There are almost as many graph file formats as there are graph processing platforms. Here, I attempt to provide a comprehensive listing. Some of these are nearly trivial, but yet, the standards are just not established. Whether the vertices are 0-indexed or 1-indexed is undefined. However, this does matter for some systems such as GraphMat. All the examples show the same directed, unweighted graph with the exception of .metis.
This is the "edge list" format. The format is one edge per line of the form . For example,
1 2
2 1
1 3