Quick Sort Algorithm in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to swap two elements in the array.

DSA Rust

arr.swap([1], high);

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Amid

Bhigh

Clow

Dpivot

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'high' instead of 'low' causes no change in array.

Using 'pivot' as index causes compile error.

The swap function exchanges the element at index low with the element at index high to place the pivot correctly.

2fill in blank

medium

Complete the code to compare the current element with the pivot value.

DSA Rust

if arr[j] [1] pivot_value {

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A<

B<=

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' reverses the partition logic.

Using '>=' or '<=' includes equal elements incorrectly.

We check if the current element is less than the pivot value to decide if it should be moved to the left partition.

3fill in blank

hard

Fix the error in the recursive call to sort the left partition.

DSA Rust

quick_sort(arr, low, [1]);

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Apivot_index

Bpivot_index - 1

Cpivot_index + 1

Dhigh

Attempts:

3 left

💡 Hint

Common Mistakes

Using pivot_index includes the pivot again causing infinite recursion.

Using pivot_index + 1 skips elements.

The left partition ends at pivot_index - 1 because the pivot is already in correct place.

4fill in blank

hard

Fill both blanks to complete the partition function header and return statement.

DSA Rust

fn partition(arr: &mut [i32], [1]: usize, [2]: usize) -> usize {

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Alow

Bhigh

Cpivot

Dmid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pivot' or 'mid' as parameters causes confusion.

Swapping parameter order breaks logic.

The partition function takes the low and high indices to work on the subarray and returns the pivot index.

5fill in blank

hard

Fill all three blanks to complete the recursive quick_sort calls after partitioning.

DSA Rust

quick_sort(arr, low, [1]);
quick_sort(arr, [2], high);

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

Apivot_index - 1

Bpivot_index + 1

Cpivot_index

Dlow

Attempts:

3 left

💡 Hint

Common Mistakes

Using pivot_index in both calls causes infinite recursion.

Using low or high incorrectly swaps subarray boundaries.

After partitioning, quick_sort is called on the left subarray ending at pivot_index - 1 and the right subarray starting at pivot_index + 1.

Practice

1. What is the main idea behind the Quick Sort algorithm?

easy

A. Merge two sorted lists into one sorted list

B. Compare every element with every other element and swap if needed

C. Sort the list by repeatedly finding the minimum element

D. Divide the list into smaller parts using a pivot and sort them recursively

Solution

1. Step 1: Understand Quick Sort's approach

   Quick Sort picks a pivot element and divides the list into parts smaller and larger than the pivot.

2. Step 2: Recursive sorting of parts

   It then sorts these parts recursively, combining them to get the sorted list.

3. Final Answer:

   Divide the list into smaller parts using a pivot and sort them recursively -> Option D

4. Quick Check:

   Quick Sort = Divide and conquer [OK]

Hint: Remember: pivot divides list, then sort parts recursively [OK]

Common Mistakes:

• Confusing Quick Sort with Bubble Sort
• Thinking Quick Sort merges lists like Merge Sort
• Believing Quick Sort always picks the first element as pivot

2. Which of the following is the correct Rust function signature for a Quick Sort function that sorts a mutable slice of integers?

easy

A. fn quick_sort(arr: &mut [i32])

B. fn quick_sort(arr: Vec<i32>) -> Vec<i32>

C. fn quick_sort(arr: &[i32]) -> &[i32]

D. fn quick_sort(arr: &mut Vec<i32>) -> ()

Solution

1. Step 1: Identify correct parameter type for in-place sorting

   Quick Sort usually sorts in place, so it needs a mutable slice reference: &mut [i32].

2. Step 2: Check function signature correctness

   fn quick_sort(arr: &mut [i32]) matches this: it takes &mut [i32] and returns nothing (unit type implied).

