Edge cases with lists and strings

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. Which list is an edge case you should test for a function that finds the biggest number?

A [] (an empty list)

B [4, 9, 2] (a few different numbers)

2. A program finds the largest score in a list. Which test checks the 'maximum at the start' edge case?

A [40, 55, 90, 12]

B [90, 40, 55, 12]

3. A program finds the largest score in a list. Which test checks the 'maximum at the end' edge case?

A [12, 40, 55, 90]

B [12, 90, 55, 40]

4. Which test data is a single-item edge case?

A [7, 7, 7]

B [7]

5. True or false: an edge case must be invalid input that the program should reject.

A False: edge cases are unusual but allowed inputs

B True: edge cases are always invalid inputs

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1max([])		Anums = [42] print(nums[0] == nums[-1])
2scores = [7, 7, 7, 7]		Btest = [3, 8, 2, 9] print(max(test) == test[-1])
3A one-item list is an edge case because the first item is also the last item and the maximum.		CThe code never handles an empty list, so items[0] causes an IndexError. It needs a check such as if len(items) == 0 first.
4The maximum at the end of a list is an edge case that tests whether a loop checks every item.		DEdge case: an empty list. Python raises a ValueError because there is no largest item to return.
5biggest = nums[0] for i in range(1, len(nums) - 1): if nums[i] > biggest: biggest = nums[i]		EEdge case: duplicate values. Every item is the same, so tests should check that code handles ties.
6def first_item(items): return items[0] print(first_item([]))		FThe loop stops one item early, so it never checks the last item. It fails when the maximum is at the end, for example [1, 5, 9].

Definitions

edge case

Fill in the Blanks

Complete each line of code by filling in the missing parts.

def biggest(nums): if ___(nums) == 0: return None

scores = [42] print(scores[_]) print(scores[-1])

nums = [5, 9, 9, 2] print(nums._____(9))

nums = [2, 4, 9] print(nums[__] == max(nums))

word = "" if ___ word: print("Empty string")

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A def average(nums):

B if len(nums) == 0:

C return nums[0]

D return sum(nums) / len(nums)

Question 2

A def last_item(items):

B return items[len(items)]

C print(last_item([7]))

D print(last_item([3, 3, 3]))

Question 3

A nums = [9, 4, 2]

B pos = 1

C for i in range(len(nums)):

D if nums[i] > nums[pos]:

E pos = i

Question 4

A def count_a(text)

B n = 0

C for ch in text:

D if ch == "a":

E n += 1

F return n

Question 5

A word = "level"

B rev = word[::-1]

C if word = rev:

D print("Palindrome")

Answer Key

Edge cases with lists and strings

Section A: Quick Questions

A — [] (an empty list)

B — [90, 40, 55, 12]

A — [12, 40, 55, 90]

B — [7]

A — False: edge cases are unusual but allowed inputs

Section B: Match the Pairs

max([]) → Edge case: an empty list. Python raises a ValueError because there is no largest item to return.

scores = [7, 7, 7, 7] → Edge case: duplicate values. Every item is the same, so tests should check that code handles ties.

A one-item list is an edge case because the first item is also the last item and the maximum. → nums = [42] print(nums[0] == nums[-1])

The maximum at the end of a list is an edge case that tests whether a loop checks every item. → test = [3, 8, 2, 9] print(max(test) == test[-1])

biggest = nums[0] for i in range(1, len(nums) - 1): if nums[i] > biggest: biggest = nums[i] → The loop stops one item early, so it never checks the last item. It fails when the maximum is at the end, for example [1, 5, 9].

def first_item(items): return items[0] print(first_item([])) → The code never handles an empty list, so items[0] causes an IndexError. It needs a check such as if len(items) == 0 first.

Section C: Definitions

edge case — An unusual but allowed input, such as an empty list, a one-item list, repeated values, or the maximum at the start or end, that you test because it can break code.

Section D: Fill in the Blanks

def biggest(nums):
    if len(nums) == 0:
        return None

scores = [42]
print(scores[0])
print(scores[-1])

nums = [5, 9, 9, 2]
print(nums.count(9))

nums = [2, 4, 9]
print(nums[-1] == max(nums))

word = ""
if not word:
    print("Empty string")

Section E: Spot the Error

C — For an empty list there is no item at index 0, so nums[0] raises an IndexError. The empty-list case should return a safe value such as 0.

B — Indexes stop at len(items) - 1, so items[len(items)] is always out of range, even for a one-item list. Use items[-1] or items[len(items) - 1].

B — Starting pos at 1 skips the first item, so a maximum at the start of the list (like 9) is never chosen. pos should start at 0.

A — The def line is missing its colon, so the function will not run at all and edge cases like an empty string cannot be tested. It should be def count_a(text):

C — A single = is assignment, not comparison, so this is a syntax error. Use == to compare the word with its reverse, which also handles one-letter strings.