Character Frequency Counting in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize an empty object to store character counts.

DSA Javascript

const freq = [1];

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

A{}

B[]

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using an array [] instead of an object {}

Initializing with null or 0 which are not containers

We use an empty object {} to store character frequencies as key-value pairs.

2fill in blank

medium

Complete the code to loop through each character in the string.

DSA Javascript

for (const char [1] str) {

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

Ato

Bin

Cof

Dfrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'in' which loops over indices, not characters

Using 'to' or 'from' which are invalid in this context

Use for...of to iterate over characters in a string.

3fill in blank

hard

Fix the error in updating the frequency count for each character.

DSA Javascript

freq[char] = (freq[char] [1] 0) + 1;

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

A||

B-

C+

D&&

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' which causes NaN when freq[char] is undefined

Using '-' or '&&' which do not provide default values

The || operator returns the existing count or 0 if undefined, then adds 1.

4fill in blank

hard

Fill both blanks to create a function that returns the frequency object for a given string.

DSA Javascript

function charFreq(str) {
  const freq = [1];
  for (const char [2] str) {
    freq[char] = (freq[char] || 0) + 1;
  }
  return freq;
}

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

A{}

B[]

Cin

Dof

Attempts:

3 left

💡 Hint

Common Mistakes

Using array [] instead of object {}

Using 'in' which loops over indices, not characters

Initialize freq as an empty object {} and loop over characters using for...of.

5fill in blank

hard

Fill all three blanks to create a function that returns a formatted string showing character counts.

DSA Javascript

function formatFreq(str) {
  const freq = charFreq(str);
  let result = '';
  for (const [1] in freq) {
    result += [2] + freq[[3]] + '\n';
  }
  return result;
}

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

Achar

Bchar + ': '

Cfreq

Dchar + ' -> '

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'freq' as loop variable which is the object, not key

Concatenating without separator or wrong format

Accessing freq with wrong variable

Loop over keys with char, append char + ' -> ' plus count, and access count with freq[char].

Practice

1. What does character frequency counting mean in simple terms?

easy

A. Sorting characters alphabetically

B. Counting how many times each character appears in a string

C. Replacing characters with numbers

D. Removing duplicate characters from a string

Solution

1. Step 1: Understand the meaning of frequency

   Frequency means how often something appears.

2. Step 2: Apply to characters in a string

   Counting frequency means counting how many times each character shows up in the string.

3. Final Answer:

   Counting how many times each character appears in a string -> Option B

4. Quick Check:

   Frequency counting = Counting characters [OK]

Hint: Frequency means count occurrences of each character [OK]

Common Mistakes:

• Confusing frequency with sorting
• Thinking frequency means removing duplicates
• Mixing frequency with character replacement

2. Which of the following JavaScript code snippets correctly initializes an empty object to store character frequencies?

easy

A. const freq = '';

B. const freq = [];

C. const freq = {};

D. const freq = null;

Solution

1. Step 1: Identify the data structure for frequency counting

   We use an object to store characters as keys and counts as values.

2. Step 2: Check correct syntax for empty object

   Empty object is created with curly braces {} in JavaScript.

3. Final Answer:

   const freq = {}; -> Option C

4. Quick Check:

   Empty object = {} [OK]

Hint: Use curly braces {} to create empty object [OK]

Common Mistakes:

• Using array [] instead of object {}
• Using empty string '' instead of object
• Assigning null which is not an object

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

const str = 'hello';
const freq = {};
for (const ch of str) {
  freq[ch] = (freq[ch] || 0) + 1;
}
console.log(freq);

medium

A. {"h":1,"e":1,"l":2,"o":1}

B. {"h":1,"e":1,"l":1,"o":1}

C. {"h":0,"e":0,"l":0,"o":0}

D. SyntaxError

Solution

1. Step 1: Loop through each character in 'hello'

   Characters are 'h', 'e', 'l', 'l', 'o'.

2. Step 2: Count frequency using freq[ch] = (freq[ch] || 0) + 1

   For each character, add 1 to its count or start at 1 if not present.

3. Final Answer:

   {"h":1,"e":1,"l":2,"o":1} -> Option A

4. Quick Check:

   Count 'l' twice, others once [OK]

Hint: Use freq[ch] = (freq[ch] || 0) + 1 to count [OK]

Common Mistakes:

• Counting 'l' only once
• Getting syntax error from code
• Confusing keys and values in output

4. Find the error in this code that counts character frequency:

const str = 'abc';
const freq = {};
for (let i = 0; i <= str.length; i++) {
  const ch = str[i];
  freq[ch] = (freq[ch] || 0) + 1;
}
console.log(freq);

medium

A. Loop condition should be i < str.length, not i <= str.length

B. freq should be initialized as an array, not object

C. Use let instead of const for freq

D. Missing semicolon after freq[ch] assignment

Solution

1. Step 1: Check loop range

   Loop runs from i = 0 to i <= str.length, which means i goes up to str.length (3 for 'abc').

2. Step 2: Identify out-of-bounds access

   str[3] is undefined, so freq[undefined] is updated causing wrong key.

3. Final Answer:

   Loop condition should be i < str.length, not i <= str.length -> Option A

4. Quick Check:

   Array index max is length - 1 [OK]

Hint: Use i < length to avoid out-of-bounds errors [OK]

Common Mistakes:

• Using <= instead of < in loop
• Initializing freq as array instead of object
• Ignoring undefined character access

5. Given the string "aabbccddeeffgghhii", which code snippet correctly counts only characters that appear more than once and stores them in freq object?

hard

A. const freq = {};
for (const ch of str) {
freq[ch] = (freq[ch] || 0) + 1;
}
const filtered = {};
for (const ch in freq) {
if (freq[ch] > 1) filtered[ch] = freq[ch];
}

B. const freq = {};
for (const ch of str) {
if (freq[ch]) freq[ch]++;
else freq[ch] = 1;
}
for (const ch in freq) {
if (freq[ch] < 2) freq[ch] = 0;
}

C. const freq = {};
for (const ch of str) {
freq[ch] = 1;
}
for (const ch in freq) {
if (freq[ch] > 1) delete freq[ch];
}

D. const freq = {};
for (const ch of str) {
freq[ch] = (freq[ch] || 0) + 1;
}
for (const ch in freq) {
if (freq[ch] <= 1) delete freq[ch];
}

Solution

1. Step 1: Count all character frequencies

   Use freq[ch] = (freq[ch] || 0) + 1 to count all characters.

2. Step 2: Delete characters with frequency <= 1 from freq

   Iterate and delete keys where freq[ch] <= 1 to leave only duplicates in freq.

3. Final Answer:

   Code snippet that counts all then deletes singles from freq -> Option D

4. Quick Check:

   Delete freq <= 1 from freq [OK]

Hint: Count all, then delete characters with count <= 1 [OK]

Common Mistakes:

• Deleting keys while iterating freq object
• Setting frequency to 0 instead of removing
• Overwriting counts incorrectly