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fixed error in factorial.py #1888

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Merged
merged 5 commits into from
Aug 5, 2020
Merged

fixed error in factorial.py #1888

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49654da
Update factorial.py
SandersLin Apr 17, 2020
545fc89
Merge branch 'master' into patch-33
l3str4nge Jun 23, 2020
e95fb72
updating DIRECTORY.md
Jun 23, 2020
b7ecbf6
Update dynamic_programming/factorial.py
cclauss Aug 5, 2020
6a038a1
Update factorial.py
cclauss Aug 5, 2020
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24 changes: 11 additions & 13 deletions dynamic_programming/factorial.py
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Original file line number Diff line number Diff line change
@@ -1,6 +1,4 @@
# Factorial of a number using memoization
result = [-1] * 10
result[0] = result[1] = 1


def factorial(num):
Expand All @@ -12,26 +10,26 @@ def factorial(num):
>>> [factorial(i) for i in range(5)]
[1, 1, 2, 6, 24]
"""
result = [-1] * (num + 1)

if num < 0:
return "Number should not be negative."

return factorial_aux(num, result)


def factorial_aux(num, result):
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Type hints? Doctests?


if num == 0 or num == 1:
return 1

if result[num] != -1:
return result[num]
else:
result[num] = num * factorial(num - 1)
# uncomment the following to see how recalculations are avoided
# print(result)
result[num] = num * factorial_aux(num - 1, result)
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Technically, this is more of a "memoization" approach. While being perfectly correct, the more appropriate algorithm for this section may look as follows

Suggested change
result[num] = num * factorial_aux(num - 1, result)
result[0] = 1
for i in xrange(num):
result.append((i + 1) * result[i])
return result[num]

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xrange() was removed from Python on 1/1/2020.

The canonical way to do memoization in Python is with the lru_cache function decorator.

return result[num]


# factorial of num
# uncomment the following to see how recalculations are avoided
##result=[-1]*10
##result[0]=result[1]=1
##print(factorial(5))
# print(factorial(3))
# print(factorial(7))

if __name__ == "__main__":
import doctest

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