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Recipe 577241 revision 3 

_FAC_TABLE = [1, 1]
def factorial(n):
    if n < len(_FAC_TABLE):
        return _FAC_TABLE[n]

    last = len(_FAC_TABLE) - 1
    total = _FAC_TABLE[last]
    for i in range(last + 1, n + 1):
        total *= i
        _FAC_TABLE.append(total)

    return total


def main():
    from timeit import Timer
    print("factorial:      %.5f s" %
            Timer("factorial(500)", "from __main__ import factorial")
            .timeit(1000))

    import math
    if hasattr(math, "factorial"):
        print("math.factorial: %.5f s" %
                Timer("factorial(500)", "from math import factorial")
                .timeit(1000))


if __name__ == "__main__":
    main()

Diff to Previous Revision

--- revision 2 2010-05-23 08:22:50
+++ revision 3 2010-05-23 16:06:33
@@ -5,10 +5,9 @@
 
     last = len(_FAC_TABLE) - 1
     total = _FAC_TABLE[last]
-    _FAC_TABLE.extend([0] * (n - last))
     for i in range(last + 1, n + 1):
         total *= i
-        _FAC_TABLE[i] = total
+        _FAC_TABLE.append(total)
 
     return total

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