How should one loop over two elements of a Python list at a time while building a new list?

Question

Let's say I have a list of numbers and that I want to "extend" that list to beyond a certain number of elements. How should I extend the list by adding elements that are respectively the mean of the preceding and following element?

numbers = [1, 2, 3, 4, 5]
minimumNumberOfElementsRequired = 15

# do magic here
# first iteration: [1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5]
# second iteration: [1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5, 3.75, 4, 4.25, 4.5, 4.75, 5]
# have sufficient number of elements => return list

print(numbers_extended)
# output: [1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5, 3.75, 4, 4.25, 4.5, 4.75, 5]

The beginning of my attempt is as follows:

list1 = [1, 2, 3, 4, 5]
list2 = [1, 2, 3, 4, 5]
index = -1
iterator = iter(list1)
for x, y in zip(iterator, iterator):
    index += 1
    list2.insert(index, (x + y) / float(2))

Have you tried anything? – Morgan Thrapp Sep 01 '15 at 14:00 — Morgan Thrapp, Sep 01 '15 at 14:00
@MorganThrapp Hi. I have added the beginning of my attempt. – d3pd Sep 01 '15 at 14:10 — d3pd, Sep 01 '15 at 14:10

jonrsharpe · Accepted Answer · 2015-09-01T15:02:38.490

1

I guess you could do something like:

while len(numbers) < minimumNumberOfElementsRequired:
    index = 1
    while index < len(numbers):
        numbers.insert(index, (numbers[index-1] + numbers[index]) / 2.0)
        index += 2

It seems a bit awkward, but it's the easiest way around the shifting indices caused by insertion.

edited Sep 01 '15 at 15:02

answered Sep 01 '15 at 14:11

jonrsharpe

115,751
26
228
437

score 1 · Answer 2 · answered Sep 01 '15 at 14:11

It's not the most elegant solution, but it works.

numbers = [1,2,3,4,5]
while len(numbers) < 15:
    new_numbers = []
    for i, value in enumerate(numbers[:-1]):
        mean = (value + numbers[i+1])/2.0
        new_numbers.extend([value, mean])
    numbers = new_numbers + numbers[-1:]

score 0 · Answer 3 · answered Sep 01 '15 at 14:12

You can iterate through the numbers using index, compute mean of following elements, and insert it to the list at the right position. At the same time, you need to notice than index is actually i*2 because element are added to the list while ierating.

while len(numbers) < minimum_length:
    for i in range(len(numbers) - 1):
        j = i*2
        a, b = numbers[j], numbers[j + 1]
        mean = (a + b)/2
        numbers.insert(j + 1, mean)

score 0 · Answer 4 · answered Sep 01 '15 at 14:31

With reduce (as suggested by Yaroslav Admin).

addnew function accepts a list and an new item to append.

from functools import reduce    # if python3

numbers = [1, 2, 3, 4, 5]
minlen = 15

def addnew(lst, x):
    if lst:
        lst.append((lst[-1] + x)/2)
    lst.append(x)

    return lst

while len(numbers) <= minlen:
    numbers = reduce(addnew, numbers, [])

print(list(numbers))

score 0 · Answer 5 · answered Sep 01 '15 at 14:45

Here's an iterative way to do it:

try:
    from itertools import izip
except ImportError:  # Python 3
    izip = zip

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = iter(iterable), iter(iterable)
    next(b, None)
    return izip(a, b)

def add_means(num_list):
    r = [num_list[0]]
    for pair in pairwise(num_list):
        r.extend([sum(pair)/2.0, pair[1]])
    return r

def extend_list(numbers, min_elements):
    while len(numbers) < min_elements:
        numbers = add_means(numbers)

    return numbers

numbers = [1, 2, 3, 4, 5]
minimumNumberOfElementsRequired = 15

print(extend_list(numbers, minimumNumberOfElementsRequired))

Output:

[1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5, 3.75, 4, 4.25, 4.5, 4.75, 5]

How should one loop over two elements of a Python list at a time while building a new list?

5 Answers5