factorial recursion and iteration in python

Question

please help me understand where i went wrong:

this is the question:Create both a recursive function called recursive_factorial and iterative function called iterative_factorial that does the following

Accepts as parameter an Integer n
Computes the factorial of n
Returns the factorial of n

this is the test i am using for the question:

import unittest

class RecursiveTestCase(unittest.TestCase):

  def test_recursive_factorial_one(self):
    result = recursive_factorial(4)
    self.assertEqual(result, 24, msg="Inaccurate value")

  def test_recursive_factorial_two(self):
    result = recursive_factorial(0)
    self.assertEqual(result, 1, msg="Inaccurate value")

  def test_iterative_factorial_one(self):
    result = iterative_factorial(5)
    self.assertEqual(result, 120, msg="Inaccurate value")

  def test_iterative_factorial_two(self):
    result = iterative_factorial(0)
    self.assertEqual(result, 1, msg="Inaccurate value")

this is the code i have written:

def recursive_factorial(n):
    if n == 0:
        return 1
    else:
        return n * recursive_factorial(n-1)
def iterative_factorial(n):
    x = 1
    li = list(range(1, n + 1))
    for each in li:
        x = x * each

this is the error i am getting:

1 . test_iterative_factorial_one

Failure in line 21, in test_iterative_factorial_one self.assertEqual(result, 120, msg="Inaccurate value") AssertionError: Inaccurate value

2 . test_iterative_factorial_two

Failure in line 25, in test_iterative_factorial_two self.assertEqual(result, 1, msg="Inaccurate value") AssertionError: Inaccurate value

please help me understand where i went wrong.

Can you add some debug to print the result of recursive_factorial(0). The code looks correct. — Martin Bonner supports Monica
– Martin Bonner supports Monica, Commented Jan 8, 2016 at 11:53

NPE · Accepted Answer · 2016-01-08 14:22:02Z

5

You're forgetting to return x from iterative_factorial(), so the function is implicitly returning None.

As an aside, you can iterate of the result of range() directly:

for each in range(1, n + 1):
   ...

Finally, this might be a good opportunity to learn about Python's reduce() function.

import operator

def another_iterative_factorial(n):
   return reduce(operator.mul, range(1, n + 1))

edited Jan 8, 2016 at 14:22

answered Jan 8, 2016 at 11:45

NPE

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1 Comment

Copperfield Over a year ago

and if you are using python3 reduce was moved to the functools module. Also, good chance to learn about lambda function as you can write reduce(lambda x,y:x*y,range(1,n+1))

Martin Bonner supports Monica · Accepted Answer · 2016-01-08 11:52:18Z

1

iterative_factorial needs to end with

    return x

Also, iterative_factorial doesn't need li. Better to just write:

    for each in range(1,n+1):

edited Jan 8, 2016 at 11:52

answered Jan 8, 2016 at 11:47

Martin Bonner supports Monica

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3 Comments

Psytho Over a year ago

test_recurisve_factorial_two tests 0!, not 2!

Martin Bonner supports Monica Over a year ago

D'oh! In that case I don't understand.

Martin Bonner supports Monica Over a year ago

@Davismuriungi - make sure to accept NPE's answer. It was written first; it didn't start out with errors; and it includes reduce.

albert · Accepted Answer · 2016-07-04 20:53:50Z

0

def recursive_factorial(n):
    if n == 0:
        return 1
    else:
        return n * recursive_factorial(n-1)


def iterative_factorial(n):        
    x = 1
    li = list(range(2, n+1))
    for each in li:
        x = x*each
        yield x

edited Jul 4, 2016 at 20:53

albert

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answered Jul 4, 2016 at 19:01

fmk

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2 Comments

Tonechas Over a year ago

You need to fix the indentation of the return x sentence in iterative_factorial.

Tonechas Over a year ago

You could simplify iterative_factorial by writing for each in range(2, n + 1):

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