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I keep getting this error in Octave (though it should also show up in 
MATLAB).
\begin{verbatim}
    error: =: nonconformant arguments (op1 is 1x14, op2 is 1x15)
\end{verbatim}

I'm trying to build a text-representation of 2D array.
The 2D array is a bit complicated, in that each element is a subset of
an image, and the image uses the same few tiles over and over. 
I want a program to read in the image, then write out the tiles,
like 'w....w' for a line indicating a wall, 4 floor spaces, and another wall.

The problem does not really stem from the image, but is a problem with 
the assignment of a string to a 2D array called \verb"myroom".
Putting in some debugging code, I found that 
variable \verb"rowCount" is 8.
This means that \verb"myroom" is almost
complete, and that the problem shows up on the last line.
That is, it set \verb"myroom(7,:) = walls;" just fine, 
but setting \verb"myroom(8,:) = walls;" causes the error. 
Here is some debug code output.
\begin{verbatim}
                 walls   is "e111111111111e".
          default_line   is "ZZZZZZ::ZZZZZZ".
    myroom(rowCount-1,:) is "e111111111111e".
\end{verbatim}
The strings appear to be the exact same lengths. 

I added some more debugging code, which generated the following output:
\begin{verbatim}
    len myroom(rc-1,:) is 14, while len walls is 15
    walls
    0 90 90 90 90 90 90 58 58 90 90 90 90 90 90
    default_line
    0 90 90 90 90 90 90 58 58 90 90 90 90 90 90
    myroom(rc-1,:)
    101 49 49 49 49 49 49 49 49 49 49 49 49 101
\end{verbatim}
The ASCII representations look fine at first, until I noticed that the
leading value is 0. That should not be there. 

Once I saw the leading 0, the fix became obvious.
Going back to the code, there is an index range of 
\verb"0:MAX_TILE_COL-1" that is used to iterate over the tiles.
However, starting the index at 0 causes a problem in MATLAB/Octave,
so the index has \verb"+1", i.e. there is a line with a
statement like \verb"default_line(tile_col+1) = ..."
Another issue is that the tiles should be surrounded by walls on all
sides, so there should be 2 more columns.
While that is being done, we could shift the index such that it starts at
one. Putting this together, we get this:
\begin{verbatim}
    for tile_col = 1:length(0:MAX_TILE_COL-1) + 2
        % ... more stuff ...
        default_line(tile_col+1) = 'Z';
        % ... more stuff ...
    end
\end{verbatim}
Thus, \verb"default_line(1)" is not assigned a value by the code, 
and \verb"default_line" has a length with one more element than it should.
The 0 characters do not show up when \verb"disp" outputs the lines to 
the command window. Thus, the strings look to be the correct size, but
are not. 

The fix is simple: replace \verb"tile_col+1" with \verb"tile_col".
This illustrates a problem that takes a lot of time to understand exactly
what is going on, but only seconds to fix.


-Michael Weeks, 2023