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Python List Practice Problems
Write a Python program to remove negative values from a list.
[4, -2, 0, 7, -9, 3]
[4, 0, 7, 3]
numbers = [4, -2, 0, 7, -9, 3]print([n for n in numbers if n >= 0])[4, 0, 7, 3]
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Write a Python program to remove the 1st, 3rd and 6th elements of a list.
["a", "b", "c", "d", "e", "f", "g"]
['b', 'd', 'e', 'g']
items = ["a", "b", "c", "d", "e", "f", "g"]remove = {0, 2, 5}print([x for i, x in enumerate(items) if i not in remove])['b', 'd', 'e', 'g']
Positions 1, 3 and 6 are indexes 0, 2 and 5.
Deleting with del one by one shifts later indexes; building a new list avoids that trap.
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Write a Python program that returns True if two lists have at least one common member.
[1, 2, 3] and [5, 3, 9]; [1, 2] and [7, 8]
True False
def has_common(a, b): return not set(a).isdisjoint(b) print(has_common([1, 2, 3], [5, 3, 9]), has_common([1, 2], [7, 8]))True False
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Write a Python program to copy a list so that changing the copy does not change the original.
[1, 2, 3]
[1, 2, 3] [1, 2, 3, 4] [1, 2, 3]
copy(), list() or slicing [:].original = [1, 2, 3]copy1 = original.copy()copy2 = original[:]copy1.append(4)print(original, copy1, copy2)[1, 2, 3] [1, 2, 3, 4] [1, 2, 3]
Both create new lists.
Changing the copy leaves the original alone.
copy2 = original would not copy; both names would refer to the same list.
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Write a Python program to reverse a list with reversed() and with reverse().
[1, 2, 3, 4, 5]
[5, 4, 3, 2, 1] [5, 4, 3, 2, 1]
reversed() gives an iterator; reverse() changes the list in place.numbers = [1, 2, 3, 4, 5]print(list(reversed(numbers)))numbers.reverse()print(numbers)[5, 4, 3, 2, 1] [5, 4, 3, 2, 1]
reversed() returns an iterator, so wrap it in list() to see the items.
reverse() returns None and reverses numbers itself.
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Write a Python program to reverse a list using slicing.
[1, 2, 3, 4, 5]
[5, 4, 3, 2, 1] [1, 2, 3, 4, 5]
[::-1] returns a reversed copy.numbers = [1, 2, 3, 4, 5]print(numbers[::-1], numbers)[5, 4, 3, 2, 1] [1, 2, 3, 4, 5]
The slice creates a new list; the original is unchanged.
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Write a Python program to reverse a list using a while loop.
[1, 2, 3, 4, 5]
[5, 4, 3, 2, 1]
numbers = [1, 2, 3, 4, 5]result = []i = len(numbers) - 1while i >= 0: result.append(numbers[i]) i -= 1print(result)[5, 4, 3, 2, 1]
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Write a Python program to reverse a list using a for loop.
[1, 2, 3, 4, 5]
[5, 4, 3, 2, 1]
numbers = [1, 2, 3, 4, 5]reversed_list = []for n in numbers: reversed_list.insert(0, n)print(reversed_list)[5, 4, 3, 2, 1]
Inserting at index 0 puts each new item before the earlier ones.
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Write a Python program to get the common elements of two lists, in the order of the first list.
[4, 5, 7, 9, 2, 1], [2, 5, 8, 3, 4, 7]
[4, 5, 7, 2]
list1 = [4, 5, 7, 9, 2, 1]list2 = [2, 5, 8, 3, 4, 7]lookup = set(list2)print([x for x in list1 if x in lookup])[4, 5, 7, 2]
Membership checks on a set are fast, even for long lists.
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Write a Python program to rearrange a list so all odd values come first and all even values after them, keeping their order.
[5, 7, 2, 8, 11, 12, 17, 19, 22]
[5, 7, 11, 17, 19, 2, 8, 12, 22]
numbers = [5, 7, 2, 8, 11, 12, 17, 19, 22]odds = [n for n in numbers if n % 2]evens = [n for n in numbers if n % 2 == 0]print(odds + evens)[5, 7, 11, 17, 19, 2, 8, 12, 22]
n % 2 is 1 (truthy) for odd numbers.
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