Sorting algorithms (bubble, selection) in Intro to Computing - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to swap two elements in a list.

Intro to Computing

temp = arr[[1]]
arr[[1]] = arr[[2]]
arr[[2]] = temp

Drag options to blanks, or click blank then click option'

Ai

Bj

C0

Dlen(arr)

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping with wrong indices

Using the same index twice

To swap elements at positions i and j, you first save the value at i, then assign the value at j to i, and finally assign the saved value to j.

2fill in blank

medium

Complete the code to compare two adjacent elements in bubble sort.

Intro to Computing

if arr[[1]] > arr[[2]]:

Drag options to blanks, or click blank then click option'

Ai

Bi+1

Cj

Dj+1

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing wrong indices

Using outer loop variable instead of inner loop variable

In bubble sort, you compare element at position j with the next element j+1 to decide if they need to be swapped.

3fill in blank

hard

Fix the error in the selection sort inner loop condition.

Intro to Computing

for j in range(i+1, [1]):

Drag options to blanks, or click blank then click option'

Ai

Blen(arr)-1

Clen(arr)

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(arr)-1 which skips last element

Using wrong start index

The inner loop in selection sort runs from i+1 to the end of the list, which is len(arr). Using len(arr)-1 would miss the last element.

4fill in blank

hard

Fill both blanks to complete the bubble sort outer and inner loops.

Intro to Computing

for i in range([1]):
    for j in range([2] - i - 1):

Drag options to blanks, or click blank then click option'

Alen(arr)

Blen(arr) - 1

Clen(arr) - i

Dlen(arr) + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(arr) - 1 for outer loop

Incorrect inner loop range causing index errors

The outer loop runs from 0 to len(arr). The inner loop runs from 0 to len(arr) - i - 1 to avoid checking already sorted elements at the end.

5fill in blank

hard

Fill all three blanks to complete the selection sort swap step.

Intro to Computing

min_idx = [1]
for j in range(i+1, [2]):
    if arr[j] < arr[min_idx]:
        min_idx = j
arr[[3]], arr[i] = arr[i], arr[[3]]

Drag options to blanks, or click blank then click option'

Ai

Blen(arr)

Cmin_idx

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping wrong indices

Incorrect loop range

Not updating min_idx correctly

In selection sort, you start with min_idx = i. The inner loop runs to len(arr). After finding the minimum index, you swap arr[min_idx] with arr[i].

Practice

1. Which of the following best describes how bubble sort works?

easy

A. It repeatedly swaps neighboring items to move the largest to the end.

B. It finds the smallest item and places it at the start each time.

C. It divides the list into halves and sorts each half separately.

D. It uses a pivot to partition the list into smaller parts.

Solution

1. Step 1: Understand bubble sort's swapping method

   Bubble sort compares neighbors and swaps them if out of order, pushing the largest to the end.

2. Step 2: Compare with other sorting methods

   Selection sort finds smallest items, quicksort uses pivots, so bubble sort matches It repeatedly swaps neighboring items to move the largest to the end.

3. Final Answer:

   It repeatedly swaps neighboring items to move the largest to the end. -> Option A

4. Quick Check:

   Bubble sort = neighbor swaps [OK]

Hint: Bubble sort swaps neighbors to push largest out [OK]

Common Mistakes:

• Confusing bubble sort with selection sort
• Thinking bubble sort uses pivots
• Assuming bubble sort divides list into halves

2. Which of the following is the correct way to start a selection sort on a list named arr in Python?

easy

A. for i in range(1, len(arr)+1):

B. for i in arr:

C. while i < len(arr):

D. for i in range(len(arr)):

Solution

1. Step 1: Identify the loop for selection sort

   Selection sort uses an index loop from 0 to length-1 to select positions.

2. Step 2: Check Python syntax correctness

   Using for i in range(len(arr)): correctly loops over indices; others are incorrect or off-by-one.

3. Final Answer:

   for i in range(len(arr)): -> Option D

4. Quick Check:

   Selection sort loops over indices 0 to n-1 [OK]

Hint: Use range(len(arr)) to loop over list indices [OK]

Common Mistakes:

• Using for i in arr (loops over values, not indices)
• Using while without initializing i
• Using range starting at 1 causing off-by-one errors

3. What is the output of the following bubble sort pass on the list [4, 2, 5, 1]?

Initial list: [4, 2, 5, 1]
Pass 1: Compare and swap neighbors if needed

medium

A. [2, 4, 5, 1]

B. [2, 4, 1, 5]

C. [4, 2, 1, 5]

D. [1, 2, 4, 5]

Solution

1. Step 1: Perform neighbor comparisons and swaps

   Compare 4 & 2: swap -> [2, 4, 5, 1]; compare 4 & 5: no swap; compare 5 & 1: swap -> [2, 4, 1, 5]

2. Step 2: Confirm final list after pass 1

   After one pass, largest number 5 is bubbled to the end, list is [2, 4, 1, 5]

3. Final Answer:

   [2, 4, 1, 5] -> Option B

4. Quick Check:

   Bubble pass 1 swaps neighbors -> [2, 4, 1, 5] [OK]

Hint: Swap neighbors if left is bigger, largest moves right [OK]

Common Mistakes:

• Not swapping 5 and 1 at the end
• Swapping 4 and 5 incorrectly
• Assuming full sort after one pass

4. The following selection sort code has a bug. What is the error?

arr = [3, 1, 4]
for i in range(len(arr)):
    min_idx = i
    for j in range(i+1, len(arr)):
        if arr[j] < arr[min_idx]:
            min_idx = j
    arr[i], arr[min_idx] = arr[min_idx], arr[i]
print(arr)

medium

A. The inner loop should start from i, not i+1

B. The swap line is incorrect; it should not swap

C. No bug; the code correctly sorts the list

D. min_idx should be initialized outside the outer loop

Solution

1. Step 1: Analyze the selection sort logic

   min_idx starts at i, inner loop finds smallest element index after i, then swaps with i.

2. Step 2: Verify correctness with example

   For arr=[3,1,4], code finds min at index 1 and swaps with index 0, resulting in sorted list [1,3,4].

3. Final Answer:

   No bug; the code correctly sorts the list -> Option C

4. Quick Check:

   Selection sort code correct as given [OK]

Hint: Check if code sorts example list correctly [OK]

Common Mistakes:

• Thinking inner loop must start at i
• Believing swap line is wrong
• Misunderstanding min_idx initialization

5. You have a list [7, 3, 5, 2, 9]. After two full passes of selection sort, what will the list look like?

hard

A. [2, 3, 7, 5, 9]

B. [3, 2, 5, 7, 9]

C. [2, 3, 5, 7, 9]

D. [7, 3, 5, 2, 9]

Solution

1. Step 1: Perform first pass of selection sort

   Find smallest in [7, 3, 5, 2, 9] is 2 at index 3; swap with index 0 -> [2, 3, 5, 7, 9]

2. Step 2: Perform second pass on sublist from index 1 [3, 5, 7, 9]

   Find smallest is 3 at index 1; swap with index 1 (no change) -> [2, 3, 5, 7, 9]

3. Final Answer:

   [2, 3, 7, 5, 9] -> Option A

4. Quick Check:

   Selection sort places smallest at start each pass [OK]

Hint: Selection sort fixes one smallest item per pass [OK]

Common Mistakes:

• Assuming list is unchanged after passes
• Mixing bubble sort behavior with selection sort
• Swapping incorrectly during passes