persistent variable error in code generation

Hi everyone,

I encountered a problem when I convert a matlab file into a Mex file. The code generation throws me the following error:

Persistent variable 'default_options' must be assigned before it is used. 
The only exception is a check using 'isempty(default_options)' 
that can be performed prior to assignment.

In particular, the Matlab code I am converting is one of the functions in the MatlabBGL package: please find the source code below (I cleared some comments in the source code for simplification). This package is useful to find the largest connected set in a (large) binary matrix.

function old_default = set_matlab_bgl_default(varargin)
% SET_MATLAB_BGL_DEFAULT Sets a default option for the Matlab BGL interface
% Example:
%   % tranpose the matrix initially...
%   At = A'
%   old_options = set_matlab_bgl_default(struct('istrans',1));
%   % perform a bunch of graph work with At...
%   d1 = dfs(At,1); d2 = dfs(At,2); ...
%   % restore the old options 
%   set_matlab_bgl_default(old_options);

% David Gleich
% Copyright, Stanford University, 2006-2008

persistent default_options;
if ~isa(default_options,'struct')
    % initial default options
    default_options = struct('istrans', 0, 'nocheck', 0, 'full2sparse', 0);
end

if nargin == 0
    old_default = default_options;
else
    old_default = default_options;
    default_options = merge_options(default_options,varargin{:});
end

Since I have never used "persistent" variables before, I am not sure how I could fix this error so that I can proceed to code generation in Matlab coder.

One alternative I tried is to use the official graphic functions (graph and conncomp), but neither of them supports the code generation.

Thanks, and I look forward to hearing from you!

Best,

Long

[Ji Lee]

The error is stating that MATLAB Coder is expecting the generic idiom "isempty(default_options)" and not the more specific "~isa(default_options, 'struct')" that the code is currently using. Thus, modifying that "if" condition should eliminate that particular error.

https://www.mathworks.com/help/coder/ug/working-with-persistent-variables.html

https://www.mathworks.com/help/matlab/ref/persistent.html