MATLAB is exceptionally strong in linear algebra, numerical methods, and graphical interpretation of data. It is easily programmed and relatively easy to learn to use. Hence, it has proven invaluable to engineers and scientists who rely on the scientific techniques and methods at which MATLAB excels. Very often the individuals and groups that so employ MATLAB are primarily interested in the numbers and graphs that emerge from MATLAB commands, processes and programs. Therefore, it is enough for them to work in a MATLAB Command Window, from which they can easily print or export their desired output.

However, other practitioners of mathematical software find themselves with two additional requirements. First, they need a mathematical software package embedded in an interactive environment, in which it is easy to make changes and regenerate results. Second, they need a higher-level presentation mode, which integrates computation and graphics with text, uses different formats for input and output, and communicates effortlessly with other software applications. These additional requirements can be accomplished using either cells and the publish command, or else the M-book interface, both of which were briefly described in Chapter 3. The present chapter goes into more detail and discusses some of the fine points of these methods.

Fine Points of Publishing

As we mentioned Chapter 3, the simplest way to produce a finished presentation with MATLAB is to prepare your work in a script M-file and then publish the result.

That statement encapsulates the view of The MathWorks, Inc., the developer of MATLAB®. MATLAB 7 is an ambitious program. It contains hundreds of commands to do mathematics. You can use it to graph functions, solve equations, perform statistical tests, and much more. It is a high-level programming language that can communicate with its cousins, e.g., Fortran and C. You can produce sound and animate graphics. You can do simulations and modeling (especially if you have access not just to basic MATLAB but also to its accessory Simulink®). You can prepare materials for export to the World Wide Web. In addition, you can use MATLAB to combine mathematical computations with text and graphics in order to produce a polished, integrated, interactive document.

A program this sophisticated contains many features and options. There are literally hundreds of useful commands at your disposal. The MATLAB help documentation contains thousands of entries. The standard references, whether the MathWorks User's Guide for the product, or any of our competitors, contain a myriad of tables describing an endless stream of commands, options, and features that the user might be expected to learn or access.

In this chapter we describe an effective procedure for working with MATLAB, and for preparing and presenting the results of a MATLAB session. In particular we discuss some features of the MATLAB interface and the use of M-files. We introduce a new command in MATLAB 7, publish, which produces formatted output. We also give some simple hints for debugging your M-files.

The MATLAB Interface

Starting with version 6, MATLAB has an interface called the MATLAB Desktop. Embedded inside it is the Command Window that we described in Chapter 2.

The Desktop

By default, the MATLAB Desktop (Figure 1.1 in Chapter 1) contains four windows inside it, the Command Window on the right, the Current Directory Browser and the Workspace Browser in the upper left, and the Command History Window in the lower left. Notice that there are tabs for alternating between the Current Directory and Workspace Browsers. You can change which windows are currently visible with the Desktop menu (in MATLAB 6, the View menu) at the top of the Desktop, and you can adjust the sizes of the windows by dragging their edges with the mouse. The Command Window is where you type the commands and instructions that cause MATLAB to evaluate, compute, draw, and perform all the other wonderful magic that we describe in this book. We will discuss the other windows in separate sections below.

With MATLAB you can create your own Graphical User Interface, or GUI, which consists of a Figure window containing menus, buttons, text, graphics, etc., that a user can manipulate interactively with the mouse and keyboard. There are two main steps in creating a GUI: one is designing its layout, and the other is writing callback functions that perform the desired operations when the user selects different features.

GUI Layout and GUIDE

Specifying the location and properties of various objects in a GUI can be done with commands such as uicontrol, uimenu, and uicontextmenu in an M-file. MATLAB also provides an interactive tool (a GUI itself!) called GUIDE (this stands for Graphical User Interface Development Environment) that greatly simplifies the task of building a GUI. We will describe here how to get started writing GUIs with the MATLAB 7 version of GUIDE, which has some significant enhancements over earlier versions. The version of GUIDE in MATLAB 6 is roughly similar, but some of the menu items and options are different or missing.

Every time you create an M-file, you are writing a computer program using the MATLAB programming language. You can do quite a lot in MATLAB using no more than the most basic programming techniques that we have already introduced. In particular, we discussed simple loops (using for) and a rudimentary approach to debugging in Chapter 3. In this chapter, we will cover some further programming commands and techniques that are useful for attacking more complicated problems with MATLAB. If you are already familiar with another programming language, much of this material will be quite easy for you to pick up!

Branching

For many user-defined functions, you can use a function M-file that executes the same sequence of commands for each input. However, one often wants a function to perform a different sequence of commands in different cases, depending on the input. You can accomplish this with a branching command, and, as in many other programming languages, branching in MATLAB is usually done with the command if, which we will discuss now. Later we will describe the other main branching command, switch.

Branching with if

For a simple illustration of branching with if, consider the following function M-file absval.m, which computes the absolute value of a real number.

In this chapter, we offer advice for dealing with some common problems that you may encounter. We also list and describe the most common mistakes that MATLAB users make. Finally, we offer some simple but useful techniques for debugging your M-files.

Common Problems

Problems manifest themselves in various ways: totally unexpected or plainly wrong output appears; MATLAB produces an error message (or at least a warning); MATLAB refuses to process an input line; something that worked earlier stops working; or, worst of all, the computer freezes. Fortunately, these problems are often caused by several easily identifiable and correctable mistakes. What follows is a description of some common problems, together with a presentation of likely causes, suggested solutions, and illustrative examples. We also refer to places in the book where related issues are discussed.

Here is a list of the problems:

• Wrong or unexpected output

• Syntax error

• Spelling error

• Error or warning messages when plotting

• A previously saved M-file evaluates differently

• Computer won't respond.

Wrong or Unexpected Output

There are many possible causes for this problem, but they are likely to be among the following.

CAUSE: Forgetting to clear or reset variables.

SOLUTION: Clear or initialize variables before using them, especially in a long session.

In this chapter we describe more of MATLAB's graphics commands and the most common ways of manipulating and customizing graphics. For an overview of commands, type help graphics (for general graphics commands), help graph2d (for two-dimensional graphics commands), help graph3d (for three-dimensional graphics commands), and help specgraph (for specialized graphing commands).

We have already discussed the commands plot and ezplot in Chapter 2. We will begin this chapter by discussing more uses of these commands, as well as some of the other most commonly used plotting commands. Then we will discuss methods for customizing and manipulating graphics. Finally, we will introduce some commands and techniques for creating and modifying images and sounds.

Two-Dimensional Plots

Often one wants to draw a curve in the x-y plane, but with y not given explicitly as a function of x. There are two main techniques for plotting such curves: parametric plotting and contour or implicit plotting.