Debugging :: Programming Tips (Programming)

Programming

Debugging

This section covers the following topics:

The MATLAB Debug Functions

For a brief description of the main debug functions in MATLAB, type

```
help debug
```

For more information: See Debugging M-Files in the MATLAB Desktop Tools and Development Environment documentation.

More Debug Functions

Other functions you may find useful in debugging are listed below.

Function
Description

echo
Display function or script code as it executes.

disp
Display specified values or messages.

sprintf,
fprintf
Display formatted data of different types.

whos
List variables in the workspace.

size
Show array dimensions.

keyboard
Interrupt program execution and allow input from keyboard.

return
Resume execution following a keyboard interruption.

warning
Display specified warning message.

error
Display specified error message.

lasterr
Return error message that was last issued.

lasterror
Return last error message and related information.

lastwarn
Return warning message that was last issued.

Function	Description
`echo`	Display function or script code as it executes.
`disp`	Display specified values or messages.
`sprintf`, `fprintf`	Display formatted data of different types.
`whos`	List variables in the workspace.
`size`	Show array dimensions.
`keyboard`	Interrupt program execution and allow input from keyboard.
`return`	Resume execution following a keyboard interruption.
`warning`	Display specified warning message.
`error`	Display specified error message.
`lasterr`	Return error message that was last issued.
`lasterror`	Return last error message and related information.
`lastwarn`	Return warning message that was last issued.

The MATLAB Graphical Debugger

Learn to use the MATLAB graphical debugger. You can view the function and its calling functions as you debug, set and clear breakpoints, single-step through the program, step into or over called functions, control visibility into all workspaces, and find and replace strings in your files.

Start out by opening the file you want to debug using File -> Open or the open function. Use the debugging functions available on the toolbar and pull-down menus to set breakpoints, run or step through the program, and examine variables.

For more information: See Debugging M-Files and Using Debugging Features in the MATLAB Desktop Tools and Development Environment documentation.

A Quick Way to Examine Variables

To see the value of a variable from the Editor/Debugger window, hold the mouse cursor over the variable name for a second or two. You will see the value of the selected variable displayed.

Setting Breakpoints from the Command Line

You can set breakpoints with dbstop in any of the following ways:

Break at a specific M-file line number.
Break at the beginning of a specific subfunction.
Break at the first executable line in an M-file.
Break when a warning, or error, is generated.
Break if any infinite or NaN values are encountered.

For more information: See Setting Breakpoints in the MATLAB Desktop Tools and Development Environment documentation.

Finding Line Numbers to Set Breakpoints

When debugging from the command line, a quick way to find line numbers for setting breakpoints is to use dbtype. The dbtype function displays all or part of an M-file, also numbering each line. To display copyfile.m, use

```
dbtype copyfile
```

To display only lines 70 through 90, use

```
dbtype copyfile 70:90
```

Stopping Execution on an Error or Warning

Use dbstop if error to stop program execution on any error and enter debug mode. Use warning debug to stop execution on any warning and enter debug mode.

For more information: See Debug, Backtrace, and Verbose Modes in the MATLAB Programming documentation.

Locating an Error from the Error Message

Click on the underlined text in an error message, and MATLAB opens the M-file being executed in its editor and places the cursor at the point of error.

For more information: See Types of Errors in the MATLAB Desktop Tools and Development Environment documentation.

Using Warnings to Help Debug

You can detect erroneous or unexpected behavior in your programs by inserting warning messages that MATLAB will display under the conditions you specify. See the section on Warning Control in the MATLAB "Programming and Data Types" documentation to find out how to selectively enable warnings.

For more information: See the warning function reference page.

Making Code Execution Visible

An easy way to see the end result of a particular line of code is to edit the program and temporarily remove the terminating semicolon from that line. Then, run your program and the evaluation of that statement is displayed on the screen.

For more information: See Finding Errors in the MATLAB Desktop Tools and Development Environment documentation.

Debugging Scripts

Scripts store their variables in a workspace that is shared with the caller of the script. So, when you debug a script from the command line, the script uses variables from the base workspace. To avoid errors caused by workspace sharing, type clear all before starting to debug your script to clear the base workspace.

Program Development Variables