Lab: Lists

Lab: Lists#

Note

Source: Adapted from the C# edition (arrays/lab-array1d.rst). Exercises translated to Python idioms; C# array syntax replaced by Python list syntax.

Goals for this lab:

  • Practice creating and modifying lists.

  • Use loops and list methods to process sequences.

  • Write functions that take and return lists.

1. Filter Positives#

Write a function filter_positives(nums) that returns a new list containing only the positive numbers from nums.

>>> filter_positives([3, -1, 4, -1, 5, -9, 2])
[3, 4, 5, 2]

Write it first with a for loop and append, then as a list comprehension.

2. Running Total#

Write a function running_total(nums) that returns a new list where each element is the cumulative sum of the original list up to that index.

>>> running_total([1, 2, 3, 4])
[1, 3, 6, 10]

3. Manual Min and Max#

Without using the built-in min() or max() functions, write:

  • list_min(nums) — returns the smallest element

  • list_max(nums) — returns the largest element

Test with lists containing negative numbers, duplicates, and a single-element list.

4. Rotate Left#

Write a function rotate_left(lst, k) that returns a new list with all elements shifted left by k positions (elements that fall off the left end wrap around to the right).

>>> rotate_left([1, 2, 3, 4, 5], 2)
[3, 4, 5, 1, 2]
>>> rotate_left([1, 2, 3], 0)
[1, 2, 3]

Use slicing: the result is lst[k:] + lst[:k]. Then verify this by also writing an explicit loop version.

5. Palindrome Check#

Write a function is_palindrome_list(lst) that returns True if the list reads the same forwards and backwards.

>>> is_palindrome_list([1, 2, 3, 2, 1])
True
>>> is_palindrome_list([1, 2, 3])
False

Try two approaches:

  1. Compare lst to lst[::-1].

  2. Use two indices walking inward from both ends.