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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the vector in ascending order using a stable sort.

DSA Rust

let mut numbers = vec![3, 1, 4, 1, 5];
numbers.[1]();

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

Ashuffle

Bsort

Creverse

Dsort_unstable

Attempts:

3 left

💡 Hint

Common Mistakes

Using `sort_unstable()` which is not stable.

Using methods that do not sort like `reverse()` or `shuffle()`.

The `sort()` method in Rust performs a stable sort, preserving the order of equal elements.

2fill in blank

medium

Complete the code to sort a vector of tuples by the second element using a stable sort.

DSA Rust

let mut pairs = vec![(1, 3), (2, 1), (3, 3)];
pairs.sort_by_key(|k| k.[1]);

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

A1

B3

C2

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 0 which sorts by the first element.

Using an index out of range like 2 or 3.

Index 1 accesses the second element of the tuple, which is used as the key for sorting.

3fill in blank

hard

Fix the error in the code to perform an unstable sort on the vector.

DSA Rust

let mut data = vec![5, 3, 8, 1];
data.[1]();

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

Asort_by

Bsort

Csort_by_key

Dsort_unstable

Attempts:

3 left

💡 Hint

Common Mistakes

Using `sort()` which is stable, not unstable.

Using `sort_by` or `sort_by_key` without unstable suffix.

The `sort_unstable()` method performs an unstable sort which is faster but does not preserve order of equal elements.

4fill in blank

hard

Fill both blanks to sort a vector of strings by length using a stable sort.

DSA Rust

let mut words = vec!["apple", "bat", "carrot"];
words.sort_by_key(|word| word.[1]());
// This sort is [2]

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

Alen

Blength

Cunstable

Dstable

Attempts:

3 left

💡 Hint

Common Mistakes

Using `length()` which does not exist in Rust strings.

Saying the sort is unstable when it is stable.

The `len()` method returns the length of a string slice. The `sort_by_key` method is stable.

5fill in blank

hard

Fill all three blanks to create a vector of tuples sorted by the second element in descending order using an unstable sort.

DSA Rust

let mut items = vec![("a", 2), ("b", 3), ("c", 1)];
items.sort_unstable_by(|a, b| b.[1].cmp(&a.[2]));
// The sort is [3]

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

A1

B0

Cunstable

Dstable

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 0 which accesses the first element.

Saying the sort is stable when it is unstable.

Index 1 accesses the second element of the tuple. Using `sort_unstable_by` makes the sort unstable.

Practice

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

easy

A. It uses less memory than unstable sorts

B. It sorts elements in ascending order only

C. It always runs faster than unstable sorts

D. It keeps the original order of equal elements unchanged

Solution

1. Step 1: Understand stability in sorting

   A stable sort means if two elements are equal, their order stays the same as before sorting.

2. Step 2: Compare options with definition

   Only It keeps the original order of equal elements unchanged matches this meaning. Other options describe unrelated properties.

3. Final Answer:

   It keeps the original order of equal elements unchanged -> Option D

4. Quick Check:

   Stable sort = keeps equal elements order [OK]

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

Common Mistakes:

• Confusing stability with sorting order
• Thinking stability means faster sorting
• Assuming stability means less memory use

2. Which Rust standard library method sorts a slice with a stable sort without requiring a comparator closure?

easy

A. slice.sort_unstable_by()

B. slice.sort_unstable()

C. slice.sort()

D. slice.sort_by()

Solution

1. Step 1: Recall Rust slice sorting methods

   Rust's slice.sort() uses a stable sorting algorithm.

2. Step 2: Differentiate stable and unstable methods

   sort_unstable() and sort_unstable_by() are unstable sorts. sort_by() is stable but requires a comparator.

3. Final Answer:

   slice.sort() -> Option C

4. Quick Check:

   slice.sort() = stable sort [OK]

Hint: Use sort() for stable sorting in Rust [OK]

Common Mistakes:

• Choosing sort_unstable() thinking it's stable
• Confusing sort_by() with unstable sort
• Not knowing sort() is stable by default

3. Given the Rust code below, what is the output?

let mut data = [(3, 'a'), (1, 'b'), (3, 'c'), (2, 'd')];
data.sort();
println!("{:?}", data);

medium

A. [(1, 'b'), (2, 'd'), (3, 'a'), (3, 'c')]

B. [(1, 'b'), (2, 'd'), (3, 'c'), (3, 'a')]

C. [(3, 'a'), (3, 'c'), (2, 'd'), (1, 'b')]

D. [(2, 'd'), (1, 'b'), (3, 'a'), (3, 'c')]

Solution

1. Step 1: Understand sorting of tuples in Rust

   Rust sorts tuples by first element, then second if first are equal. Here, sorting by integer first.

2. Step 2: Apply stable sort to keep order of equal keys

   Elements with key 3 are (3, 'a') then (3, 'c'). Stable sort keeps this order.

3. Final Answer:

   [(1, 'b'), (2, 'd'), (3, 'a'), (3, 'c')] -> Option A

4. Quick Check:

   Stable sort keeps (3, 'a') before (3, 'c') [OK]

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

Common Mistakes:

• Swapping order of equal keys
• Ignoring tuple sorting rules
• Assuming unstable sort by default

4. The following Rust code uses sort_unstable() but the order of equal elements is important. What is the main issue?

let mut items = [(2, 'x'), (1, 'y'), (2, 'z')];
items.sort_unstable();
println!("{:?}", items);

medium

A. The code will not compile because sort_unstable() requires a comparator

B. The order of (2, 'x') and (2, 'z') may change, breaking stability

C. The code will panic at runtime due to unstable sort

D. The output will always be sorted and stable

Solution

1. Step 1: Understand sort_unstable() behavior

   sort_unstable() sorts faster but does not guarantee stable order of equal elements.

2. Step 2: Identify problem with equal keys

   Elements with key 2 may swap order, so (2, 'x') and (2, 'z') order can change, which breaks stability.

3. Final Answer:

   The order of (2, 'x') and (2, 'z') may change, breaking stability -> Option B

4. Quick Check:

   Unstable sort = order of equal keys can change [OK]

Hint: Unstable sort can reorder equal elements [OK]

Common Mistakes:

• Thinking unstable sort always panics
• Assuming unstable sort requires comparator
• Believing unstable sort keeps order

5. You have a list of student records with fields (score: i32, name: &str). You want to sort by score ascending but keep the original order of students with the same score. Which sorting method should you use in Rust and why?

hard

A. Use sort() because it is stable and preserves order of equal scores

B. Use sort_unstable() because it is faster and order doesn't matter

C. Use sort_unstable_by() with a custom comparator for stability

D. Use sort_by() but it is unstable so order may change

Solution

1. Step 1: Identify requirement for stability

   We want to keep original order of students with same score, so stable sort is needed.

2. Step 2: Choose Rust method that is stable

   sort() is stable and sorts ascending by default. sort_unstable() is faster but unstable. sort_unstable_by() is unstable even with comparator. sort_by() is stable but Use sort_by() but it is unstable so order may change claims it is unstable incorrectly.

3. Final Answer:

   Use sort() because it is stable and preserves order of equal scores -> Option A

4. Quick Check:

   Stable sort needed = use sort() [OK]

Hint: Stable sort keeps order of equal keys; use sort() [OK]

Common Mistakes:

• Choosing unstable sort for speed ignoring order
• Thinking sort_unstable_by() is stable
• Misunderstanding sort_by() stability