User-Defined Functions and Function Handles in MATLAB, Slides of Computational Methods

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Chp3 MATLAB

Functions: Part

Functions (ReDeaux)

• MATLAB Has Two Types of Functions

1. Built-In Functions Provided by the

Softeware

• e.g.; max, min, median

2. User-Defined

Functions are .m-files

that can accept InPut

Arguments/Parameters and

Return OutPut Values

Function Handles

• You can create a function handle to any function by using the “at” sign, @, before the function name. You can then name the handle if you wish, and use the handle to reference the function. For example, to create a handle to the sine function, you type >> sine_handle = @sin;

 where sine_handle is a user-selected name for the handle (or NickName).

Function Handles cont

• A common use of a function handle is to pass the function as an argument to another function. For example, plot sinx over 0 ≤ x ≤ 6 as follows:

>> plot([0:0.01:6], sine_handle([0:0.01:6]))

 The Result

Function Handles cont

• Can Pass a Function with @ sign handle

>> gen_plot(@sech,0:.02:10)

Calling Functions

• There are four ways to invoke, or “call,” a function into action. These are:

1. As a character string identifying the appropriate function .m-file
2. As a function handle
3. As an “inline” function object
4. As a string expression.

• Examples of these ways follow for the fzero Built-in fcn which acts on the user-defined function parab, which computes y = x^2 − 4

Calling Functions cont

1. As a character string identifying the appropriate function .m-file, which is

 The function may be called as follows, to compute the zero over the range: 0 ≤ x ≤ 3 >> [x, value] = fzero('parab', [0 3]) x = 2 value = 0

Calling Functions cont

2. As a function handle to an existing function .m-file: >> [z, val0] = fzero(@parab,[0, 3])
3. As an “inline” function object:

**>> parab1 = 'x.^2-4';

parab_inline = inline(parab1); [w, val_0] = fzero(parab_inline,[0, 3]) w = 2 val_0 = 0**

Calling Functions cont

• The function handle method (method 2) is the fastest method, followed by method 1.
• In addition to speed improvement, another advantage of using a function handle is that it provides access to subfunctions , which are normally not visible outside of their defining .m- file.
• If we give a “SubFunction” a handle, or nickname, then we can “reach into” the function file to engage the useful SubFcn withOUT using the main function

Types of User Defined Fcns

• The PRIMARY function is the first function in a .m-file and typically contains the main program. Following the primary function in the same file can be any number of subfunctions , which can serve as subroutines that support the primary function.
• That is, .m-files can contain code for more than one function. Additional functions within the file are called subfunctions, and these are only visible to the primary function or to other subfunctions in the same file.

Types of User Defined Fcns

• ANONYMOUS functions enable creation of a simple function withOUT needing to write a separate a .m-file for it.
• You can construct an anonymous function either at the MATLAB command line or from within another function or script.
• Thus, anonymous functions provide a quick way of making a function from any MATLAB expression withOUT the need to create, name, and save a file.
• More on anonymous fcns in a few slides

Types of User Defined Fcns

• SUBFUNCTIONS are placed in the primary function and are called by the primary fcn
• You can use multiple functions within a single primary function m-file
• NESTED functions are functions defined within another function.
• They can help to improve the readability of your program and also give you more flexible access to variables in the .m-file. Produce Nesting “ Levels ”
• The difference between nested functions and subfunctions is that subfunctions normally cannot be accessed outside of their primary function file

Function Function

• The term function function is not a separate

function type but refers to any function that

accepts another function as an input

argument, such as the function fzero or

fminbnd

• You can pass a function to another function

using a function handle.

Anonymous Functions

• The syntax for creating an anonymous function from an expression is fhandle = @(arglist) expr

 Where

• The Term arglist is a comma-separated list of input arguments to be passed to the function
• The Term expr is any single , valid MATLAB expression.