# How to construct an undirected graph by reading edge by edge from a text file

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TR RAO on 6 Sep 2017
Answered: TR RAO on 28 Sep 2017
Suppose I have a list of edges in a text file(dont know how many edges are there in a text file). I have to create an undirected graph from it. Finally I have to construct first order transition probability matrix from that graph. for example for the edges={a,b}, {a,c}, {a,d}, the transition matrix is [0 1/3 1/3 1/3;1 0 0 0; 1 0 0 0;1 0 0 0]
José-Luis on 6 Sep 2017
Are you talking about building tree structures in Matlab?

Josh Meyer on 6 Sep 2017
You can use textscan to read the text file into a cell array, with the first entry listing the source nodes and the second entry listing the target nodes. Then you can create the graph directly from the cell array columns.
fid = fopen('textfile.txt');
C = textscan(fid,'%s%s')
fclose(fid);
G = graph(C{1},C{2})
If the file lists duplicate edges (for an undirected graph a->b and b->a is a duplicate edge, so the example file you provided contains duplicates) you'll need to trim those away first. One method is to convert to a char array, sort the edges, find the unique rows, then convert back to a cell array:
C = cell2mat([C{1} C{2}]);
C = unique(sort(C,2),'rows');
C = num2cell(C)
G = graph(C(:,1),C(:,2))
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TR RAO on 18 Sep 2017
Thank You

TR RAO on 28 Sep 2017
clc; clear; fileID = fopen('C:\Users\TR RAO\Desktop\rao1.txt','r'); C = textscan(fileID, '%s %s'); fclose(fileID); C1 =cell2mat(C{1,1}); C2 =cell2mat(C{1,2}); CC = [C1,C2] s=C1;t=C2; nodes = unique(CC); n_nodes = size(nodes,1) S0=eye(n_nodes); adjacency_matrix = false(n_nodes); for idx = 1:size(C1,1) adjacency_matrix(C1(idx)==nodes,C2(idx)==nodes) = 1; end adjacency_matrix n_elements = sum(adjacency_matrix,2); transition_matrix = adjacency_matrix./repmat(n_elements, 1, n_nodes); transition_matrix(isnan(transition_matrix)) = 0; P=transition_matrix;
I am reading a list of edges from a textfile. For small file with six edges it is forming a transition matrix. but for large file of 50000 edges it is giving the following error. Error using cat Dimensions of matrices being concatenated are not consistent.
Error in cell2mat (line 83) m{n} = cat(1,c{:,n});
Error in readingfile1 (line 14) C1 =cell2mat(C{1,1});