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python takes list and returns only if negative value also exists using set

I’m having problems with another homework problem.

By using a set, write a method negated(a) that takes a list, a, as an
argument and returns a list containing only the elements x such that
-x is also in a

His example shows that the input is

[-6, 8, 7, 3, 2, -9, 1, -3, 2, -4, 4, -8, 7, 8, 2, -2, -7, 0, 1, -9,
-3, -7, -3, -5, 6, -3, 6, -3, -10, -8]

and the output is

[-6, 8, 7, 3, 2, -3, 2, -4, 4, -8, 7, 8, 2, -2, -7, 0, -3, -7, -3, 6,
-3, 6, -3, -8]

I was able to figure out how to do it without using a set with

return [x for x in a if -x in a]

I’m just having problems implementing a set into the problem.
Can someone give me the steps to take, how I should tackle the problem… I’m not looking for the complete work, but it would be nice to see how you do it also.

Answer 1

Here’s the algorithm that you need to implement:

1. iterate through all elements in the list
2. when you consider each individual element, check if its negation exists in the list
3. if the negation exists in the list, keep it
4. if the negation does not exist in the list, don’t keep it.

It’s usually a bad idea to delete elements from a list as you’re iterating over it, so it’s best to make a new empty list and keep adding (or not) to it.

def filterNegs(L):
    answer = []
    for i in L:
        if -1*i in L:
            answer.append(i)
    return answer

Here’s a list comprehension for the same:

return [i for i in a if -1*i in a]

There’s a performance issue when you do this though. Checking if an element is in a list is an O(n) operation and O(1) in a set. So, it’s better to turn L into a set first:

def filterNegs(L):
    L = set(L)
    answer = []
    for i in L:
        if -1*i in L:
            answer.append(i)
    return answer

Here’s a list comprehension for the same:

L = set(L)
return [i for i in a if -1*i in a]

Hope this helps