Priority Queue Introduction and Concept in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new empty priority queue using Rust's standard library.

DSA Rust

let mut pq = std::collections::BinaryHeap::[1]();

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

Apush

Bnew

Cpop

Dpeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using push or pop instead of new to create the priority queue.

Trying to use peek to create the queue.

The new function creates an empty BinaryHeap, which is Rust's priority queue.

2fill in blank

medium

Complete the code to add an element 10 to the priority queue.

DSA Rust

pq.[1](10);

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

Apeek

Bnew

Cpop

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using new instead of push to add elements.

Using pop or peek which are for removing or viewing elements.

The push method adds an element to the priority queue.

3fill in blank

hard

Fix the error in the code to remove the highest priority element from the priority queue.

DSA Rust

let max = pq.[1]().unwrap();

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

Apop

Bpush

Cpeek

Dnew

Attempts:

3 left

💡 Hint

Common Mistakes

Using peek which only views the element without removing.

Using push or new which do not remove elements.

The pop method removes and returns the highest priority element from the queue.

4fill in blank

hard

Fill both blanks to check if the priority queue is empty and print a message.

DSA Rust

if pq.[1]() [2] true {
    println!("Priority queue is empty");
}

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

Ais_empty

B==

C!=

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using len instead of is_empty for checking emptiness.

Using != instead of == in the condition.

The is_empty method checks if the queue has no elements, and == compares the result to true.

5fill in blank

hard

Fill all three blanks to create a priority queue from a vector, pop the top element, and peek at the next top element.

DSA Rust

let mut pq = std::collections::BinaryHeap::from([1]);
let top = pq.[2]().unwrap();
let next = pq.[3]().unwrap();

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

Avec![5, 3, 8, 1]

Bpop

Cpeek

Dnew

Attempts:

3 left

💡 Hint

Common Mistakes

Using new instead of from to create from a vector.

Using push instead of pop to remove elements.

Using pop instead of peek to view the next element.

We create a BinaryHeap from a vector using from, then remove the top element with pop, and finally look at the next top element with peek.

Practice

1. What is the main feature of a Priority Queue compared to a regular queue?

easy

A. It only stores elements of the same value

B. Elements are always dequeued in the order they were inserted

C. Elements are dequeued based on priority, not just order of insertion

D. It does not allow duplicate elements

Solution

1. Step 1: Understand queue behavior

   A regular queue removes elements in the order they were added (FIFO).

2. Step 2: Understand priority queue behavior

   A priority queue removes elements based on their priority, not just insertion order.

3. Final Answer:

   Elements are dequeued based on priority, not just order of insertion -> Option C

4. Quick Check:

   Priority Queue = Priority-based removal [OK]

Hint: Priority queue removes highest priority first, not oldest [OK]

Common Mistakes:

• Confusing priority queue with FIFO queue
• Thinking priority queue sorts elements permanently
• Assuming priority queue disallows duplicates

2. Which of the following is the correct way to create a max-heap priority queue in Rust using std::collections::BinaryHeap?

easy

A. let mut pq = std::collections::BinaryHeap::min_heap();

B. let mut pq = std::collections::BinaryHeap::with_capacity(10i32);

C. let mut pq = std::collections::BinaryHeap::default(10);

D. let mut pq = std::collections::BinaryHeap::new();

Solution

1. Step 1: Recall Rust BinaryHeap creation

   The standard way to create a max-heap priority queue is using BinaryHeap::new().

2. Step 2: Check other options

   with_capacity(10i32) type mismatch (usize expected); default(10) invalid arguments; min_heap() does not exist.

3. Final Answer:

   let mut pq = std::collections::BinaryHeap::new(); -> Option D

4. Quick Check:

   BinaryHeap::new() creates max-heap [OK]

Hint: Use BinaryHeap::new() for max-heap priority queue [OK]

Common Mistakes:

• Using with_capacity with i32 instead of usize
• Passing arguments to default()
• Using non-existent min_heap() method

3. What will be the output of the following Rust code using a priority queue?

use std::collections::BinaryHeap;

fn main() {
    let mut pq = BinaryHeap::new();
    pq.push(10);
    pq.push(5);
    pq.push(20);
    while let Some(val) = pq.pop() {
        print!("{} ", val);
    }
}

medium

A. 20 10 5

B. 5 20 10

C. 10 5 20

D. 5 10 20

Solution

1. Step 1: Understand BinaryHeap behavior

   Rust's BinaryHeap is a max-heap by default, so pop() returns the largest element first.

2. Step 2: Trace the pop order

   Inserted values: 10, 5, 20. Popping returns 20, then 10, then 5.

3. Final Answer:

   20 10 5 -> Option A

4. Quick Check:

   Max-heap pop order = descending [OK]

Hint: BinaryHeap pops largest first, so output is descending [OK]

Common Mistakes:

• Assuming pop returns elements in insertion order
• Thinking BinaryHeap is min-heap by default
• Mixing up push and pop order

4. Identify the error in this Rust code that tries to create a min-heap priority queue:

use std::collections::BinaryHeap;

fn main() {
    let mut pq = BinaryHeap::new();
    pq.push(3);
    pq.push(1);
    pq.push(2);
    while let Some(val) = pq.pop() {
        println!("{}", val);
    }
}

medium

A. BinaryHeap does not support min-heap by default

B. pq.push() method does not exist

C. pop() returns elements in insertion order

D. println! macro is used incorrectly

Solution

1. Step 1: Check BinaryHeap default behavior

   Rust's BinaryHeap is a max-heap by default, so it does not create a min-heap without extra work.

2. Step 2: Verify code correctness

   push() and pop() methods exist and println! is correct; the issue is the heap type.

3. Final Answer:

   BinaryHeap does not support min-heap by default -> Option A

4. Quick Check:

   BinaryHeap default = max-heap, no min-heap [OK]

Hint: BinaryHeap is max-heap by default; min-heap needs wrapper [OK]

Common Mistakes:

• Assuming BinaryHeap is min-heap by default
• Thinking push() or pop() are missing
• Misreading println! syntax

5. You want to implement a task scheduler in Rust that always runs the highest priority task first. Which approach correctly uses a priority queue to achieve this?

use std::collections::BinaryHeap;

struct Task {
    priority: i32,
    name: String,
}

fn main() {
    let mut pq = BinaryHeap::new();
    pq.push(Task { priority: 2, name: String::from("Clean") });
    pq.push(Task { priority: 5, name: String::from("Cook") });
    pq.push(Task { priority: 1, name: String::from("Shop") });

    while let Some(task) = pq.pop() {
        println!("{}", task.name);
    }
}

What must you do to make this code work as intended?

hard

A. Use a Vec<Task> instead of BinaryHeap

B. Implement Ord and Eq traits for Task to order by priority

C. Change priority to u32 type

D. Replace pop() with peek() in the loop

Solution

1. Step 1: Understand BinaryHeap requirements

   BinaryHeap requires elements to implement Ord and Eq to compare and order them.

2. Step 2: Check code for ordering

   Task struct has no ordering traits implemented, so BinaryHeap cannot order tasks by priority.

3. Step 3: Correct approach

   Implement Ord and Eq for Task to order by priority, so highest priority pops first.

4. Final Answer:

   Implement Ord and Eq traits for Task to order by priority -> Option B

5. Quick Check:

   BinaryHeap needs Ord/Eq for custom types [OK]

Hint: Custom types need Ord/Eq traits for BinaryHeap ordering [OK]

Common Mistakes:

• Using Vec instead of priority queue
• Changing data type without ordering
• Using peek() instead of pop() to remove elements