Substring Search Patterns in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the pattern exists in the text using Java's contains method.

DSA Java

boolean found = text.[1](pattern);

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

Aequals

BindexOf

Ccontains

DstartsWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using indexOf returns an int, not a boolean.

Using equals checks full string equality, not substring presence.

The contains method returns true if the pattern is found anywhere in the text.

2fill in blank

medium

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

DSA Java

int index = text.[1](pattern);

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

AindexOf

Bsubstring

ClastIndexOf

Dcontains

Attempts:

3 left

💡 Hint

Common Mistakes

Using contains returns boolean, not index.

Using substring extracts part of the string, not search.

The indexOf method returns the first position where the pattern appears or -1 if not found.

3fill in blank

hard

Fix the error in the code to correctly check if the pattern starts the text.

DSA Java

boolean starts = text.[1](pattern);

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

AstartsWith

Bequals

Ccontains

DendsWith

Attempts:

3 left

💡 Hint

Common Mistakes

Using contains checks anywhere, not just start.

Using equals checks full string equality.

The startsWith method checks if the text begins with the given pattern.

4fill in blank

hard

Fill both blanks to extract a substring from text starting at index and ending before endIndex.

DSA Java

String sub = text.[1]([2], endIndex);

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

Asubstring

Bindex

Clength

DstartIndex

Attempts:

3 left

💡 Hint

Common Mistakes

Using length instead of start index.

Using startIndex which is not defined.

substring(start, end) extracts the part of text from start index to endIndex-1. The variable index holds the start position.

5fill in blank

hard

Fill all three blanks to create a map of pattern lengths to patterns that appear in text with length > 2.

DSA Java

Map<Integer, String> map = new HashMap<>();
for (String pattern : patterns) {
    if (pattern.[1]() > 2 && text.[2](pattern)) {
        map.put(pattern.[3](), pattern);
    }
}

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

Alength

Bcontains

DindexOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using indexOf instead of contains for substring check.

Using length as a variable instead of method call.

We check pattern length with length(), check if text contains pattern with contains(), and use length() again to put the key in map.

Practice

1. What does the indexOf method in Java's String class return when searching for a substring that exists in the string?

easy

A. The starting index of the first occurrence of the substring

B. The length of the substring

C. The number of times the substring appears

D. A boolean indicating if the substring exists

Solution

1. Step 1: Understand the purpose of indexOf

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

2. Step 2: Identify the return value when substring exists

   If the substring is found, indexOf returns the starting index (0-based) of that substring in the string.

3. Final Answer:

   The starting index of the first occurrence of the substring -> Option A

4. Quick Check:

   indexOf returns index, not count or boolean [OK]

Hint: Remember: indexOf returns position, not count or boolean [OK]

Common Mistakes:

• Thinking indexOf returns count of occurrences
• Assuming it returns true/false
• Confusing indexOf with substring length

2. Which of the following is the correct syntax to find the first occurrence of substring "cat" in string str in Java?

easy

A. int pos = str.find("cat");

B. int pos = str.search("cat");

C. int pos = str.indexOf("cat");

D. int pos = str.contains("cat");

Solution

1. Step 1: Recall Java String methods for substring search

   Java's String class uses indexOf to find substring positions.

2. Step 2: Check method names and return types

   find and search are not valid String methods; contains returns boolean, not int.

3. Final Answer:

   int pos = str.indexOf("cat"); -> Option C

4. Quick Check:

   Use indexOf for substring position [OK]

Hint: Use indexOf for position, contains for boolean check [OK]

Common Mistakes:

• Using non-existent methods like find or search
• Using contains which returns boolean, not index
• Confusing method names from other languages

3. What is the output of the following Java code?

String text = "hello world";
int pos = text.indexOf("lo");
System.out.println(pos);

medium

A. -1

B. 3

C. 4

D. 5

Solution

1. Step 1: Locate substring "lo" in "hello world"

   Counting indices: h(0), e(1), l(2), l(3), o(4). The substring "lo" starts at index 3 (l at 3, o at 4).

2. Step 2: Understand indexOf return value

   indexOf returns the starting index of the substring, which is 3 here.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   Substring "lo" starts at index 3 [OK]

Hint: Count string indices from 0 to find substring start [OK]

Common Mistakes:

• Counting from 1 instead of 0
• Returning index of second character in substring
• Assuming -1 means substring found

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

String s = "banana";
int index = s.indexOf('an');
System.out.println(index);

medium

A. Using single quotes for substring instead of double quotes

B. Missing semicolon after indexOf call

C. indexOf method does not exist for String

D. Printing index before assignment

Solution

1. Step 1: Check substring argument type in indexOf

   The indexOf method expects a String argument, but single quotes denote a char.

2. Step 2: Identify correct syntax for substring

   Substring literals must be in double quotes, so 'an' should be "an".

3. Final Answer:

   Using single quotes for substring instead of double quotes -> Option A

4. Quick Check:

   Substring literals need double quotes [OK]

Hint: Use double quotes for substrings, single for chars [OK]

Common Mistakes:

• Using single quotes for multi-character substrings
• Confusing char and String types
• Ignoring compiler error messages

5. Given a string text, which code snippet correctly finds all starting indices of substring "ab" in text using indexOf?

hard

A. int index = 0;
while (index < text.length()) {
index = text.indexOf("ab");
System.out.print(index + " ");
index++;
}

B. for (int i = 0; i < text.length(); i++) {
if (text.indexOf("ab") == i) System.out.print(i + " ");
}

C. int index = text.indexOf("ab");
while (index != -1) {
System.out.print(index + " ");
index = text.indexOf("ab");
}

D. int index = 0;
while ((index = text.indexOf("ab", index)) != -1) {
System.out.print(index + " ");
index++;
}

Solution

1. Step 1: Understand how to find multiple occurrences with indexOf

   Use indexOf(substring, fromIndex) to search from a moving start index to find all occurrences.

2. Step 2: Analyze each option's logic

   int index = 0; while ((index = text.indexOf("ab", index)) != -1) { System.out.print(index + " "); index++; } correctly updates index after each found occurrence to avoid infinite loop and find all positions. Others either do not update index properly or repeatedly search from start.

3. Final Answer:

   int index = 0; while ((index = text.indexOf("ab", index)) != -1) { System.out.print(index + " "); index++; } -> Option D

4. Quick Check:

   Use indexOf with start index and increment after each find [OK]

Hint: Increment start index after each found substring to continue search [OK]

Common Mistakes:

• Not updating start index causing infinite loop
• Always searching from start index 0
• Confusing indexOf usage with for-loop index