fzero

Root of nonlinear function

Syntax

x = fzero(fun,x0)

x = fzero(fun,x0,options)

x = fzero(problem)

[x,fval,exitflag,output] = fzero(___)

Description

x = fzero(fun,x0) tries to find a point x where fun(x) = 0. This solution is where fun(x) changes sign—fzero cannot find a root of a function such as x^2.

x = fzero(fun,x0,options) uses options to modify the solution process.

x = fzero(problem) solves a root-finding problem specified by problem.

[x,fval,exitflag,output] = fzero(___) returns fun(x) in the fval output, exitflag encoding the reason fzero stopped, and an output structure containing information on the solution process.

Examples

Root Starting From One Point

Calculate $$\pi$$ by finding the zero of the sine function near 3.

fun = @sin; % function
x0 = 3; % initial point
x = fzero(fun,x0)

x = 3.1416

Root Starting from an Interval

Find the zero of cosine between 1 and 2.

fun = @cos; % function
x0 = [1 2]; % initial interval
x = fzero(fun,x0)

x = 1.5708

Note that $$\cos (1)$$ and $$\cos (2)$$ differ in sign.

Root of a Function Defined by a File

Find a zero of the function f(x) = x³ – 2x – 5.

First, write a file called f.m.

function y = f(x)
y = x.^3-2*x-5;

Save f.m on your MATLAB^® path.

Find the zero of f(x) near 2.

fun = @f; % function
x0 = 2; % initial point
z = fzero(fun,x0)

z =
    2.0946

Since f(x) is a polynomial, you can find the same real zero, and a complex conjugate pair of zeros, using the roots command.

roots([1 0 -2 -5])

   ans =
   2.0946          
  -1.0473 + 1.1359i
  -1.0473 - 1.1359i

Root of Function with Extra Parameter

Find the root of a function that has an extra parameter.

myfun = @(x,c) cos(c*x);  % parameterized function
c = 2;                    % parameter
fun = @(x) myfun(x,c);    % function of x alone
x = fzero(fun,0.1)

x = 0.7854

Nondefault Options

Plot the solution process by setting some plot functions.

Define the function and initial point.

fun = @(x)sin(cosh(x));
x0 = 1;

Examine the solution process by setting options that include plot functions.

options = optimset('PlotFcns',{@optimplotx,@optimplotfval});

Run fzero including options.

x = fzero(fun,x0,options)

x = 1.8115

Solve Problem Structure

Solve a problem that is defined by a problem structure.

Define a structure that encodes a root-finding problem.

problem.objective = @(x)sin(cosh(x));
problem.x0 = 1;
problem.solver = 'fzero'; % a required part of the structure
problem.options = optimset(@fzero); % default options

Solve the problem.

x = fzero(problem)

x = 1.8115

More Information from Solution

Find the point where exp(-exp(-x)) = x, and display information about the solution process.

fun = @(x) exp(-exp(-x)) - x; % function
x0 = [0 1]; % initial interval
options = optimset('Display','iter'); % show iterations
[x fval exitflag output] = fzero(fun,x0,options)

 
 Func-count    x          f(x)             Procedure
    2               1     -0.307799        initial
    3        0.544459     0.0153522        interpolation
    4        0.566101    0.00070708        interpolation
    5        0.567143  -1.40255e-08        interpolation
    6        0.567143   1.50013e-12        interpolation
    7        0.567143             0        interpolation
 
Zero found in the interval [0, 1]

x = 0.5671

fval = 0

exitflag = 1

output = struct with fields:
    intervaliterations: 0
            iterations: 5
             funcCount: 7
             algorithm: 'bisection, interpolation'
               message: 'Zero found in the interval [0, 1]'

fval = 0 means fun(x) = 0, as desired.

Input Arguments

fun — Function to solve

function handle | function name

Function to solve, specified as a handle to a scalar-valued function or the name of such a function. fun accepts a scalar x and returns a scalar fun(x).

fzero solves fun(x) = 0. To solve an equation fun(x) = c(x), instead solve fun2(x) = fun(x) - c(x) = 0.

To include extra parameters in your function, see the example Root of Function with Extra Parameter and the section Passing Extra Parameters.

