read edges from stdin instead of argv

README.md

@@ -11,25 +11,26 @@ Algorithm by R. Tarjan
 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
 
-First argument is the number of vertices. Subsequent arguments are ordered
+First argument is the number of vertices. Ordered pairs of space separated
-pairs of comma separated vertices that make up the directed edges of the
+vertices are given via standard input and make up the directed edges of the
 graph.
 
 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;
@@ -42,6 +43,11 @@ 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

@@ -61,13 +61,13 @@ procedure circuit_enumeration;
 
 import sys
 
-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])
 
 A = [[] for a in range(int(sys.argv[1]))]
 
-for edge in sys.argv[2:]:
+for edge in sys.stdin.readlines():
-    v1,v2 = edge.split(',', 1)
+    v1,v2 = edge.split(' ', 1)
     A[int(v1)].append(int(v2));
 
 def print_point_stack():