MATLAB®

Internal MATLAB^® Functions

Overview

Many of the functions provided with the MATLAB^® software are implemented as M-files just like the M-files that you will create with MATLAB. Other MATLAB functions are precompiled executable programs called built-ins that run much more efficiently. Many of the MATLAB functions are also overloaded so that they handle different classs appropriately.

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M-File Functions

If you look in the subdirectories of the toolbox\matlab directory, you can find the M-file sources to many of the functions supplied with MATLAB. You can locate your toolbox\matlab directory by typing

dir([matlabroot '\toolbox\matlab\'])

MATLAB functions with an M-file source are just like any other functions coded with MATLAB. When one of these M-file functions is called, MATLAB parses and executes each line of code in the M-file. It saves the parsed version of the function in memory, eliminating parsing time on any further calls to this function.

Identifying M-File Functions

To find out if a function is implemented with an M-file, use the exist function. The exist function searches for the name you enter on the MATLAB path and returns a number identifying the source. If the source is an M-file, then exist returns the number 2. This example identifies the source for the repmat function as an M-file:

exist repmat
ans =
     2

The exist function also returns 2 for files that have a file type unknown to MATLAB. However, if you invoke exist on a MATLAB function name, the file type will be known to MATLAB and will return 2 only on M-files.

Viewing the Source Code

One advantage of functions implemented as M-files is that you can look at the source code. This may help when you need to understand why the function returns a value you did not expect, if you need to figure out how to code something in MATLAB that is already coded in a function, or perhaps to help you create a function that overloads one of the MATLAB functions.

To find the source code for any MATLAB M-file function, use which:

which repmat
   D:\matlabR14\toolbox\matlab\elmat\repmat.m

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Built-In Functions

Functions that are frequently used or that can take more time to execute are often implemented as executable files. These functions are called built-ins.

Unlike M-file functions, you cannot see the source code for built-ins. Although most built-in functions do have an M-file associated with them, this file is there mainly to supply the help documentation for the function. Take the reshape function, for example, and find it on the MATLAB path:

which reshape
   D:\matlabR14\toolbox\matlab\elmat\reshape.m

If you type this M-file out, you will see that it consists almost entirely of help text. At the bottom is a call to the built-in executable image.

Identifying Built-In Functions

As with M-file functions, you can identify which functions are built-ins using the exist function. This function identifies built-ins by returning the number 5:

exist reshape
ans =
     5

Forcing a Built-In Call

If you overload any of the MATLAB built-in functions to handle a specific class, then MATLAB will always call the overloaded function on that type. If, for some reason, you need to call the built-in version, you can override the usual calling mechanism using a function called builtin. The expression

builtin('reshape', arg1, arg2, ..., argN);

forces a call to MATLAB built-in reshape, passing the arguments shown, even though an overload exists for the class in this argument list.

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Overloaded MATLAB^® Functions

An overloaded function is an additional implementation of an existing function that has been designed specifically to handle a certain class. When you pass an argument of this type in a call to the function, MATLAB looks for the function implementation that handles that type and executes that function code.

Each overloaded MATLAB function has an M-file on the MATLAB path. The M-files for a certain class are placed in a directory named with an @ sign followed by the class name. For example, to overload the MATLAB plot function to plot expressions of a class named polynom differently than other class, you would create a directory called @polynom and store your own version of plot.m in that directory.

You can add your own overloads to any function by creating a class directory for the class you wish to support for that function, and creating an M-file that handles that type in a manner different from the default. See Defining Classes — Syntax Overview and Developing Classes — Typical Workflow.

When you use the which command with the -all option, MATLAB returns all occurrences of the file you are looking for. This is an easy way to find functions that are overloaded:

which -all set          % Show all implementations for 'set'

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Symbol Reference

Functions and Scripts