Why Hash Map Exists and What Problem It Solves in DSA Typescript - Quick Recap

Recall & Review

beginner

What problem does a hash map solve in data storage and retrieval?

A hash map solves the problem of quickly finding, adding, or removing data by using a key to directly access the stored value, avoiding slow searches through all data.

beginner

How does a hash map improve performance compared to a list when searching for an item?

A hash map uses a key to jump directly to the data location, making search operations very fast (close to constant time), while a list requires checking items one by one, which takes longer as the list grows.

beginner

What is a 'key' in the context of a hash map?

A key is a unique identifier used to store and find a value in a hash map. It acts like a label or name that points directly to the data.

intermediate

Why can't we just use arrays or lists for all data storage needs instead of hash maps?

Arrays or lists require checking each item one by one to find a value, which is slow for large data. Hash maps use keys and hashing to find data instantly, making them better for fast lookups.

intermediate

What is the role of a hash function in a hash map?

A hash function converts a key into an index or address where the value is stored, allowing quick access without searching through all data.

What is the main advantage of using a hash map over a list for data retrieval?

A Automatically sorts data

B Faster access using keys

C Stores data in order

D Uses less memory

Hash maps allow fast access to data using keys, unlike lists which require searching through items.

In a hash map, what does the key do?

A Stores the value

B Deletes the value

C Sorts the data

D Identifies where the value is stored

The key identifies the location of the value in the hash map for quick access.

Why is searching in a list slower than in a hash map?

A Lists require checking each item one by one

B Lists store data randomly

C Lists use keys for access

D Lists have no values

Lists require checking each item until the target is found, which takes more time as the list grows.

What does a hash function do in a hash map?

A Sorts the keys alphabetically

B Deletes duplicate keys

C Converts a key into an index for storage

D Stores values in a list

A hash function converts a key into an index to quickly find where the value is stored.

Which of these is NOT a reason to use a hash map?

A Maintaining data order

B Direct access using keys

C Fast data lookup

D Efficient insertion and deletion

Hash maps do not maintain data order; they focus on fast access and efficient operations.

Explain why hash maps exist and what problem they solve in simple terms.

Think about how you find a friend's phone number quickly without checking every contact.

Describe how a hash function helps a hash map work efficiently.

Imagine a librarian who uses a code to find a book's shelf instantly.

Practice

1. Why do we use a Hash Map instead of a simple list to store key-value pairs?

easy

A. Because it allows fast lookup of values by keys

B. Because it stores data in sorted order automatically

C. Because it uses less memory than a list

D. Because it can only store numbers

Solution

1. Step 1: Understand the problem with lists

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

2. Step 2: How hash maps solve this

   Hash maps use a special function to find the exact place of a key quickly, making lookup fast.

3. Final Answer:

   Because it allows fast lookup of values by keys -> Option A

4. Quick Check:

   Fast key lookup = Because it allows fast lookup of values by keys [OK]

Hint: Hash maps speed up key-based searches [OK]

Common Mistakes:

• Thinking hash maps sort data automatically
• Believing hash maps use less memory always
• Assuming hash maps only store numbers

2. Which of the following is the correct way to create a hash map (Map) in TypeScript?

easy

A. const map = new Map();

B. const map = Map();

C. const map = new map();

D. const map = new Map(); string, number;

Solution

1. Step 1: Recall TypeScript Map syntax

   In TypeScript, a Map is created using new Map<KeyType, ValueType>().

2. Step 2: Check each option

   const map = new Map(); uses correct syntax with new Map<string, number>(). Others have syntax errors or wrong casing.

3. Final Answer:

   const map = new Map<string, number>(); -> Option A

4. Quick Check:

   Correct Map creation syntax = const map = new Map<string, number>(); [OK]

Hint: Use 'new Map<KeyType, ValueType>()' to create hash maps [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using lowercase 'map' instead of 'Map'
• Incorrect generic syntax

3. What will be the output of the following TypeScript code?

const map = new Map();
map.set('apple', 5);
map.set('banana', 3);
map.set('apple', 10);
console.log(map.get('apple'));

medium

A. Error

B. 5

C. undefined

D. 10

Solution

1. Step 1: Understand map.set behavior

   When setting a key that already exists, the value is updated to the new one.

2. Step 2: Trace the code

   First 'apple' is set to 5, then updated to 10. So map.get('apple') returns 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Updated value for 'apple' = 10 [OK]

Hint: Last set value for a key overwrites previous [OK]

Common Mistakes:

• Assuming first value remains
• Thinking get returns undefined if key repeated
• Expecting error on duplicate keys

4. Identify the error in this TypeScript code that tries to use a hash map:

const map = new Map();
map['key'] = 'value';
console.log(map.get('key'));

medium

A. map.get() cannot be used to retrieve values

B. Map constructor requires key and value types

C. Using bracket notation to add key-value pairs does not work with Map

D. Map keys must be numbers only

Solution

1. Step 1: Understand how to add entries to Map

   Map entries must be added using map.set(key, value), not bracket notation.

2. Step 2: Analyze the code behavior

   Using map['key'] = 'value' adds a property to the object, not a Map entry, so map.get('key') returns undefined.

3. Final Answer:

   Using bracket notation to add key-value pairs does not work with Map -> Option C

4. Quick Check:

   Use map.set() to add entries [OK]

Hint: Use map.set() to add entries, not bracket notation [OK]

Common Mistakes:

• Adding entries with bracket notation
• Expecting map.get() to find bracket-added keys
• Thinking Map keys must be numbers

5. You want to count how many times each word appears in a list of words using a hash map in TypeScript. Which approach correctly solves this problem?

hard

A. Use a Set to store words and count size for each word

B. Use a Map, for each word check if it exists, then increment or set to 1

C. Use a Map with word as value and count as key

D. Use an array and push words, then count duplicates manually later

Solution

1. Step 1: Understand the counting problem

   We need to store each word as a key and count as a value, so Map<string, number> fits well.

2. Step 2: How to update counts

   For each word, check if it exists in the map; if yes, increment count, else set count to 1.

3. Step 3: Why other options fail

   Use an array and push words, then count duplicates manually later is inefficient and manual. Use a Map with word as value and count as key swaps key and value wrongly. Use a Set to store words and count size for each word's Set only stores unique words without counts.

4. Final Answer:

   Use a Map<string, number>, for each word check if it exists, then increment or set to 1 -> Option B

5. Quick Check:

   Counting words = Map with word keys and number counts [OK]

Hint: Map keys = words, values = counts; increment or set 1 [OK]

Common Mistakes:

• Using Set which can't count duplicates
• Swapping key and value types in Map
• Counting manually with arrays inefficiently