3. Final Answer:

   fn quick_sort(arr: &mut [i32]) -> Option A

4. Quick Check:

   Mutable slice parameter = fn quick_sort(arr: &mut [i32]) [OK]

Hint: Quick Sort sorts in place, so use &mut slice parameter [OK]

Common Mistakes:

• Using immutable slice &[] which cannot be changed
• Returning a new vector instead of sorting in place
• Using Vec<i32> without mutability for in-place sort

3. What will be the output of the following Rust code snippet after calling quick_sort(&mut arr)?

fn quick_sort(arr: &mut [i32]) {
    if arr.len() <= 1 { return; }
    let pivot = arr[0];
    let mut left = 1;
    let mut right = arr.len() - 1;
    while left <= right {
        if arr[left] <= pivot {
            left += 1;
        } else {
            arr.swap(left, right);
            if right == 0 { break; }
            right -= 1;
        }
    }
    arr.swap(0, right);
    quick_sort(&mut arr[0..right]);
    quick_sort(&mut arr[right+1..]);
}

fn main() {
    let mut arr = [4, 2, 7, 1, 3];
    quick_sort(&mut arr);
    println!("{:?}", arr);
}

medium

A. [1, 2, 3, 7, 4]

B. [1, 2, 3, 4, 7]

C. [4, 2, 7, 1, 3]

D. [7, 4, 3, 2, 1]

Solution

1. Step 1: Understand partitioning around pivot

   The pivot is 4. Elements <=4 move left, >4 move right. After partition, array becomes [3, 2, 1, 4, 7] with pivot at index 3.

2. Step 2: Recursive calls sort left and right parts

   Left part [3, 2, 1] sorts to [1, 2, 3], right part [7] is already sorted. Combined final array is [1, 2, 3, 4, 7].

3. Final Answer:

   [1, 2, 3, 4, 7] -> Option B

4. Quick Check:

   Sorted array = [1, 2, 3, 4, 7] [OK]

Hint: Trace pivot swaps and recursive calls carefully [OK]

Common Mistakes:

• Ignoring pivot swaps and recursive sorting
• Assuming array remains unchanged
• Mixing up left and right partitions

4. Identify the bug in this Rust Quick Sort implementation:

fn quick_sort(arr: &mut [i32]) {
    if arr.len() <= 1 { return; }
    let pivot = arr[0];
    let mut left = 1;
    let mut right = arr.len() - 1;
    while left < right {
        if arr[left] <= pivot {
            left += 1;
        } else {
            arr.swap(left, right);
            right -= 1;
        }
    }
    arr.swap(0, right);
    quick_sort(&mut arr[0..right]);
    quick_sort(&mut arr[right+1..]);
}

medium

A. The while loop condition should be <= instead of <

B. Pivot should be chosen as the last element, not the first

C. The recursive calls should use inclusive ranges

D. The swap after the loop should be with left, not right

Solution

1. Step 1: Analyze the while loop condition

   The loop uses while left < right, but it must include the case when left == right to correctly partition.

2. Step 2: Effect of incorrect loop condition

   Stopping early misses some comparisons, causing incorrect partition and sorting errors.

3. Final Answer:

   The while loop condition should be <= instead of < -> Option A

4. Quick Check:

   Loop condition must cover all indices [OK]

Hint: Check loop boundaries carefully in partition step [OK]

Common Mistakes:

• Ignoring edge case when left equals right
• Changing pivot choice unnecessarily
• Misusing slice ranges in recursive calls

5. You want to sort a large list of integers using Quick Sort in Rust. Which strategy helps avoid worst-case performance when the input is already sorted or nearly sorted?

hard

A. Always pick the first element as pivot

B. Use Bubble Sort instead for sorted inputs

C. Pick a random element as pivot each time

D. Sort only the first half of the list

Solution

1. Step 1: Understand worst-case in Quick Sort

   Picking the first element as pivot on sorted input causes unbalanced partitions, leading to worst-case O(n²) time.

2. Step 2: Use random pivot to avoid worst-case

   Choosing a random pivot spreads partitions more evenly, improving average performance and avoiding worst-case.

3. Final Answer:

   Pick a random element as pivot each time -> Option C

4. Quick Check:

   Random pivot = better average case [OK]

Hint: Randomize pivot to prevent worst-case input issues [OK]

Common Mistakes:

• Always picking first element as pivot
• Switching to Bubble Sort unnecessarily
• Sorting only part of the list