read edges from stdin instead of argv

README.md

@@ -15,7 +15,7 @@ vertices. Three lines now contain a sorted() statement.
 Usage
 -----
 
-    python cycles.py 4 0,1 0,2 1,0 1,3 2,0 3,0 3,1 3,2
+    echo "0 1\n0 2\n1 0\n1 3\n2 0\n3 0\n3 1\n3 2" | python cycles.py 4
 
 First argument is the number of vertices. Subsequent arguments are ordered
 pairs of comma separated vertices that make up the directed edges of the
@@ -25,15 +25,16 @@ DOT file input
 --------------
 
 For simplicity, there is no DOT file parser included but the following allows
-to create a suitable argument string for simple DOT graphs.
+to create a suitable argument string and standard input for simple DOT graphs.
 
 Given a DOT file of a simple (no labels, colors, styles, only pairs of
-vertices...) directed graph, the following line produces commandline
+vertices...) directed graph, the following lines generate the number of
-arguments in the above format for that graph.
+vertices as well as the edge list expected on standard input.
 
-	echo `sed -n -e '/^\s*[0-9]\+;$/p' graph.dot | wc -l` `sed -n -e 's/^\s*\([0-9]\) -> \([0-9]\);$/\1,\2/p' graph.dot`
+        sed -n -e '/^\s*[0-9]\+;$/p' graph.dot | wc -l
+        sed -n -e 's/^\s*\([0-9]\) -> \([0-9]\);$/\1 \2/p' graph.dot
 
-The above line works on DOT files like the following:
+The above lines works on DOT files like the following:
 
     digraph G {
       0;
@@ -46,6 +47,11 @@ The above line works on DOT files like the following:
       2 -> 1;
       }
 
-It would produce the following output:
+They would produce the following output:
 
-    3 0,1 0,2 1,0 2,0 2,1
+    3
+    0 1
+    0 2
+    1 0
+    2 0
+    2 1

cycles.py

@@ -12,8 +12,8 @@ import sys
 import networkx as nx
 from collections import defaultdict
 
-if len(sys.argv) < 3:
+if len(sys.argv) != 2:
-    print "usage: %s num_vertices [v1,v2...]"%(sys.argv[0])
+    print "usage: echo \"v1 v2\nv1 v3\n...\" | %s num_vertices"%(sys.argv[0])
 
 def simple_cycles(G):
     def _unblock(thisnode):
@@ -73,9 +73,9 @@ def simple_cycles(G):
 
 G = nx.DiGraph()
 
-for edge in sys.argv[2:]:
+for edge in sys.stdin.readlines():
-    v1,v2 = edge.split(',', 1)
+    v1,v2 = edge.split(' ', 1)
-    G.add_edge(v1,v2)
+    G.add_edge(v1.strip(),v2.strip())
 
 for c in simple_cycles(G):
     print " ".join(c[:-1])