Sorting Stability and When to Use Which Sort in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the array of objects by age using a stable sort.

DSA Javascript

people.sort((a, b) => a.age [1] b.age);

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

A-

B+

C===

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using comparison operators like '>' or '===' which do not return the expected numeric value for sort.

The subtraction operator - returns a negative, zero, or positive number which is required for the sort callback to work correctly and maintain stability.

2fill in blank

medium

Complete the code to keep the original order of items with the same score using a stable sort.

DSA Javascript

items.sort((a, b) => a.score [1] b.score);

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

A-

B===

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using comparison operators that return boolean values instead of numbers.

Subtracting b.score from a.score returns a number that the sort method uses to order items, preserving stability for equal scores.

3fill in blank

hard

Fix the error in the code to ensure stable sorting by name when ages are equal.

DSA Javascript

people.sort((a, b) => a.age - b.age || a.name [1] b.name);

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

A===

B>

C-

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction on strings which causes NaN and breaks sorting.

When ages are equal, comparing names with < returns a boolean which can be converted to a number to decide order. This helps keep the sort stable by name.

4fill in blank

hard

Fill both blanks to create a dictionary of word lengths for words longer than 3 characters.

DSA Javascript

{ [1]: [2] for word in words if word.length > 3 }

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

Aword

Blen(word)

Cword.length

Dword.upper()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Python's len() function instead of JavaScript's length property.

The key is the word itself, and the value is its length using word.length in JavaScript.

5fill in blank

hard

Fill all three blanks to filter and map an array of numbers to their squares if they are even.

DSA Javascript

const result = numbers.filter(n => n [1] 2 === 0).map(n => n [2] [3]);

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

A%

B**

C2

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect operators for checking even numbers or squaring.

Use % to check even numbers, then n ** 2 to get the square.

Practice

1. What does it mean when a sorting algorithm is stable?

easy

A. It keeps the original order of equal elements after sorting

B. It sorts elements faster than unstable algorithms

C. It uses less memory than unstable algorithms

D. It always sorts in ascending order

Solution

1. Step 1: Understand stability in sorting

   A stable sort keeps the relative order of items with equal keys unchanged.

2. Step 2: Compare with other options

   Speed, memory use, or sorting order do not define stability.

3. Final Answer:

   It keeps the original order of equal elements after sorting -> Option A

4. Quick Check:

   Stable sort = keeps equal elements order [OK]

Hint: Stable means equal items keep their original order [OK]

Common Mistakes:

• Confusing stability with speed
• Thinking stability means sorting ascending only
• Assuming stable sorts use less memory

2. Which of the following is the correct way to call JavaScript's built-in sort method to sort numbers in ascending order?

easy

A. arr.sort()

B. arr.sort((a, b) => a + b)

C. arr.sort((a, b) => b - a)

D. arr.sort((a, b) => a - b)

Solution

1. Step 1: Understand JavaScript's default sort

   By default, arr.sort() sorts elements as strings, not numbers.

2. Step 2: Use compare function for numeric ascending

   The compare function (a, b) => a - b sorts numbers ascending correctly.

3. Final Answer:

   arr.sort((a, b) => a - b) -> Option D

4. Quick Check:

   Numeric ascending sort uses (a, b) => a - b [OK]

Hint: Use (a, b) => a - b for ascending numeric sort [OK]

Common Mistakes:

• Using arr.sort() alone for numbers
• Using (a, b) => b - a for ascending order
• Using (a, b) => a + b which is invalid

3. Consider the array of objects:

const items = [
  {name: 'apple', price: 50},
  {name: 'banana', price: 20},
  {name: 'cherry', price: 50},
  {name: 'date', price: 20}
];
items.sort((a, b) => a.price - b.price);

What will be the order of items after sorting?

medium

A. [{banana,20}, {date,20}, {cherry,50}, {apple,50}]

B. [{date,20}, {banana,20}, {cherry,50}, {apple,50}]

C. [{banana,20}, {date,20}, {apple,50}, {cherry,50}]

D. [{date,20}, {banana,20}, {apple,50}, {cherry,50}]

Solution

1. Step 1: Understand the sort comparator

   The comparator sorts by price ascending: 20 then 50.

2. Step 2: Check stability of JavaScript sort

   JavaScript's sort is stable in modern engines, so equal price items keep original order.

3. Step 3: Apply stability to equal price items

   For price 20, original order is banana then date; for 50, apple then cherry.

4. Final Answer:

   [{banana,20}, {date,20}, {apple,50}, {cherry,50}] -> Option C

5. Quick Check:

   Stable sort keeps equal price order [OK]

Hint: Stable sort keeps original order of equal keys [OK]

Common Mistakes:

• Assuming unstable sort changes order of equal items
• Mixing up banana and date order
• Ignoring stability of JavaScript sort

4. The following code tries to sort an array of strings by length, which may not keep the original order of equal length strings:

const arr = ['bat', 'cat', 'apple', 'dog'];
arr.sort((a, b) => a.length - b.length);

What is the main reason the original order of equal length strings might not be preserved?

medium

A. Strings cannot be sorted by length

B. JavaScript's sort is unstable in some engines

C. The comparator function is incorrect

D. The array contains duplicates

Solution

1. Step 1: Check comparator correctness

   The comparator (a, b) => a.length - b.length correctly compares string lengths.

2. Step 2: Understand JavaScript sort stability

   JavaScript's sort is stable in modern engines but may be unstable in older engines, causing order change.

3. Final Answer:

   JavaScript's sort is unstable in some engines -> Option B

4. Quick Check:

   Unstable sort breaks equal element order [OK]

Hint: Old JS engines have unstable sort, losing order [OK]

Common Mistakes:

• Blaming comparator function
• Thinking strings can't be sorted by length
• Assuming duplicates cause instability

5. You have a list of student records with name and score. You want to sort by score ascending but keep the original order for students with the same score. Which sorting algorithm should you choose and why?

hard

A. MergeSort, because it is stable and sorts efficiently

B. QuickSort, because it is fast but unstable

C. HeapSort, because it uses less memory

D. BubbleSort, because it is simple and stable

Solution

1. Step 1: Identify need for stability

   We want to keep original order for equal scores, so stability is required.

2. Step 2: Choose algorithm with stability and efficiency

   MergeSort is stable and efficient (O(n log n)), suitable for this task.

3. Step 3: Compare other options

   QuickSort is fast but unstable; HeapSort is unstable; BubbleSort is stable but inefficient.

4. Final Answer:

   MergeSort, because it is stable and sorts efficiently -> Option A

5. Quick Check:

   Stable + efficient = MergeSort [OK]

Hint: Stable and efficient sort for equal keys: MergeSort [OK]

Common Mistakes:

• Choosing QuickSort ignoring instability
• Picking BubbleSort despite inefficiency
• Confusing HeapSort as stable