rosgenmsg(folderpath) from custom ros msgs add on causes Index exceeds the number of array elements (0).

Hello,

I'm trying to get Matlab to be able to use some custom ROS msgs from a ROS package that follows the required structure has worked in other environments. I've followed the instructions at https://www.mathworks.com/help/ros/ug/create-custom-messages-from-ros-package.html but will get a 'Index exceeds the number of array elements (0).' Error on the rosgenmsg(folderpath)

Any one run into this issue before or is this a bug.

[Dhananjay Kumar]

Hey daniel, Can you write the complete error message you see or attach a snip?

[Daniel Levy]

The code I used was straight from the instructions at https://www.mathworks.com/help/ros/ug/create-custom-messages-from-ros-package.html I also made sure that folder exists.

examplePackages = fullfile(fileparts(which('rosgenmsg')), 'examples', 'packages');
userFolder = 'C:\MATLAB\CustomMessages\';
copyfile(examplePackages, userFolder)
folderpath = userFolder;
rosgenmsg(folderpath)

Error message is below, breaks at rosgenmsg(folderpath)

Error using ROSScratchPad (line 8)
Index exceeds the number of array elements (0).

Thanks,

Answer 1

[Cam Salzberger]

Daniel,

This kind of error message generally comes up (in any MATLAB product) when you are trying to call a function but have shadowed the function name with a variable name. For example, if you do this:

rosgenmsg = 1;
myPath = 'C:\ros';
rosgenmsg(myPath)

MATLAB is trying to index into the variable "rosgenmsg" using the character values in "myPath" as index values. Since 'C' is valued at a larger index than the length of "rosgenmsg", you see this error.

Run this command to see if you have "rosgenmsg" shadowed by a variable name:

which -all rosgenmsg

-Cam

[Daniel Levy]

Cam,

Yes, that looks like it's the problem

>> which -all rosgenmsg
C:\Users\######\Documents\MATLAB\rosgenmsg.m
C:\ProgramData\MATLAB\SupportPackages\R2019b\toolbox\ros\supportpackages\roscustommsg\rosgenmsg.m  % Shadowed

So I can remove the rosgenmsg in my MATLAB documents (don't remember putting that there) and the only one that remains is in the toolbox, but I get the same error when I use it. Do I need to remove the toolbox?

-Dan

[Cam Salzberger]

That is odd. I was expecting a variable in the workspace more than I was expecting a shadowing function. Could just be that you need to rehash (after removing the function from your Documents folder) to get it to target the correct function. Try this, just to be safe:

rehash toolboxcache

Restarting MATLAB wouldn't go amiss either.

[Daniel Levy]

So now I only get the tool box function, but it did not get rid of the same error after the rehash and restart.

>> which -all rosgenmsg
C:\ProgramData\MATLAB\SupportPackages\R2019b\toolbox\ros\supportpackages\roscustommsg\rosgenmsg.m

[Cam Salzberger]

Hmm, we're now exploring odd territory. If "rosgenmsg" were indeed a variable, then I would expect to see an error message more like this:

Index exceeds the number of array elements (1).

'rosgenmsg' appears to be both a function and a variable. If this is unintentional, use 'clear rosgenmsg' to remove the variable 'rosgenmsg' from the workspace.

And this would be result of which -all rosgenmsg:

rosgenmsg is a variable.
C:\ProgramData\MATLAB\SupportPackages\R2019b\toolbox\ros\supportpackages\roscustommsg\rosgenmsg.m  % Shadowed

If it was calling the correct rosgenmsg, but there was some issue with it, I'd expect to see a stack trace that shows at which line in rosgenmsg there was an issue. However, a lot of the ROS 1 functionality uses custom error messages which can sometimes disguise exactly where the error occurs in the toolbox code. Try giving this a shot, and see if the report looks more useful:

try
    rosgenmsg(folderpath)
catch ME
    getCustomReport(ME)
end

Another thing to try after that is to simply debug-break at first point the error occurs. Sometimes this is a bit down in the weeds, but it can help situations like this. First run this:

dbstop if error

Then run your code. See where the error is occurring, what the error is, and what the workspace looks like while in debug mode.

-Cam

[Daniel Levy]

Cam,

So, I found the error, there was a +ros folder that got copied into another folder and was creating the duplications of functions within rosgenmsg. It wasn't getting deleted/rehashed because it was saved in the windows environment and I had to restart the computer to get rid of it.

It wasn't actually rosgenmsg that was the problem but the generate custom Jar files and some others that was throwing the error. So your original intuition was correct, I must have bungled something that copied over that +ros folder. When I was using the custom messages example's 'copyfile(examplePackages, userFolder)' command.

Thanks for the help/effort,

-Dan

[Cam Salzberger]

That's good to know, and hopefully anyone else running into the same issue can find it now. Thanks for posting the answer!