CODEWITHSUNDEEP
pythonpython-3.xrecursion
cumin
cumin

Reputation: 491

next steps to make this recursive and more pythonic as well

My ultimate goal is to sum a list of values by summing each pair in the list, producing a smaller list, summing each of those pairs, and so on. For now, I have a list with an even number of elements, but eventually I want to be able to handle an arbitrary sized list.

With some help from previous posts, I have this function:

def collapse_sum(lst):
    sums = list()
    rems = list()
    acc = 0
    n = 2
    print(lst)
    print("final sum: {}\n".format(sum(lst)))
    pairs = [lst[i:i + n] for i in range(0, len(lst), n)]
    while len(pairs) > 0:
        if len(pairs) % 2 == 1:
            rems.append(pairs[-1])
            del pairs[-1]
        for a,b in pairs:
            sums.append(a + b)
        pairs = [sums[i:i + 2] for i in range(0,len(sums),n)]
        sums = list()
        if len(pairs) == 1:
            for a,b in pairs:
                acc += a + b
            rems = [item for sublist in rems for item in sublist]
            pairs = [rems[i:i + n] for i in range(0, len(rems), n)]
            if len(pairs) == 1:
                for a,b in pairs:
                    acc += a + b
                del pairs[-1]
                return acc

            rems = list()
    print(acc)
    return acc

Upvotes: 0

Views: 35

Answers (1)

Brad Solomon
Brad Solomon

Reputation: 40888

My ultimate goal is to sum a list of values by summing each pair in the list, producing a smaller list, summing each of those pairs, and so on.

Here's a solution that does that for even or odd-sized lists, without using any library dependencies.

# Helper function to split iterable obj into blocks/chunks
def blocks(obj, n):
    for i in range(0, len(obj), n):
        yield obj[i:i + n]

def collapse_sum(obj):
    while len(obj) > 1:
        obj = blocks(obj, 2)      
        obj = [sum(i) for i in obj]
    return obj[0]

Some examples:

a = [1, 2, 3, 4, 5]    
b = [1, 2, 3, 4]

collapse_sum(a)
15

collapse_sum(b)
10

You can visualize this here.

Upvotes: 1

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