Mike on the MATLAB Desktop

May 19th, 2008

Stop in the name of error

Recently I was performing some maintenance on the Dependency Report tool. The report has logic to display something reasonable in the web browser even if the scanned directory contains invalid MATLAB files. To make that work, there are several try/catch blocks that the process various errors that may be encountered.

When I tested my new changes in a directory with some “bad” MATLAB files…let’s just say that the code was not handling the errors properly! Normally in these situations I type:

dbstop if caught error

and inspect the state of program in the editor. However due to the recursive, iterative, and nested nature of this code, there were too many caught errors in different files to make this an efficient process. That’s when I remembered that we can stop on a particular type of error, but I could not remember the syntax to do so.

Debug menu to the rescue!

I clicked Debug -> Stop if Errors/Warnings… to open the Stop if Errors/Warnings for All Files dialog.

Next, I needed a unique error identifier, so MATLAB would know where to halt execution. This is easy to do from the command prompt using the lasterror command:

>> lasterror

ans = 

       message: 'Index exceeds matrix dimensions.'
    identifier: 'MATLAB:badsubscript'
         stack: [1x1 struct]

Then, I cut and pasted the identifier into the Stop if Errors/Warnings for All Files dialog. To follow along, Click the “Try/Catch Errors” tab (1), select “Use Message Identifiers” (2), and click “Add” (3):

Finally, I pasted in the message I was interested in and clicked “OK”:

After this, I re-ran my code. Execution stopped only when this error was caught, which allowed me to debug just this issue.

Incidentally, the command-line syntax is shown in the dialog; hopefully I will remember that for next time!

By Michael Katz

Mike is a developer on the MATLAB Mobile team. When not describing himself in the third person, biking, homebrewing, or rooting for the home team, he's busy trying to make the world a better place for programming.

06:20 UTC | Posted in Editor | Permalink | 3 Comments »

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3 Responses to “Stop in the name of error”

Mike replied on May 28th, 2008 at 09:00 UTC :

I should mention, dear readers that you of course should not use the lasterror function. This function depends on global state, and its programmatic use is not encouraged.

At the command line, MException.last contains all of the information in lasterror.
Christian Hachmann replied on April 19th, 2011 at 13:48 UTC :

Hi,
when I enter debug mode via a breakpoint inside a for-loop and enter the command “break” at the prompt it does not behave as it would in the code. I get
```
 ??? Error: A BREAK statement appeared outside of a loop. Use RETURN instead.
```
The continue-statement does not seem to work in debug-mode either.
So i would appreciate a way to skip execution of the rest of the loop and continue execution of the code after the loop.
Mike replied on April 25th, 2011 at 17:34 UTC :

@Christian,

This is not possible. When you run commands from the debug prompt, they are evaluated in a separate program-flow context from the running program (although it does have the appropriate workspace).