Why Hash Map Exists and What Problem It Solves in DSA Typescript - See It Work

Why Hash Map Exists and What Problem It Solves

📖 Scenario: Imagine you have a big box of keys and you want to find the right key quickly to open a door. Without any order, you have to check each key one by one, which takes a lot of time. This is like searching for a value in a list. A hash map is like having a special box with labeled slots where each key goes directly to its own slot, so you can find it instantly.

🎯 Goal: You will create a simple example to show how a hash map helps find values quickly by using keys, compared to searching in a list.

📋 What You'll Learn

Create a list of keys and values representing items and their prices

Create a hash map (object) with the same items and prices

Write code to find the price of a specific item by searching the list

Write code to find the price of the same item using the hash map

Print both results to compare the process

💡 Why This Matters

🌍 Real World

Hash maps are used in apps and websites to quickly find user data, settings, or any information by a key like username or ID.

💼 Career

Understanding hash maps is important for software developers to write fast and efficient code for searching and storing data.

1

Create a list of items with prices

Create a variable called itemsList that holds an array of objects. Each object should have name and price properties with these exact entries: { name: 'apple', price: 100 }, { name: 'banana', price: 50 }, { name: 'orange', price: 70 }.

DSA Typescript

// Create the itemsList array with objects for apple, banana, and orange
// Your code here

Hint

Use square brackets [] to create an array. Inside, add objects with name and price keys.

2

Create a hash map for items and prices

Create a variable called itemsMap that holds an object with keys as item names and values as their prices. Use these exact entries: 'apple': 100, 'banana': 50, 'orange': 70.

DSA Typescript

const itemsList = [
  { name: 'apple', price: 100 },
  { name: 'banana', price: 50 },
  { name: 'orange', price: 70 }
];
// Create the itemsMap object with keys and prices
// Your code here

Hint

Use curly braces {} to create an object. Each key is the item name in quotes, followed by a colon and the price.

3

Find price by searching the list

Write code to find the price of 'banana' by searching itemsList. Use a for loop with variables item to check each object's name. When item.name is 'banana', save item.price to a variable called priceFromList.

DSA Typescript

const itemsList = [
  { name: 'apple', price: 100 },
  { name: 'banana', price: 50 },
  { name: 'orange', price: 70 }
];

const itemsMap = {
  'apple': 100,
  'banana': 50,
  'orange': 70
};

// Find price of 'banana' by searching itemsList
// Your code here

Hint

Use a for...of loop to check each item. Use if to compare item.name with 'banana'. Assign item.price to priceFromList and use break to stop the loop.

4

Find price using the hash map and print both results

Get the price of 'banana' from itemsMap and save it to a variable called priceFromMap. Then print both priceFromList and priceFromMap using console.log.

DSA Typescript

const itemsList = [
  { name: 'apple', price: 100 },
  { name: 'banana', price: 50 },
  { name: 'orange', price: 70 }
];

const itemsMap = {
  'apple': 100,
  'banana': 50,
  'orange': 70
};

let priceFromList = 0;
for (const item of itemsList) {
  if (item.name === 'banana') {
    priceFromList = item.price;
    break;
  }
}

// Get priceFromMap from itemsMap and print both prices
// Your code here

Hint

Access the price from itemsMap using square brackets with the key 'banana'. Use console.log to print both prices on separate lines.

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