Doc command doesn't work

I recently upgraded from R2019b to R2021b. I used to be able to type:

doc abs

or some other command and the documentation page for that command would pop up. Now when I type

doc <command>

I get this error:

Error using strjoin (line 38)
The first argument should be a cell array of strings.

Error in matlab.net.URI/get.EncodedPath (line 514)
str = strjoin(path, '/');

Error in matlab.net.URI/string (line 680)
path = obj.EncodedPath;

Error in matlab.net.URI/char (line 696)
str = string(obj);

Error in doc>displayFile (line 205)
com.mathworks.mlservices.MLHelpServices.setCurrentLocation(char(htmlFile));

Error in doc>displayDocPage (line 169)
displayFile(refPage);

Error in doc (line 120)
if ~displayDocPage(refTopics, topic) && ~showHelpwin(topic)
    
    Error in doc (line 46)
    feval( orig_doc, funcname );

I tried typing

    doc "abs"

Which does bring up the documentation popup, but only as a search for "abs" and not the actual documentation page.

This happens for all commands, not just abs.

Answer 1

[Walter Roberson]

You have a third party product installed that defines its own strjoin.

Most likely culprit would be SPM12 which if I recall correctly defines strjoin in one of its "compatibility layers" directories. For a recent matlab version such as yours, the two compatability directories for SPM12 should be removed from the matlab path.

[Daniel Zuerbig]

I don't think this is the issue. I'm able to call "help strjoin" and "open strjoin" and see the regular strjoin function.

As a separate experiment, I tried typing "doc ajdkfla" or some other command that doesn't exist, and that does indeed open the doc popup and search for the specified command. So the doc command only works for commands that don't exist

[Daniel Zuerbig]

Another update: I'm running Windows 11. I looked into the doc.m file located in: C:\Program Files\MATLAB\R2021b\toolbox\matlab\helptools\doc.m

When I call doc abs I set a breakpoint at line 205

com.mathworks.mlservices.MLHelpServices.setCurrentLocation(char(htmlFile));

The error occurs at the char command, and htmlFile contains:

htmlFile = 

URI with properties:

Scheme: "https"
UserInfo: [0×0 string]
Host: "www.mathworks.com"
Port: []
EncodedAuthority: "www.mathworks.com"
Path: [""    "help"    "releases"    "R2021b"    "matlab"    "ref"    "abs.html"]
Query: [1×1 matlab.net.QueryParameter]
EncodedQuery: "overload=abs+false"
Fragment: [0×0 string]
Absolute: 1

When I call abs asdf then this line 205 is never reached. Maybe this is a bug in MATLAB? This is a fresh install so I would think that this bug might have been fixed already.

[Steven Lord]

Let's check that you (or someone who wrote a function you downloaded) didn't shadow the char function included in MATLAB. When you reach that line of code, please run this command and show us what it displays.

which -all char

It will likely show a long list of functions and that's okay. I want to see if there's one that's not in a directory under matlabroot.

Just to forestall a potential follow-up, can you also show what this displays?

which -all strjoin

If the command above looking at char only shows versions provided by MathWorks, I'm betting this one will show a different function. The help text for the strjoin.m included with MATLAB release R2021b is written on lines 2 through 40 of the file (line 1 is the function declaration) so the line 38 in your message is a comment not an executable command in that file.

[Walter Roberson]

That is not the current error message. The current error message is

First input must be a string array or cell array of character vectors.

Support for string objects was added around R2016b-ish (it took a couple of releases to update all functions.)

That error message would be consistent with the possibility that you are somehow getting an old version of strjoin that did not support string objects.

[Daniel Zuerbig]

Thanks for your help! There was a redefinition of strjoin somewhere deep in our MATLAB repo. I changed the name of the function and it works now.