Count Words with Given Prefix in DSA Typescript - Build from Scratch

Count Words with Given Prefix

📖 Scenario: You are building a simple search feature for a book store's website. The website wants to count how many book titles start with a certain prefix typed by the user.

🎯 Goal: Build a TypeScript program that counts how many words in a list start with a given prefix.

📋 What You'll Learn

Create an array of strings called words with exact book titles

Create a string variable called prefix with the exact prefix to search

Use a for loop to count how many words start with prefix

Print the count as a number

💡 Why This Matters

🌍 Real World

Counting words with a prefix is useful in search engines, autocomplete features, and filtering lists in apps.

💼 Career

This skill helps in building efficient search and filtering functions in web and software development.

1

Create the list of book titles

Create an array called words with these exact strings: 'apple', 'app', 'application', 'banana', 'apricot'

DSA Typescript

const words = [
  // Your code here
];

Hint

Use square brackets [] to create an array and separate strings with commas.

2

Set the prefix to search

Create a string variable called prefix and set it to the exact value 'app'

DSA Typescript

const words = ['apple', 'app', 'application', 'banana', 'apricot'];

// Your code here

Hint

Use const prefix = 'app'; to create the prefix variable.

3

Count words starting with the prefix

Create a variable called count and set it to 0. Then use a for loop with variable word to go through words. Inside the loop, use word.startsWith(prefix) to check if the word starts with prefix. If yes, increase count by 1.

DSA Typescript

const words = ['apple', 'app', 'application', 'banana', 'apricot'];
const prefix = 'app';

// Your code here

Hint

Remember to initialize count before the loop and increase it inside the if block.

4

Print the count result

Use console.log(count) to print the number of words starting with the prefix.

DSA Typescript

const words = ['apple', 'app', 'application', 'banana', 'apricot'];
const prefix = 'app';

let count = 0;
for (const word of words) {
    if (word.startsWith(prefix)) {
        count++;
    }
}

// Your code here

Hint

The output should be the number 3 because three words start with 'app'.

Practice

1. What does the function countWordsStartingWith(words: string[], prefix: string): number do in the context of counting words with a given prefix?

easy

A. Counts the total number of characters in all words

B. Counts how many words contain the prefix anywhere inside

C. Counts how many words in the list start with the given prefix

D. Returns the longest word starting with the prefix

Solution

1. Step 1: Understand the function purpose

   The function is designed to count words that begin with a specific prefix, not just contain it anywhere.

2. Step 2: Compare options with function goal

   Only Counts how many words in the list start with the given prefix matches the goal of counting words starting exactly with the prefix.

3. Final Answer:

   Counts how many words in the list start with the given prefix -> Option C

4. Quick Check:

   Counting words starting with prefix = Counts how many words in the list start with the given prefix [OK]

Hint: Look for words starting exactly with prefix, not containing it [OK]

Common Mistakes:

• Confusing 'start with' and 'contain' prefix
• Thinking it returns words instead of count
• Assuming it counts characters, not words

2. Which of the following TypeScript code snippets correctly counts words starting with a prefix pre in an array words?

easy

A. words.filter(word => word.startsWith(pre)).length

B. words.map(word => word.startsWith(pre)).length

C. words.reduce((count, word) => count + word.includes(pre), 0)

D. words.forEach(word => word.startsWith(pre))

Solution

1. Step 1: Identify correct method to filter words

   The filter method returns an array of words starting with the prefix, then length counts them.

2. Step 2: Check other options for correctness

   words.map(word => word.startsWith(pre)).length returns an array of booleans, so length is total words, not count of matches. words.reduce((count, word) => count + word.includes(pre), 0) uses includes incorrectly and sums booleans without conversion. words.forEach(word => word.startsWith(pre)) does not return a count.

3. Final Answer:

   words.filter(word => word.startsWith(pre)).length -> Option A

4. Quick Check:

   Filter + length counts matches = words.filter(word => word.startsWith(pre)).length [OK]

Hint: Use filter with startsWith, then get length [OK]

Common Mistakes:

• Using map instead of filter for counting
• Using includes instead of startsWith
• Using forEach without counting

3. What is the output of the following TypeScript code?

const words = ['apple', 'app', 'application', 'banana', 'apply'];
const prefix = 'app';
const count = words.filter(word => word.startsWith(prefix)).length;
console.log(count);

medium

A. 3

B. 4

C. 5

D. 2

Solution

1. Step 1: Identify words starting with 'app'

   Words starting with 'app' are 'apple', 'app', 'application', and 'apply' (4 words).

2. Step 2: Count these words

   Counting them gives 4.

3. Final Answer:

   4 -> Option B

4. Quick Check:

   Words starting with 'app' = 4 [OK]

Hint: Count words starting exactly with prefix 'app' [OK]

Common Mistakes:

• Counting 'banana' mistakenly
• Counting only exact 'app' word
• Confusing includes with startsWith

4. The following TypeScript code is intended to count words starting with prefix pre, but it returns 0. What is the error?

const words = ['prefix', 'preach', 'prevent', 'post'];
const pre = 'pre';
let count = 0;
for (let i = 0; i < words.length; i++) {
  if (words[i].startWith(pre)) {
    count++;
  }
}
console.log(count);

medium

A. Method name should be startsWith, not startWith

B. Variable pre is not defined

C. Loop should start from 1, not 0

D. count should be initialized to 1

Solution

1. Step 1: Check method name correctness

   The method to check prefix is startsWith, but code uses startWith, which is undefined.

2. Step 2: Understand effect of typo

   Because of the typo, condition is always false, so count remains 0.

3. Final Answer:

   Method name should be startsWith, not startWith -> Option A

4. Quick Check:

   Correct method name fixes count [OK]

Hint: Check method spelling: startsWith, not startWith [OK]

Common Mistakes:

• Using startWith instead of startsWith
• Incorrect loop index start
• Wrong count initialization

5. Given a large list of words, which data structure would optimize counting how many words start with a given prefix efficiently for multiple queries?

hard

A. Array sorted alphabetically

B. Stack storing words

C. Hash Map with full words as keys

D. Trie (Prefix Tree)

Solution

1. Step 1: Understand prefix query optimization

   Trie stores words by characters in a tree, allowing fast prefix searches.

2. Step 2: Compare other data structures

   Sorted arrays require binary search but less efficient for many prefix queries. Hash maps store full words, not prefixes. Stacks do not support prefix queries efficiently.

3. Final Answer:

   Trie (Prefix Tree) -> Option D

4. Quick Check:

   Trie enables fast prefix counts [OK]

Hint: Use Trie for fast prefix counting in many queries [OK]

Common Mistakes:

• Choosing array or hash map for prefix queries
• Not knowing Trie supports prefix efficiently
• Confusing stack usage for prefix search