Count Words with Given Prefix in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a word starts with the given prefix.

DSA Typescript

function startsWith(word: string, prefix: string): boolean {
  return word.[1](prefix);
}

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

AindexOf

Bincludes

CendsWith

DstartsWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'includes' checks anywhere in the string, not just the start.

The startsWith method checks if the string begins with the given prefix.

2fill in blank

medium

Complete the code to count how many words start with the prefix.

DSA Typescript

function countWords(words: string[], prefix: string): number {
  let count = 0;
  for (const word of words) {
    if (word.[1](prefix)) {
      count++;
    }
  }
  return count;
}

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

Aincludes

BstartsWith

CendsWith

DindexOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'includes' counts words that have the prefix anywhere, not just at the start.

We use startsWith to check if each word begins with the prefix and count those that do.

3fill in blank

hard

Fix the error in the function to return the correct count of words starting with prefix.

DSA Typescript

function countWords(words: string[], prefix: string): number {
  let count = 0;
  for (const word of words) {
    if (word.[1](prefix)) {
      count += 1;
    }
  }
  return count;
}

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

AstartsWith

Bincludes

CendsWith

Dsubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'includes' or 'endsWith' instead of 'startsWith'.

The correct method to check if a word starts with a prefix is startsWith.

4fill in blank

hard

Fill both blanks to create a function that returns a list of words starting with the prefix.

DSA Typescript

function filterWords(words: string[], prefix: string): string[] {
  return words.[1](word => word.[2](prefix));
}

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

Afilter

Bmap

CstartsWith

Dincludes

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'map' instead of 'filter' changes the array instead of selecting items.

Use filter to select words, and startsWith to check the prefix.

5fill in blank

hard

Fill all three blanks to create a function that returns a dictionary with counts of words starting with each prefix in prefixes array.

DSA Typescript

function countWordsByPrefixes(words: string[], prefixes: string[]): {[key: string]: number} {
  const result: {[key: string]: number} = {};
  for (const prefix of prefixes) {
    result[[1]] = words.filter(word => word.[2](prefix)).[3];
  }
  return result;
}

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

Aprefix

BstartsWith

Clength

Dincludes

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'includes' instead of 'startsWith' causes wrong counts.

We use the prefix as the key, filter words starting with it, and get the count using length.

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