MATLAB: Problem in Finding COnnected Components: Error Matrix need to be sparse. I have 2500 cells with 150 1’s. Still showing matrix is not sparse. Please help me to generate connected compnents of the disconnected graph

Question

I am writing below the error message reflected:

??? Error using ==> check_matlab_bgl at 50
the matrix A must be sparse.  (See set_matlab_bgl_default.)
Error in ==> components at 39
if check, check_matlab_bgl(A,struct()); end
Error in ==> StoreADJMatrix at 83
comps = components(adjmat)
%%%%%%%%%%%%%%Part of the code 
% I have also attached matfile having adjmat matrix for no of nodes 50 in 200X 200 area
R=29;
row=1;
E=zeros(0,0);
format short g
for i=1:noOfNodes
for j=1:noOfNodes
distance(i,j) = sqrt((Xd(i)-Xd(j))^2 + (Yd(i)-Yd(j))^2);
if distance (i,j) <= R && (i~=j)
% row=row+1;
    matrix(i, j) = 1; % there is a link;
E(row,1)=i;
E(row,2)=j;
E(row,3)=distance(i,j);
row=row+1;
else
matrix(i,j) = 0;
end;
end;
end
save('NodeGraph.mat','matrix','distance','E','-append')
%for each pair (i,j) in the connectivity list E, set adj(i,j) to 1
fromlist = E(:,1);
tolist = E(:,2);
adjmat=zeros(noOfNodes,noOfNodes);
fromtoidx = sub2ind([noOfNodes,noOfNodes], fromlist, tolist);  %as linear indices
adjmat(fromtoidx) = 1;
tofromidx = sub2ind([noOfNodes,noOfNodes], tolist, fromlist);  %connections are bidirectional
adjmat(tofromidx) = 1;
save('NodeGraph.mat','adjmat','-append')
%nMST=grMinSpanTree(E)
gplot(adjmat,coords(:,:),'-*')
grid on
axis square
comps = components(adjmat)
%%%%%%%%%%%%%%

Answer 1

Replace

adjmat=zeros(noOfNodes,noOfNodes);

with

adjmat = sparse([], [], [], noOfNodes, noOfNodes);

However, you would be even better off using sparse() calls to do the assignments with no linear indexing and no explicit assignments:

adjmat = sparse([fromlist; tolist], [tolist; fromlist], 1);