Fixed side-effect bug in DPLL; more cleanup.

darius · darius · commit 0fbaeab54014 · 2011-10-05T06:45:56.000Z
diff --git a/logic.py b/logic.py
@@ -47,7 +47,7 @@ def __init__(self, sentence=None):
         abstract
 
     def tell(self, sentence): 
-        "Add the sentence to the KB"
+        "Add the sentence to the KB."
         abstract
 
     def ask(self, query):
@@ -671,19 +671,19 @@ def dpll(clauses, symbols, model):
     P, value = find_unit_clause(clauses, model)
     if P:
         return dpll(clauses, removeall(P, symbols), extend(model, P, value))
-    P = symbols.pop()           # XXX is this side-effect more global than desired?
+    P, symbols = symbols[0], symbols[1:]
     return (dpll(clauses, symbols, extend(model, P, True)) or
             dpll(clauses, symbols, extend(model, P, False)))
  
-def find_pure_symbol(symbols, unknown_clauses):
+def find_pure_symbol(symbols, clauses):
     """Find a symbol and its value if it appears only as a positive literal
     (or only as a negative) in clauses.
     >>> find_pure_symbol([A, B, C], [A|~B,~B|~C,C|A])
     (A, True)
     """
     for s in symbols:
         found_pos, found_neg = False, False
-        for c in unknown_clauses:
+        for c in clauses:
             if not found_pos and s in disjuncts(c): found_pos = True
             if not found_neg and ~s in disjuncts(c): found_neg = True
         if found_pos != found_neg: return s, found_pos
