Manipulate a number algorithm

Question

How can I fix this algo?

def change_num(n, direc, numMove):
    if direc == 'back':
        nums = range(10)
        return nums[n-numMove]
    elif direc == 'forward':
        nums = range(10)
        nums.reverse()
        return nums[n-numMove]-1

This doesn't work for direc == 'back' here are some sample expected returns:

0 1 2 3 4 5 6 7 8 9

change_num(1,'back', 5)  -> 6
change_num(5,'back', 1)  -> 4
change_num(7,'forward',5) -> 2
change_num(0,'forward',5) -> 5

Do you understand how the algo works? I'm just not implementing it right.

Why change_num(7,'forward',5) -> 4?

Sven Marnach
– Sven Marnach

2011-05-19 20:28:37 +00:00
Commented May 19, 2011 at 20:28 — Sven Marnach
– Sven Marnach, Commented May 19, 2011 at 20:28
ya it was typo, should be '2', fixed it

MistahX
– MistahX

2011-05-19 20:29:08 +00:00
Commented May 19, 2011 at 20:29 — MistahX
– MistahX, Commented May 19, 2011 at 20:29

Sven Marnach · Accepted Answer · 2011-05-19 22:21:56Z

4

Instead of 'back' and 'forward', it will be easier to pass +1 for forward and -1 for back. Then your function simplifies to

def change_num(n, direc, num_move):
    return (n + direc * num_move) % 10

edited May 19, 2011 at 22:21

answered May 19, 2011 at 20:30

Sven Marnach

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Comments

jon_darkstar · Accepted Answer · 2011-05-19 20:29:07Z

2

def change_num(n, direc, numMove):
   d = {'back' : -1, 'forward' : 1}
   return (n + d[direc]*numMove) % 10

Have you given thought to what this function does? this num list is pointless bc num[x] === x. You don't need if statements for 'back' and 'forward', they really just correspond to -1 and 1 respectively.

answered May 19, 2011 at 20:29

jon_darkstar

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2 Comments

MistahX Over a year ago

I take it nums is defined as nums = range(10)

jon_darkstar Over a year ago

there is no nums. i mistyped before

johnsyweb · Accepted Answer · 2011-05-22 04:37:27Z

Your function is over-complicated and won't work in Python 3.

% cat ./test.py
#!/usr/bin/env python

import unittest

def change_num(n, direc, numMove):
    directions = {
            'back': -1,
            'forward': +1,
            }
    steps = n + directions[direc] * numMove
    wrapped = steps % 10
    return wrapped

class TestChangeNum(unittest.TestCase):

    def test_back_no_wrap(self):
        self.assertEquals(6, change_num(1, 'back', 5))

    def test_back_with_wrap(self):
        self.assertEquals(4, change_num(5, 'back', 1))

    def test_forward_with_wrap(self):
        self.assertEquals(2, change_num(7, 'forward', 5))

    def test_forward_no_wrap(self):
        self.assertEquals(5, change_num(0, 'forward', 5))

if __name__ == '__main__':
    unittest.main()

% python test.py 
....
----------------------------------------------------------------------
Ran 4 tests in 0.000s

OK

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