Matlab Codegen build error - c++

Question

I am trying to convert the below Matlab code into C++ using codegen. However it fails at build and I get the error:
"??? Unless 'rows' is specified, the first input must be a vector. If the vector is variable-size, the either the first dimension or the second must have a fixed length of 1. The input [] is not supported. Use a 1-by-0 or 0-by-1 input (e.g., zeros(1,0) or zeros(0,1)) to represent the empty set."
It then points to [id,m,n] = unique(id); being the culprit. Why doesn't it build and what's the best way to fix it?
function [L,num,sz] = label(I,n) %#codegen
% Check input arguments
error(nargchk(1,2,nargin));
if nargin==1, n=8; end
assert(ndims(I)==2,'The input I must be a 2-D array')
sizI = size(I);
id = reshape(1:prod(sizI),sizI);
sz = ones(sizI);
% Indexes of the adjacent pixels
vec = #(x) x(:);
if n==4 % 4-connected neighborhood
idx1 = [vec(id(:,1:end-1)); vec(id(1:end-1,:))];
idx2 = [vec(id(:,2:end)); vec(id(2:end,:))];
elseif n==8 % 8-connected neighborhood
idx1 = [vec(id(:,1:end-1)); vec(id(1:end-1,:))];
idx2 = [vec(id(:,2:end)); vec(id(2:end,:))];
idx1 = [idx1; vec(id(1:end-1,1:end-1)); vec(id(2:end,1:end-1))];
idx2 = [idx2; vec(id(2:end,2:end)); vec(id(1:end-1,2:end))];
else
error('The second input argument must be either 4 or 8.')
end
% Create the groups and merge them (Union/Find Algorithm)
for k = 1:length(idx1)
root1 = idx1(k);
root2 = idx2(k);
while root1~=id(root1)
id(root1) = id(id(root1));
root1 = id(root1);
end
while root2~=id(root2)
id(root2) = id(id(root2));
root2 = id(root2);
end
if root1==root2, continue, end
% (The two pixels belong to the same group)
N1 = sz(root1); % size of the group belonging to root1
N2 = sz(root2); % size of the group belonging to root2
if I(root1)==I(root2) % then merge the two groups
if N1 < N2
id(root1) = root2;
sz(root2) = N1+N2;
else
id(root2) = root1;
sz(root1) = N1+N2;
end
end
end
while 1
id0 = id;
id = id(id);
if isequal(id0,id), break, end
end
sz = sz(id);
% Label matrix
isNaNI = isnan(I);
id(isNaNI) = NaN;
[id,m,n] = unique(id);
I = 1:length(id);
L = reshape(I(n),sizI);
L(isNaNI) = 0;
if nargout>1, num = nnz(~isnan(id)); end

Answer 1

Just an FYI, if you are using MATLAB R2013b or newer, you can replace error(nargchk(1,2,nargin)) with narginchk(1,2).
As the error message says, for codegen unique requires that the input be a vector unless 'rows' is passed.
If you look at the report (click the "Open report" link that is shown) and hover over id you will likely see that its size is neither 1-by-N nor N-by-1. The requirement for unique can be seen if you search for unique here:
http://www.mathworks.com/help/coder/ug/functions-supported-for-code-generation--alphabetical-list.html
You could do one of a few things:
Make id a vector and treat it as a vector for the computation. Instead of the declaration:
id = reshape(1:prod(sizI),sizI);
you could use:
id = 1:numel(I)
Then id would be a row vector.
You could also keep the code as is and do something like:
[idtemp,m,n] = unique(id(:));
id = reshape(idtemp,size(id));
Obviously, this will cause a copy, idtemp, to be made but it may involve fewer changes to your code.

Answer 2

Remove the anonymous function stored in the variable vec and make vec a subfunction:
function y = vec(x)
coder.inline('always');
y = x(:);
Without the 'rows' option, the input to the unique function is always interpreted as a vector, and the output is always a vector, anyway. So, for example, something like id = unique(id) would have the effect of id = id(:) if all the elements of the matrix id were unique. There is no harm in making the input a vector going in. So change the line
[id,m,n] = unique(id);
to
[id,m,n] = unique(id(:));