Divisible by 2: Python

Question

I am trying to write a function called splitList(myList, option) that takes a list and an option which is either 0 or 1 as parameters. If the value of the option is 0 the function returns a list consisting of the elements in myList that are negative and if the value of the option is 1 the function returns a list consisting of the elements in myList that are even (we consider 0 to be an even number, since it is evenly divisible by 2).

For example:

splitList([1,-3,5,7,-9,-11,0,2,-4], 0)

Would return the list:

[-3,-9,-11,-4]

Where as:

splitList([1,-3,5,7,-9,-11,0,2,-4], 1)

Would return the list:

[0,2,-4]

For this problem I must use a for loop.

Here is what I have:

def splitList(myList, option):
    negativeValues = []
    positiveValues = []
    evenValues = [] 
    for i in range(0,len(myList)):       
        if myList[i] < 0: 
            negativeValues.append(myList [i]) 
        else: 
            positiveValues.append(myList [i]) 

    for element in myList: 
        if option == 1: 
            myList [i] % 2 == 0 
            evenValues.append(myList [i]) 
            return evenValues 
        else: 
            return negativeValues

The only thing I cannot get it to do is to is sort the list and return all the numbers that are divisible by 2.

@Tom: How would the `slice` function help? I mean, if you can say more. — hughdbrown, Mar 08 '13 at 18:41

score 4 · Answer 1 · answered Mar 08 '13 at 18:12

4

Using a loop is a bit redundant here since there's a standard function filter that does what you want: returns a new list with these elements of a list which match a given predicate.

Let's define the predicates first:

def is_even(x):
    return x % 2 == 0

def is_negative(x):
    return x < 0

Then you can easily define your function in terms of filter:

def splitList(myList, option):
    predicate = is_negative if option == 0 else is_even
    return filter(predicate, myList)

answered Mar 08 '13 at 18:12

Kos

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As an exercise, I suggest to write the `filter` function yourself. – Kos Mar 08 '13 at 18:14
2

you could also use `lambdas` to make it even shorter. I personally prefer an if wich has two return statements over an additional variable but that is a matter of taste. an requirement was that a `for` is used, otherwise I see the beauty in your code... – ted Mar 08 '13 at 18:16
2

+1 I like your answer, but what do you make of this: "For this problem, you must use a “for loop”." Maybe: `return [x for x in myList if predicate(x)]` – hughdbrown Mar 08 '13 at 18:24
You can implement `filter` yourself using a for loop, like I said in my first comment. – Kos Mar 08 '13 at 18:33
@hughdbrown using `map` and `filter` is very much like using list comprehensions; I use `map`/`filter` when I have the function ready and a comprehension when I'd have to write a lambda. – Kos Mar 08 '13 at 18:34
I'm familiar with `map`, `filter`, list comprehensions, and more. I was just saying that OP's question (from his prof? for homework?) has a requirement for a `for-loop`. Or, I agree: we wouldn't write it with a `for-loop` because we know better, but OP Is asking for it that way for some reason. – hughdbrown Mar 08 '13 at 18:44

hughdbrown · Answer 2 · 2013-03-08T18:46:16.180

2

You can build all your variants from these primitives:

def even_list(numbers):
    return [x for x in numbers if not (x & 1)]

def odd_list(numbers):
    return [x for x in numbers if x & 1]

def negative_list(numbers):
    return [x for x in numbers if x < 0]

def positive_list(numbers):
    return [x for x in numbers if x > 0]

Then test:

>>> def test():
...     numbers = list(range(-3, 4))
...     print even_list(numbers)
...     print odd_list(numbers)
...     print positive_list(numbers)
...     print negative_list(numbers)
... 
>>> test()
[-2, 0, 2]
[-3, -1, 1, 3]
[1, 2, 3]
[-3, -2, -1]

Later: so stealing from @Kos, you could write split_list like this:

def split_list(myList, option):
    predicate = negative_list if not option else even_list
    return predicate(myList)

Or:

def split_list(myList, option):
    predicates = [negative_list, even_list]
    return predicates[option](myList)

Not sure if it meets your needs if the for-loop is in a list comprehension in a called function.

Also: "Function names should be lowercase, with words separated by underscores as necessary to improve readability."

edited Mar 08 '13 at 18:46

answered Mar 08 '13 at 18:20

hughdbrown

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+1 for list comprehension – ted Mar 08 '13 at 18:21
2

Didn't see this: 'For this problem, you must use a “for loop”.' Bah, humbug. – hughdbrown Mar 08 '13 at 18:23
1

well true list comprehension is technically not a for loop one could argue, but I think it is good to see the option and I would count it as close enough, you basically have changed the order. – ted Mar 08 '13 at 18:25
1

@Ted: "you basically have changed the order" Changed the order -- of what? The numbers are in the same order. – hughdbrown Mar 08 '13 at 18:27
1

The syntactical order, instead of `loop, check condition, add to list` it reads `make a list from all elements looped over that match condition` – ted Mar 08 '13 at 18:43

score 1 · Answer 3 · answered Mar 08 '13 at 18:18

You return too soon. You first have to complete the foor loop and return after it, not from inside the loop.

Example

for i in range(5):
    print i
    numbers.append(i)
    return numbers   //wrong: exit the function on the first pass in the loop.

for i in range(5):
    print i
    numbers.append(i)

return numbers       //right

Besides that why do you calculate the negative valuse list if you don't need it?

jurgenreza · Answer 4 · 2013-03-08T18:35:11.353

1

def splitList(myList,option):

    negative_numbers = [i for i in myList if i < 0]
    even_numbers = [i for i in myList if i % 2 == 0]

    return sorted(even_numbers) if option else sorted(negative_numbers)

edited Mar 08 '13 at 18:35

answered Mar 08 '13 at 18:22

jurgenreza

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`odds = [i for i in myList if i % 2 == 0]` -- the code does not match the variable. `i % 2 == 0` will not find the odd numbers. – hughdbrown Mar 08 '13 at 18:30
@hughdbrown you are right, wrong naming. – jurgenreza Mar 08 '13 at 18:33

score 0 · Accepted Answer · answered Mar 08 '13 at 18:18

0

I believe this is what you where trying to achieve:

def splitList(myList,option):

    result = []

    if option == 0:
        for item in myList:
            if (item < 0):
                result.append(item)
    elif option == 1:
        for item in myList:
            if (item % 2 == 0):
                result.append(item)
    else:
        return "Option Error"

    return sorted(result)

print splitList([1,-3,5,7,-9,-11,0,2,-4], 0)
print splitList([1,-3,5,7,-9,-11,0,2,-4], 1)
print splitList([1,-3,5,7,-9,-11,0,2,-4], 2)

Outputs:

[-11, -9, -4, -3]
[-4, 0, 2]
Option Error

answered Mar 08 '13 at 18:18

Chris Seymour

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This worked for me. How would you convert this and use a "while loop"? – user2149455 Mar 08 '13 at 18:31
A `while loop` wouldn't be suitable here. – Chris Seymour Mar 08 '13 at 18:37
How would a write a while loop with the circumstances that are given? – user2149455 Mar 08 '13 at 18:52
`Goto previous comment` Read and **understand** [this](http://stackoverflow.com/questions/920645/when-to-use-while-or-the-for-in-python/920692#920692) – Chris Seymour Mar 08 '13 at 18:56

Divisible by 2: Python

5 Answers5