Subsetting python list into positive/negative movements/trends

Question

Sorry for creating this question but I have been stuck on this question for a while.

Basically I'm trying to take a list:

numbers=[1, 2, -1, -2, 4, 5]

And subset this list into a list of list that display positive/negative movements (or trends)

The end result is to have:

subset_list = [[1, 2], [-1, -2], [4, 5]]

Basically I have been using nested while functions to append a positive movement to the subset, and when the condition is not met, the subset is appended to subset_list and then evaluates if there is a negative movement.

I keep getting an

IndexError

, and so far

subset_list

only contains

[[1, 2]]

Here is my code:

numbers = [1,2,-1,-2,4,5]



subset = []

subset_list = []

subset.append(numbers[0])



i = 1

while i < (len(numbers)):

    if numbers[i] <= numbers[i+1]:

        subset.append(numbers[i])

        i+= 1

        while subset[-1] <= numbers[i] or numbers[i] <= numbers[i+1]:

            subset.append(numbers[i])

            i += 1

        subset_list.append(subset)

        subset = []

        i += 1

    if numbers[i] > numbers[i+1]:

        subset.append(numbers[i])

        i+= 1

        while subset[-1] <= numbers[i] or numbers[i] <= numbers[i+1]:

            subset.append(numbers[i])

            i+= 1

        subset_list.append(subset)

        subset = []

        i += 1

Thanks!

-Jake

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Answer 1

Here is a way to re-write this:


numbers=[1,2,-1,-2,4,5] 


direction = True  # positive or negative
prevdirection = True
res = [[numbers[0]]]

for previtem, item in zip(numbers[:-1], numbers[1:]):
    direction = True if item - previtem > 0 else False
    if direction != prevdirection:
        res.append([])
    prevdirection = direction
    res[-1].append(item)

print(res)

Subsetting python list into positive/negative movements/trends

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