Substring Search Patterns in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the pattern exists in the text using includes method.

DSA Javascript

const text = 'hello world';
const pattern = 'world';
const found = text.[1](pattern);
console.log(found);

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

Amatch

BindexOf

Csearch

Dincludes

Attempts:

3 left

💡 Hint

Common Mistakes

Using indexOf returns position, not boolean.

Using search returns index or -1, not boolean.

The includes() method returns true if the pattern is found in the text, otherwise false.

2fill in blank

medium

Complete the code to find the first index of the pattern in the text.

DSA Javascript

const text = 'abracadabra';
const pattern = 'cad';
const index = text.[1](pattern);
console.log(index);

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

AindexOf

Bmatch

Csearch

Dincludes

Attempts:

3 left

💡 Hint

Common Mistakes

Using includes returns boolean, not index.

Using match returns array or null, not index.

indexOf() returns the first index where the pattern starts or -1 if not found.

3fill in blank

hard

Fix the error in the code to correctly find all occurrences of the pattern using RegExp.

DSA Javascript

const text = 'banana bandana';
const pattern = /ana/g;
const matches = text.[1](pattern);
console.log(matches);

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

Amatch

Bsearch

CmatchAll

DindexOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using search returns only first index.

Using matchAll returns an iterator, not array.

match() with global flag returns an array of all matches.

4fill in blank

hard

Fill both blanks to extract all starting indices of the pattern in the text.

DSA Javascript

const text = 'testtesttest';
const pattern = /test/g;
const indices = [];
let match;
while ((match = pattern.[1](text)) !== null) {
  indices.push(match.[2]);
}
console.log(indices);

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

Aexec

Bindex

ClastIndex

Dsearch

Attempts:

3 left

💡 Hint

Common Mistakes

Using search does not return match object.

Using lastIndex is a property of RegExp, not match.

exec() returns match object with index property showing start position.

5fill in blank

hard

Fill all three blanks to create a function that returns all indices of a substring in a string.

DSA Javascript

function findAllIndices(text, pattern) {
  const indices = [];
  let pos = 0;
  while (true) {
    const index = text.[1](pattern, pos);
    if (index === [2]) break;
    indices.push(index);
    pos = index + [3];
  }
  return indices;
}

console.log(findAllIndices('abracadabra', 'abra'));

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

AindexOf

B-1

Cpattern.length

Dincludes

Attempts:

3 left

💡 Hint

Common Mistakes

Using includes returns boolean, not index.

Not advancing pos causes infinite loop.

indexOf finds substring from position pos; -1 means no more matches; advance pos by pattern length.

Practice

1. What does the indexOf method do when used on a string in JavaScript?

easy

A. Converts the string to uppercase

B. Replaces a substring with another string

C. Splits the string into an array of substrings

D. Finds the position of the first occurrence of a substring

Solution

1. Step 1: Understand the purpose of indexOf

   The indexOf method searches a string for a specified substring and returns the position of its first occurrence.

2. Step 2: Confirm behavior on strings

   If the substring is found, it returns the zero-based index; if not found, it returns -1.

3. Final Answer:

   Finds the position of the first occurrence of a substring -> Option D

4. Quick Check:

   indexOf = position of substring [OK]

Hint: Remember: indexOf returns position or -1 if not found [OK]

Common Mistakes:

• Confusing indexOf with replace method
• Thinking indexOf splits the string
• Assuming indexOf changes the string case

2. Which of the following is the correct syntax to check if the substring 'cat' exists in the string text using includes method?

easy

A. text.include('cat')

B. includes(text, 'cat')

C. text.includes('cat')

D. text.contains('cat')

Solution

1. Step 1: Recall the correct method name

   The correct method to check substring presence is includes, called on the string object.

2. Step 2: Verify syntax

   The syntax is string.includes(substring), so text.includes('cat') is correct.

3. Final Answer:

   text.includes('cat') -> Option C

4. Quick Check:

   Correct method call syntax = text.includes('cat') [OK]

Hint: Use string.includes(substring) to check presence [OK]

Common Mistakes:

• Using wrong method name like include or contains
• Calling includes as a standalone function
• Passing arguments in wrong order

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

const text = 'hello world';
const pos = text.indexOf('o');
console.log(pos);

medium

A. 4

B. 5

C. -1

D. 6

Solution

1. Step 1: Identify the string and substring

   The string is 'hello world' and the substring searched is 'o'.

2. Step 2: Find the first occurrence of 'o'

   Counting from zero: h(0) e(1) l(2) l(3) o(4) (space 5) w(6) o(7). The first 'o' is at index 4.

3. Step 3: Verify the output

   The code uses indexOf('o') which returns the first occurrence index, which is 4.

4. Final Answer:

   4 -> Option A

5. Quick Check:

   First 'o' index = 4 [OK]

Hint: Count string index from zero to find first substring position [OK]

Common Mistakes:

• Counting index starting from 1 instead of 0
• Confusing second 'o' position with first
• Assuming indexOf returns -1 if substring exists

4. The following code is intended to check if the substring 'test' exists in str. What is the error?

const str = 'this is a test';
if (str.indexOf == 'test') {
  console.log('Found');
} else {
  console.log('Not Found');
}

medium

A. The variable str is not declared properly

B. indexOf is used without parentheses, so it compares function to string

C. The if condition should use === instead of ==

D. The substring 'test' is misspelled

Solution

1. Step 1: Analyze the use of indexOf

   The code uses str.indexOf without parentheses, so it refers to the function itself, not the result of calling it.

2. Step 2: Understand the comparison

   Comparing a function to a string ('test') is always false, so the condition is incorrect.

3. Final Answer:

   indexOf is used without parentheses, so it compares function to string -> Option B

4. Quick Check:

   Missing parentheses on indexOf causes error [OK]

Hint: Always call indexOf with parentheses and substring argument [OK]

Common Mistakes:

• Forgetting parentheses on indexOf
• Using wrong comparison operator
• Assuming indexOf returns boolean directly

5. You want to find all starting positions of the substring 'ana' in the string 'banana'. Which approach correctly finds all positions?

hard

A. Use a loop with indexOf and update the start position after each find

B. Use includes repeatedly until it returns false

C. Use lastIndexOf to find all positions from the end

D. Use replace to remove substring and count occurrences

Solution

1. Step 1: Understand the problem of multiple occurrences

   To find all positions, you must repeatedly search starting after the last found position.

2. Step 2: Use a loop with indexOf and update start

   Use indexOf('ana', start) in a loop, updating start to position + 1 each time until no more found.

3. Final Answer:

   Use a loop with indexOf and update the start position after each find -> Option A

4. Quick Check:

   Loop + indexOf with updated start = all positions [OK]

Hint: Loop with indexOf(start) to find all substring positions [OK]

Common Mistakes:

• Using includes which returns only true/false
• Using lastIndexOf which finds only last occurrence
• Using replace which changes string and loses positions