Example: 'sin'

Example: @myFunction

Example: @(x)(x-a)^5 - 3*x + a - 1

Data Types: char | function_handle | string

x0 — Initial value

scalar | 2-element vector

Initial value, specified as a real scalar or a 2-element real vector.

• Scalar — fzero begins at x0 and tries to locate a point x1 where fun(x1) has the opposite sign of fun(x0). Then fzero iteratively shrinks the interval where fun changes sign to reach a solution.

• 2-element vector — fzero checks that fun(x0(1)) and fun(x0(2)) have opposite signs, and errors if they do not. It then iteratively shrinks the interval where fun changes sign to reach a solution. An interval x0 must be finite; it cannot contain ±Inf.

Tip

Calling fzero with an interval (x0 with two elements) is often faster than calling it with a scalar x0.

Example: 3

Example: [2,17]

Data Types: double

options — Options for solution process

structure, typically created using optimset

Options for solution process, specified as a structure. Create or modify the options structure using optimset. fzero uses these options structure fields.

Display	Level of display (see Iterative Display): 'off' displays no output. 'iter' displays output at each iteration. 'final' displays just the final output. 'notify' (default) displays output only if the function does not converge.
FunValCheck	Check whether objective function values are valid. 'on' displays an error when the objective function returns a value that is complex, Inf, or NaN. The default, 'off', displays no error.
OutputFcn	Specify one or more user-defined functions that an optimization function calls at each iteration, either as a function handle or as a cell array of function handles. The default is none ([]). See Output Function and Plot Function Syntax.
PlotFcns	Plots various measures of progress while the algorithm executes. Select from predefined plots or write your own. Pass a function name, function handle, or a cell array of function names or handles. The default is none ([]): @optimplotx plots the current point. @optimplotfval plots the function value. Custom plot functions use the same syntax as output functions. See Output Functions for Optimization Toolbox and Output Function and Plot Function Syntax.
TolX	Termination tolerance on x, a positive scalar. The default is eps, 2.2204e–16. See Tolerances and Stopping Criteria.

Example: options = optimset('FunValCheck','on')

Data Types: struct

problem — Root-finding problem

structure

Root-finding problem, specified as a structure with all of the following fields.

objective	Objective function
x0	Initial point for x, scalar or 2-D vector
solver	'fzero'
options	Options structure, typically created using optimset

Data Types: struct

Output Arguments

x — Location of root or sign change

real scalar

Location of root or sign change, returned as a scalar.

fval — Function value at x

real scalar

Function value at x, returned as a scalar.

exitflag — Integer encoding the exit condition

integer

Integer encoding the exit condition, meaning the reason fzero stopped its iterations.

1	Function converged to a solution x.
-1	Algorithm was terminated by the output function or plot function.
-3	NaN or Inf function value was encountered while searching for an interval containing a sign change.
-4	Complex function value was encountered while searching for an interval containing a sign change.
-5	Algorithm might have converged to a singular point.
-6	fzero did not detect a sign change.

output — Information about root-finding process

structure

Information about root-finding process, returned as a structure. The fields of the structure are:

intervaliterations	Number of iterations taken to find an interval containing a root
iterations	Number of zero-finding iterations
funcCount	Number of function evaluations
algorithm	'bisection, interpolation'
message	Exit message

Algorithms

The fzero command is a function file. The algorithm, created by T. Dekker, uses a combination of bisection, secant, and inverse quadratic interpolation methods. An Algol 60 version, with some improvements, is given in [1]. A Fortran version, upon which fzero is based, is in [2].

Alternative Functionality

App

The Optimize Live Editor task provides a visual interface for fzero.

References

[1] Brent, R., Algorithms for Minimization Without Derivatives, Prentice-Hall, 1973.

[2] Forsythe, G. E., M. A. Malcolm, and C. B. Moler, Computer Methods for Mathematical Computations, Prentice-Hall, 1976.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

For C/C++ code generation:

• The fun input argument must be a function handle, and not a structure or character vector.

• fzero ignores all options except for TolX and FunValCheck.

• fzero does not support the fourth output argument, the output structure.

Version History

Introduced before R2006a