inc2adj.m

%##################################################################
% Convert an incidence matrix representation to an adjacency matrix representation for an arbitrary graph.
%
% INPUTs: incidence matrix, nxm (num nodes x num edges)
% OUTPUTs: adjacency matrix, nxn
%
% GB: last updated, Sep 25, 2012
%##################################################################


function adj = inc2adj(inc)

m = size(inc,2);               % number of edges
adj = zeros(size(inc,1));      % initialize adjacency matrix

if isempty(find(inc==-1))      % undirected graph
  
  for e=1:m
    
    ind=find(inc(:,e)==1);
    
    if length(ind)==2
      adj(ind(1),ind(2))=1;
      adj(ind(2),ind(1))=1;
    elseif length(ind)==1      % selfloop
      adj(ind,ind)=1;
    else
      'invalid incidence matrix'
      return
    end
    
  end
    
else                           % directed graph (there are "-1"
                               % entries in the incidence matrix)

  for e=1:m

    ind1=find(inc(:,e)==1);
    indm1=find(inc(:,e)==-1);
    
    if isempty(indm1) & length(ind1)==1  % selfloop
      adj(ind1,ind1)=1;
    elseif length(indm1)==1 & length(ind1)==1
      adj(indm1,ind1)=1;
    else
      'invalid incidence matrix'
      return
    end
    
  end

end