A connection or a string giving the file name to write the graph
Character string giving the file format. Right now
pajek, graphml, dot, gml, edgelist,
lgl, ncol and dimacs are implemented. As of igraph 0.4
this argument is case insensitive.
Other, format specific arguments, see below.
Edge list format
The edgelist format is a simple text file,
with one edge in a line, the two vertex ids separated by a space character.
The file is sorted by the first and the second column. This format has no