HashMap Implementation from Scratch in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a struct named HashMap.

DSA Rust

struct [1] {
    buckets: Vec<Vec<(String, String)>>,
}

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

AHashTable

BMap

CHashMap

DDictionary

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different struct name like Map or Dictionary.

The struct name should be HashMap to match the implementation.

2fill in blank

medium

Complete the code to initialize the buckets vector with empty vectors.

DSA Rust

impl HashMap {
    fn new(size: usize) -> Self {
        let mut buckets = Vec::with_capacity(size);
        for _ in 0..size {
            buckets.push([1]);
        }
        HashMap { buckets }
    }
}

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AVec::new()

BVec::with_capacity(1)

CVec::default()

DVec::empty()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Vec::empty() which does not exist in Rust.

Using Vec::with_capacity(1) which creates capacity but not an empty vector.

Each bucket is an empty vector of key-value pairs, so Vec::new() is correct.

3fill in blank

hard

Fix the error in the hash function to compute the bucket index.

DSA Rust

fn hash(&self, key: &str) -> usize {
    let mut hash = 0;
    for byte in key.bytes() {
        hash = hash.wrapping_mul(31).wrapping_add(byte as usize);
    }
    hash [1] self.buckets.len()
}

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A%

B/

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using division (/) which returns quotient, not suitable for indexing.

Using multiplication (*) which increases the number.

Using addition (+) which does not limit the range.

The modulo operator % is used to keep the hash within bucket range.

4fill in blank

hard

Fill both blanks to complete the insert method that adds or updates a key-value pair.

DSA Rust

fn insert(&mut self, key: String, value: String) {
    let index = self.hash(&key);
    let bucket = &mut self.buckets[index];
    for pair in bucket.iter_mut() {
        if pair.[1] == key {
            pair.[2] = value;
            return;
        }
    }
    bucket.push((key, value));
}

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

A0

B1

Cpush

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using push or pop instead of tuple indices.

Mixing up key and value positions.

pair.0 is the key, pair.1 is the value in the tuple.

5fill in blank

hard

Fill all three blanks to complete the get method that retrieves a value by key.

DSA Rust

fn get(&self, key: &str) -> Option<&String> {
    let index = self.hash(key);
    let bucket = &self.buckets[index];
    for pair in bucket.iter() {
        if pair.[1] == key {
            return Some(pair.[2]);
        }
    }
    [3]
}

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

A0

B1

CNone

DSome

Attempts:

3 left

💡 Hint

Common Mistakes

Returning Some instead of None when key is missing.

Mixing up tuple indices.

pair.0 is key, pair.1 is value, and None is returned if key not found.

Practice

1. What is the main purpose of a HashMap in data structures?

easy

A. To store elements in a queue-like manner

B. To store elements in sorted order

C. To store key-value pairs for fast lookup

D. To store elements in a stack-like manner

Solution

1. Step 1: Understand HashMap functionality

   A HashMap stores data as key-value pairs, allowing quick access by key.

2. Step 2: Compare with other data structures

   Unlike stacks or queues, HashMaps do not maintain order but focus on fast lookup.

3. Final Answer:

   To store key-value pairs for fast lookup -> Option C

4. Quick Check:

   HashMap = key-value fast access [OK]

Hint: HashMap = fast key-value storage [OK]

Common Mistakes:

• Confusing HashMap with sorted structures
• Thinking HashMap stores elements in order
• Mixing HashMap with stack or queue behavior

2. Which of the following is the correct Rust syntax to create an empty HashMap from scratch?

easy

A. let map = HashMap::empty();

B. let map = new HashMap();

C. let map = HashMap();

D. let map = HashMap::new();

Solution

1. Step 1: Recall Rust HashMap creation syntax

   Rust uses HashMap::new() to create an empty HashMap.

2. Step 2: Check other options for Rust correctness

   Options B, C, and D are not valid Rust syntax for HashMap creation.

3. Final Answer:

   let map = HashMap::new(); -> Option D

4. Quick Check:

   Rust HashMap empty = HashMap::new() [OK]

Hint: Use HashMap::new() to create empty map [OK]

Common Mistakes:

• Using constructor syntax like new HashMap()
• Calling HashMap() without ::new()
• Using non-existent methods like empty()

3. Consider this simplified Rust code snippet for a custom HashMap insert method:

let mut map = MyHashMap::new();
map.insert("apple", 3);
map.insert("banana", 5);
map.insert("apple", 7);
println!("{:?}", map.get("apple"));

What will be printed?

medium

A. Some(3)

B. Some(7)

C. None

D. Some(5)

Solution

1. Step 1: Analyze insertions for key "apple"

   First insert sets "apple" to 3, second insert updates "apple" to 7.

2. Step 2: Check the get method output

   Getting "apple" returns the latest value, which is 7, wrapped in Some.

3. Final Answer:

   Some(7) -> Option B

4. Quick Check:

   Last insert value = get output [OK]

Hint: Last inserted value for key is returned [OK]

Common Mistakes:

• Assuming first inserted value remains
• Confusing None with Some value
• Mixing values of different keys

4. Given this Rust snippet for a custom HashMap insert method, what is the bug?

fn insert(&mut self, key: K, value: V) {
    let index = self.hash(&key) % self.buckets.len();
    if self.buckets[index].is_empty() {
        self.buckets[index].push((key, value));
    } else {
        self.buckets[index].push((key, value));
    }
}

medium

A. It does not update value if key exists, causing duplicates

B. It uses wrong hash function signature

C. It does not handle empty buckets correctly

D. It forgets to resize buckets when full

Solution

1. Step 1: Review insert logic for existing keys

   The code always pushes new (key, value) pairs without checking for existing keys.

2. Step 2: Identify consequence of missing update

   This causes duplicate keys in the same bucket, breaking expected HashMap behavior.

3. Final Answer:

   It does not update value if key exists, causing duplicates -> Option A

4. Quick Check:

   Insert must update existing key, not just push [OK]

Hint: Check for existing key before inserting [OK]

Common Mistakes:

• Ignoring key existence before insert
• Assuming push replaces existing entries
• Missing bucket resizing logic

5. You are implementing a custom HashMap in Rust with separate chaining. Which approach best handles collisions and updates existing keys efficiently?

hard

A. Store key-value pairs in a vector at each bucket; on insert, search vector for key to update or append

B. Use a single vector for all keys without hashing; append all pairs sequentially

C. Replace entire bucket vector on each insert without checking keys

D. Use a linked list but never check for existing keys before inserting

Solution

1. Step 1: Understand separate chaining collision handling

   Separate chaining uses a list (vector) at each bucket to store multiple key-value pairs.

2. Step 2: Efficient insert requires checking existing keys

   Before inserting, search the bucket's vector for the key to update value or append if not found.

3. Step 3: Evaluate options

   Store key-value pairs in a vector at each bucket; on insert, search vector for key to update or append correctly describes this approach; others ignore key checks or hashing.

4. Final Answer:

   Store key-value pairs in a vector at each bucket; on insert, search vector for key to update or append -> Option A

5. Quick Check:

   Separate chaining + key check = efficient insert [OK]

Hint: Search bucket vector for key before insert [OK]

Common Mistakes:

• Ignoring key existence in bucket
• Not using hashing for bucket selection
• Replacing bucket data blindly