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

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### Answers (2)

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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