Substring Search Patterns in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of substring search in strings?

To find if a smaller string (pattern) exists inside a larger string (text), and if so, where it starts.

beginner

Explain the naive substring search approach.

Check every position in the text to see if the pattern matches starting there. Simple but can be slow for large texts.

intermediate

What is the key idea behind the Knuth-Morris-Pratt (KMP) algorithm?

Use information from previous matches to avoid re-checking characters, by building a prefix table that tells where to continue after a mismatch.

intermediate

What does the prefix table (also called lps array) represent in KMP?

For each position in the pattern, it stores the length of the longest prefix that is also a suffix up to that position.

intermediate

Why is KMP more efficient than naive substring search?

Because it never moves backward in the text, it uses the prefix table to skip unnecessary comparisons, making it run in linear time.

What does substring search try to find?

A The last character of a string

B The length of a string

C A smaller string inside a larger string

D The number of vowels in a string

Substring search looks for a smaller string (pattern) inside a bigger string (text).

In naive substring search, what do we do at each position in the text?

A Check if the pattern matches starting there

B Skip the position

C Reverse the text

D Sort the characters

Naive search checks if the pattern matches starting at each position in the text.

What does the prefix table in KMP help us avoid?

A Re-checking characters after a mismatch

B Finding the pattern

C Counting characters

D Reversing the pattern

The prefix table helps KMP avoid re-checking characters that we already know match.

What is stored in the prefix table (lps array) in KMP?

A Positions of vowels in the pattern

B Length of longest prefix which is also suffix for each pattern position

C Number of mismatches

D Length of the text

The prefix table stores the longest prefix which is also suffix length for each position in the pattern.

Why is KMP faster than naive search?

A It sorts the pattern

B It checks every character twice

C It reverses the text

D It skips unnecessary comparisons using the prefix table

KMP uses the prefix table to skip unnecessary comparisons, making it faster.

Describe how the naive substring search algorithm works step-by-step.

Think about sliding the pattern over the text one by one.

Explain the purpose and construction of the prefix table in the KMP algorithm.

Focus on how the pattern itself helps avoid re-checking.

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