Lists

A list is an ordered collection of items. You can add, remove, sort, and iterate over items — they're one of the most useful data structures in Python.

What you'll learn

• Creating lists and accessing items by index
• Slicing to get sublists
• Adding and removing items
• Iterating with for loops
• List comprehensions for concise transformations

Instructions

You'll write two small functions that the grader will test with several inputs.

1. Write clean(numbers) — returns a NEW list with 0 appended to the end and any negative numbers removed. Don't modify the input list.
2. Write average(numbers) — returns the mean of the list (sum divided by length).
3. The starter has a driver block that reads a comma-separated list from stdin and calls your functions. Leave it as-is — it only runs when you click Run.

Example for input 3, -1, 7, 2, -4, 10:

Original: [3, -1, 7, 2, -4, 10]
Cleaned: [3, 7, 2, 10, 0]
Sum: 22
Average: 4.4

Key concepts

Creating and accessing:

nums = [10, 20, 30]
first = nums[0]      # 10
last = nums[-1]      # 30
sub = nums[1:3]      # [20, 30]

Modifying:

nums.append(40)       # add to end
nums.remove(20)       # remove first occurrence
nums.pop()            # remove and return last item

Iterating:

for n in nums:
    print(n)

List comprehension — a compact way to build a new list:

positives = [n for n in nums if n > 0]

len(nums) gives you the number of items. sum(nums) adds them all up.

Hints

• To split a comma-separated string: "3, -1, 7".split(",") gives ["3", " -1", " 7"].
• You'll need int(x.strip()) to convert each piece to a number.
• Don't modify a list while iterating over it — build a new one instead (list comprehension is perfect for this).