MATLAB: How to find maximum of a multivariable function using max()

MATLAB: Find the least positive root of the equation y=cos(sqrt​(abs(polyv​al(p,x))))​-sin(x)/2 with the use of fzero

Related Question

MATLAB: Find the least positive root of the equation y=cos(sqrt(abs(polyval(p,x))))-sin(x)/2 with the use of fzero