Development Environment

Debugging M-Files

This section introduces general techniques for finding errors, and then illustrates MATLAB debugger features found in the Editor/Debugger and equivalent debugging functions using a simple example. It includes these topics:

• Types of Errors
• Finding Errors
• Debugging Example--The Collatz Problem
• Trial Run for Example
• Using Debugging Features

In addition to the Debugger and debugging functions, the Profiler included with MATLAB can be a useful tool to help you improve performance and detect problems in your M-files. For details, see Measuring Performance in the Programming and Data Types section of the MATLAB documentation.

Types of Errors

Debugging is the process by which you isolate and fix problems with your code. Debugging helps to correct two kinds of errors:

• Syntax errors--For example, misspelling a function name or omitting a parenthesis. Syntax Highlighting helps you identify these problems, as does the process of setting breakpoints.

• When you run an M-file with a syntax error, MATLAB will most likely detect it and display an error message in the Command Window describing the error and showing its line number in the M-file. Click the underlined portion of the error message, or position the cursor within the message and press Ctrl+Enter. The offending M-file opens in the Editor, scrolled to the line containing the error. Use the pcode function to check for syntax errors in your M-file without running the M-file.

• Run-time errors--These errors are usually algorithmic in nature. For example, you might modify the wrong variable or perform a calculation incorrectly. Run-time errors are apparent when an M-file produces unexpected results.

Finding Errors

Usually, it is easy to find syntax errors based on MATLAB error messages. Run-time errors are more difficult to track down because the function's local workspace is lost when the error forces a return to the MATLAB base workspace. Use the following techniques to isolate the causes of run-time errors:

• Remove selected semicolons from the statements in your M-file. Semicolons suppress the display of intermediate calculations in the M-file. By removing the semicolons, you instruct MATLAB to display these results on your screen as the M-file executes.
• Add keyboard statements to the M-file. Keyboard statements stop M-file execution at the point where they appear and allow you to examine and change the function's local workspace. This mode is indicated by a special prompt:

• K>>
  

• Resume function execution by typing return and pressing the Return key.

• Comment out the leading function declaration and run the M-file as a script. This makes the intermediate results accessible in the base workspace.
• Use the depfun function to see the dependent functions.
• Use the MATLAB Editor/Debugger or debugging functions. They are useful for correcting run-time errors because you can access function workspaces and examine or change the values they contain. You can set and clear breakpoints, lines in an M-file at which execution halts. You can change workspace contexts, view the function call stack, and execute the lines in an M-file one by one.

The remainder of this section on debugging M-files describes the use of the Editor/Debugger and debugging functions using an example.

Closing M-Files

Debugging Example--The Collatz Problem