Substring Search Patterns in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C#

bool ContainsPattern(string text, string pattern) {
    return text.[1](pattern) != -1;
}

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

AIndexOf

BContains

CStartsWith

DEndsWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using Contains which returns a bool, not an index.

Using StartsWith or EndsWith which check only start or end.

The IndexOf method returns the starting index of the pattern in the text or -1 if not found.

2fill in blank

medium

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

DSA C#

int FindPatternIndex(string text, string pattern) {
    return text.[1](pattern);
}

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

ASubstring

BContains

CLastIndexOf

DIndexOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using Contains which returns bool.

Using LastIndexOf which returns last occurrence.

IndexOf returns the first index of the pattern or -1 if not found.

3fill in blank

hard

Fix the error in the code to correctly check if the pattern is at the start of the text.

DSA C#

bool StartsWithPattern(string text, string pattern) {
    return text.[1](pattern);
}

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

AContains

BIndexOf

CStartsWith

DSubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using Contains which checks anywhere.

Using IndexOf which returns an index.

StartsWith checks if the text begins exactly with the pattern.

4fill in blank

hard

Fill both blanks to extract the substring from the first occurrence of the pattern to the end.

DSA C#

string ExtractFromPattern(string text, string pattern) {
    int index = text.[1](pattern);
    return index == -1 ? "" : text.[2](index);
}

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

AIndexOf

BContains

CSubstring

DLastIndexOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using Contains which returns bool, not index.

Using LastIndexOf which finds last occurrence.

IndexOf finds the start index, and Substring extracts from that index to the end.

5fill in blank

hard

Fill all three blanks to create a dictionary of pattern counts in a list of texts.

DSA C#

Dictionary<string, int> CountPatternOccurrences(List<string> texts, string pattern) {
    var counts = new Dictionary<string, int>();
    foreach (var text in texts) {
        int count = 0, index = 0;
        while ((index = text.[1](pattern, index)) != -1) {
            count++;
            index += [2];
        }
        counts[[3]] = count;
    }
    return counts;
}

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

AIndexOf

Bpattern.Length

Ctext

DContains

Attempts:

3 left

💡 Hint

Common Mistakes

Using Contains which returns bool, not index.

Incrementing index by 1 causing infinite loops.

Using pattern instead of text as dictionary key.

Use IndexOf with a start index to find occurrences, increment index by pattern length to avoid overlap, and store count keyed by the text.

Practice

1. What is the main purpose of substring search algorithms in strings?

easy

A. To sort characters in a string alphabetically

B. To find if a smaller string exists inside a larger string

C. To reverse the order of characters in a string

D. To count the total number of characters in a string

Solution

1. Step 1: Understand substring search goal

   Substring search algorithms check if a smaller string (pattern) appears inside a bigger string (text).

2. Step 2: Compare with other options

   Sorting, reversing, and counting characters are different string operations unrelated to substring search.

3. Final Answer:

   To find if a smaller string exists inside a larger string -> Option B

4. Quick Check:

   Substring search = find smaller string inside bigger string [OK]

Hint: Substring search means finding a smaller string inside a bigger one [OK]

Common Mistakes:

• Confusing substring search with sorting or reversing strings
• Thinking substring search counts characters
• Assuming substring search changes the string

2. Which of the following is the correct way to check if string text contains substring pattern in C#?

easy

A. bool found = Contains(text, pattern);

B. bool found = text.Find(pattern);

C. bool found = text.Contains(pattern);

D. bool found = text.Index(pattern);

Solution

1. Step 1: Recall C# string method for substring check

   The correct method to check if a string contains another is Contains, called on the string instance.

2. Step 2: Validate method usage

   text.Contains(pattern) returns a boolean indicating if pattern is inside text. Other options are invalid or do not exist.

3. Final Answer:

   bool found = text.Contains(pattern); -> Option C

4. Quick Check:

   Use Contains method on string to check substring [OK]

Hint: Use text.Contains(pattern) to check substring presence [OK]

Common Mistakes:

• Using non-existent methods like Find or Index
• Calling Contains as a standalone function
• Confusing method names or parameters

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

string text = "hello world";
string pattern = "wor";
bool result = text.Contains(pattern);
Console.WriteLine(result);

medium

A. True

B. Runtime exception

C. Compilation error

D. False

Solution

1. Step 1: Check if pattern "wor" is in text "hello world"

   The substring "wor" appears starting at index 6 in "hello world".

2. Step 2: Evaluate Contains method result

   Since the substring exists, text.Contains(pattern) returns true, so result is true.

3. Final Answer:

   True -> Option A

4. Quick Check:

   Substring "wor" in "hello world" = true [OK]

Hint: Check if substring is inside text literally to get true/false [OK]

Common Mistakes:

• Assuming Contains returns index instead of bool
• Thinking substring is not present due to partial match
• Confusing output with string printing

4. Identify the error in the following C# code snippet for substring search:

string text = "data structures";
string pattern = "struct";
int index = text.IndexOf(pattern);
if (index)
{
    Console.WriteLine("Found at " + index);
}

medium

A. The if condition should check if index >= 0

B. IndexOf method does not exist for strings

C. Pattern variable is not declared

D. Console.WriteLine syntax is incorrect

Solution

1. Step 1: Understand IndexOf return value

   IndexOf returns the starting index of the substring or -1 if not found.

2. Step 2: Check if condition usage

   The if condition if (index) is invalid because index is an int, not a boolean. It should check index >= 0 to confirm substring found.

3. Final Answer:

   The if condition should check if index >= 0 -> Option A

4. Quick Check:

   Check index >= 0 to confirm substring found [OK]

Hint: Check if index >= 0 to confirm substring found [OK]

Common Mistakes:

• Using int directly as boolean in if condition
• Assuming IndexOf returns boolean
• Ignoring negative index meaning not found

5. Given a list of strings words and a pattern string p, which approach best finds all words containing p as a substring efficiently in C#?

hard

A. Use words.IndexOf(p) directly

B. Sort the list and use binary search for the pattern

C. Replace all occurrences of p in each word with empty string

D. Use a loop to check each word with word.Contains(p) and collect matches

Solution

1. Step 1: Understand substring search in multiple strings

   To find all words containing p, we must check each word individually for the substring.

2. Step 2: Evaluate options for efficiency and correctness

   Using a loop with word.Contains(p) is straightforward and efficient. Sorting and binary search do not help substring search. words.IndexOf(p) is invalid because words is a list of strings, not a string. Replacing substrings does not find matches.

3. Final Answer:

   Use a loop to check each word with word.Contains(p) and collect matches -> Option D

4. Quick Check:

   Loop + Contains per word finds all substring matches [OK]

Hint: Loop through words and use Contains to find substring matches [OK]

Common Mistakes:

• Trying binary search for substring matching
• Using IndexOf on list instead of strings
• Replacing substrings instead of checking presence