Count Words with Given Prefix in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

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

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

Aincludes

BstartsWith

CendsWith

DindexOf

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using 'endsWith' which checks the end of the word.

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 Javascript

function countWords(words, prefix) {
  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'

AstartsWith

BindexOf

CendsWith

Dincludes

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'includes' which counts words containing prefix anywhere.

Using 'endsWith' which checks the end of the word.

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

3fill in blank

hard

Fix the error in the code to correctly count words with the prefix.

DSA Javascript

function countWords(words, prefix) {
  return words.filter(word => word.[1](prefix)).length;
}

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

AendsWith

Bincludes

CstartsWith

DindexOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'includes' which matches anywhere in the word.

Using 'endsWith' which checks the end of the word.

The filter method keeps words starting with the prefix using startsWith, then counts them.

4fill in blank

hard

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

DSA Javascript

function filterWords(words, prefix) {
  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' which changes the array instead of selecting.

Using 'includes' which matches prefix anywhere, not just start.

filter selects words where startsWith returns true for the prefix.

5fill in blank

hard

Fill all three blanks to create a function that returns the count of words starting with the prefix using reduce.

DSA Javascript

function countWordsReduce(words, prefix) {
  return words.reduce((count, word) => {
    return count + (word.[1](prefix) ? [2] : [3]);
  }, 0);
}

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

AstartsWith

B1

C0

Dincludes

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'includes' instead of 'startsWith'.

Adding wrong values instead of 1 or 0.

The reduce method adds 1 to count if the word starts with prefix, else adds 0.

Practice

1. What does the function countWordsWithPrefix(words, prefix) do when given a list of words and a prefix?

easy

A. Counts how many words contain the prefix anywhere

B. Counts the total number of words in the list

C. Counts how many words end with the given prefix

D. Counts how many words start with the given prefix

Solution

1. Step 1: Understand the function purpose

   The function checks each word to see if it starts with the given prefix.

2. Step 2: Count matching words

   It counts only those words that begin exactly with the prefix, ignoring others.

3. Final Answer:

   Counts how many words start with the given prefix -> Option D

4. Quick Check:

   Starts with prefix = Count [OK]

Hint: Look for words starting exactly with prefix [OK]

Common Mistakes:

• Counting words containing prefix anywhere
• Counting words ending with prefix
• Counting total words instead of filtered ones

2. Which of the following is the correct syntax to check if a string word starts with a prefix prefix in JavaScript?

easy

A. word.beginWith(prefix)

B. word.hasPrefix(prefix)

C. word.startsWith(prefix)

D. word.starts(prefix)

Solution

1. Step 1: Recall JavaScript string methods

   JavaScript strings have a method called startsWith() to check prefix.

2. Step 2: Match method name

   Only startsWith() is the correct method; others do not exist.

3. Final Answer:

   word.startsWith(prefix) -> Option C

4. Quick Check:

   Correct method name = startsWith [OK]

Hint: Use startsWith() method for prefix check [OK]

Common Mistakes:

• Using non-existent methods like hasPrefix
• Using incorrect method names like beginWith
• Confusing startsWith with other string methods

3. What is the output of the following code?

const words = ['apple', 'app', 'application', 'banana'];
const prefix = 'app';
function countWordsWithPrefix(words, prefix) {
  let count = 0;
  for (const word of words) {
    if (word.startsWith(prefix)) {
      count++;
    }
  }
  return count;
}
console.log(countWordsWithPrefix(words, prefix));

medium

A. 3

B. 2

C. 1

D. 4

Solution

1. Step 1: Check each word against prefix 'app'

   'apple', 'app', and 'application' all start with 'app'. 'banana' does not.

2. Step 2: Count matching words

   There are 3 words starting with 'app', so count is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Words starting with 'app' = 3 [OK]

Hint: Count words starting exactly with prefix [OK]

Common Mistakes:

• Counting words containing prefix anywhere
• Missing one of the matching words
• Counting all words regardless of prefix

4. The following code is intended to count words starting with a prefix, but it has a bug. What is the bug?

function countWordsWithPrefix(words, prefix) {
  let count = 0;
  for (let i = 0; i <= words.length; i++) {
    if (words[i].startsWith(prefix)) {
      count++;
    }
  }
  return count;
}

medium

A. startsWith method is used incorrectly

B. Loop goes out of bounds causing an error

C. count variable is not initialized

D. Prefix is not passed as a parameter

Solution

1. Step 1: Analyze loop condition

   The loop runs with i from 0 to words.length inclusive, but array indices go from 0 to words.length - 1.

2. Step 2: Identify out of bounds access

   At i = words.length, words[i] is undefined, so calling startsWith causes an error.

3. Final Answer:

   Loop goes out of bounds causing an error -> Option B

4. Quick Check:

   Loop index must be less than length [OK]

Hint: Use i < words.length, not <= [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Not checking array bounds before access
• Assuming startsWith works on undefined

5. Given the array words = ['Test', 'testing', 'Tester', 'tent', 'team'] and prefix 'te', which code snippet correctly counts words starting with the prefix ignoring case sensitivity?

hard

A. words.filter(w => w.toLowerCase().startsWith(prefix.toLowerCase())).length

B. words.filter(w => w.startsWith(prefix)).length

C. words.filter(w => w.includes(prefix)).length

D. words.filter(w => w.endsWith(prefix)).length

Solution

1. Step 1: Understand case sensitivity issue

   Direct startsWith is case sensitive, so 'Test' and 'test' differ.

2. Step 2: Use toLowerCase() to normalize

   Convert both word and prefix to lowercase before checking startsWith to ignore case.

3. Final Answer:

   words.filter(w => w.toLowerCase().startsWith(prefix.toLowerCase())).length -> Option A

4. Quick Check:

   Normalize case before prefix check [OK]

Hint: Convert both strings to lowercase before startsWith [OK]

Common Mistakes:

• Ignoring case sensitivity
• Using includes instead of startsWith
• Using endsWith instead of startsWith