Why Hash Map Exists and What Problem It Solves in DSA Rust - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty hash map in Rust.

DSA Rust

let mut scores = std::collections::HashMap::[1]();

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

Anew

Bcreate

Cinit

Dbuild

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods like create or init which do not exist for HashMap.

Trying to instantiate HashMap without calling a method.

The HashMap::new() function creates an empty hash map in Rust.

2fill in blank

medium

Complete the code to insert a key-value pair into the hash map.

DSA Rust

scores.[1]("Blue", 10);

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

Aadd

Binsert

Cpush

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using add or push which are not methods for HashMap.

Using set which is not a Rust HashMap method.

The insert method adds a key-value pair to a HashMap in Rust.

3fill in blank

hard

Fix the error in the code to get the value for a key from the hash map.

DSA Rust

let score = scores.[1]("Blue");

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

Alookup

Bget_value

Cfind

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using get_value or find which do not exist for HashMap.

Using lookup which is not a Rust HashMap method.

The get method returns an Option with the value for the given key in a HashMap.

4fill in blank

hard

Fill both blanks to check if a key exists and then print its value.

DSA Rust

if scores.[1]("Blue") {
    println!("Score: {}", scores.[2]("Blue").unwrap());
}

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

Aget

Bcontains_key

Cinsert

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert or remove instead of contains_key or get.

Mixing up the order of methods.

contains_key checks if the key exists, and get retrieves the value.

5fill in blank

hard

Fill all three blanks to create a hash map from a vector of tuples and print all key-value pairs.

DSA Rust

let teams = vec![("Blue", 10), ("Yellow", 50)];
let scores: std::collections::HashMap<_, _> = teams.[1]().[2]();
for (key, value) in scores.[3]() {
    println!("{}: {}", key, value);
}

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

Ainto_iter

Bcollect

Citer

Dkeys

Attempts:

3 left

💡 Hint

Common Mistakes

Using keys() instead of iter() to loop over entries.

Using iter() instead of into_iter() when collecting.

Forgetting to collect into a HashMap.

into_iter converts the vector into an iterator, collect gathers items into a HashMap, and iter iterates over key-value pairs.

Practice

1. Why do we use a HashMap in Rust instead of a simple list or array?

easy

A. To save memory by compressing data

B. To store values in a sorted order automatically

C. To quickly find values by keys without searching the entire collection

D. To allow duplicate keys with different values

Solution

1. Step 1: Understand the problem with lists/arrays

   Lists require checking each item one by one to find a value, which is slow for large data.

2. Step 2: How HashMap solves this

   HashMap uses keys to jump directly to the value, making lookup very fast.

3. Final Answer:

   To quickly find values by keys without searching the entire collection -> Option C

4. Quick Check:

   Fast key-based lookup = C [OK]

Hint: HashMap = fast key lookup, not ordered or duplicates [OK]

Common Mistakes:

• Thinking HashMap sorts data automatically
• Believing HashMap compresses data
• Assuming HashMap allows duplicate keys

2. Which of the following is the correct way to create an empty HashMap in Rust?

easy

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

B. let map = HashMap();

C. let map = new HashMap();

D. let map = HashMap[];

Solution

1. Step 1: Recall Rust syntax for creating HashMap

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

2. Step 2: Check each option

   Only HashMap::new() is valid syntax; others are incorrect.

3. Final Answer:

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

4. Quick Check:

   Correct Rust HashMap creation = B [OK]

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

Common Mistakes:

• Using parentheses instead of ::new()
• Trying to use new keyword like other languages
• Using square brackets for initialization

3. What will be the output of this Rust code?

use std::collections::HashMap;

fn main() {
    let mut map = HashMap::new();
    map.insert("apple", 3);
    map.insert("banana", 2);
    map.insert("apple", 5);
    println!("{:?}", map.get("apple"));
}

medium

A. Some(3)

B. Compilation error

C. None

D. Some(5)

Solution

1. Step 1: Understand insert behavior in HashMap

   Inserting a key that already exists updates its value to the new one.

2. Step 2: Trace the insertions and get

   "apple" first gets 3, then updated to 5. So, map.get("apple") returns Some(5).

3. Final Answer:

   Some(5) -> Option D

4. Quick Check:

   Updated value returned = D [OK]

Hint: Inserting same key updates value, get returns latest [OK]

Common Mistakes:

• Thinking insert adds duplicate keys
• Expecting original value after update
• Confusing None with Some

4. Identify the error in this Rust code using HashMap:

use std::collections::HashMap;

fn main() {
    let mut map = HashMap::new();
    map.insert("key1", 10);
    let value = map["key2"];
    println!("{}", value);
}

medium

A. HashMap::new() is not the correct way to create a map

B. Using square brackets to access a non-existent key causes panic

C. Cannot insert values into a mutable HashMap

D. Missing semicolon after insert statement

Solution

1. Step 1: Check how map access works

   Using square brackets with a key that does not exist causes the program to panic at runtime.

2. Step 2: Identify the error in code

   Accessing "key2" which was never inserted triggers panic, causing runtime error.

3. Final Answer:

   Using square brackets to access a non-existent key causes panic -> Option B

4. Quick Check:

   Non-existent key access panics = A [OK]

Hint: Square brackets panic if key missing; use get() for safe access [OK]

Common Mistakes:

• Thinking HashMap::new() is wrong
• Believing insert needs immutable map
• Ignoring runtime panic on missing key

5. You want to count how many times each word appears in a list using a HashMap in Rust. Which approach correctly solves this problem?

hard

A. Use entry() method to update count or insert 1 if missing

B. Insert each word with value 1, then insert again with value 1 for duplicates

C. Store words in a vector and count manually later

D. Use a HashMap but clear it after each insertion

Solution

1. Step 1: Understand counting with HashMap

   We need to increase count if word exists, or add with count 1 if new.

2. Step 2: Use entry() API for efficient update

   The entry() method allows checking and updating count in one step safely.

3. Final Answer:

   Use entry() method to update count or insert 1 if missing -> Option A

4. Quick Check:

   Counting words with entry() = A [OK]

Hint: Use entry().or_insert(0) to count frequencies easily [OK]

Common Mistakes:

• Inserting 1 repeatedly without updating count
• Using vector instead of HashMap for counting
• Clearing map after each insert