String comparison (equals, compareTo) in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if two strings are equal using the correct method.

Kotlin

val result = str1.[1](str2)
println(result)

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

AcompareTo

Bequals

C==

DequalsIgnoreCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using compareTo instead of equals causes confusion because it returns an integer.

Using == which compares content in Kotlin, but is an operator not a method call.

The equals method checks if two strings have the same content exactly.

2fill in blank

medium

Complete the code to compare two strings lexicographically and print the result.

Kotlin

val comparison = str1.[1](str2)
println(comparison)

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

AcompareTo

Bequals

C==

DequalsIgnoreCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using equals instead of compareTo when needing order comparison.

Expecting compareTo to return a boolean.

The compareTo method compares strings lexicographically and returns an integer.

3fill in blank

hard

Fix the error in the code to correctly check if two strings are equal ignoring case.

Kotlin

val isEqual = str1.[1](str2)
println(isEqual)

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

A==

BcompareTo

Cequals

DequalsIgnoreCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using equals which is case sensitive.

Using compareTo which returns an integer, not boolean.

The equalsIgnoreCase method compares strings ignoring letter case differences.

4fill in blank

hard

Fill both blanks to create a map of words to their lengths only if the length is greater than 3.

Kotlin

val lengths = buildMap<String, Int> {
  for ([1] in words) {
    if ([2] > 3) {
      this[[1]] = [2]
    }
  }
}

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

Aword

Blen(word)

Cword.length

Dword.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(word) which is Python syntax, not Kotlin.

Using word.size which is not a valid property for String length in Kotlin.

We use word as the key and word.length as the value. The condition checks length > 3.

5fill in blank

hard

Fill all three blanks to create a map of uppercase words to their lengths only if length is greater than 4.

Kotlin

val result = buildMap<String, Int> {
  for ([3] in words) {
    if ([2] > 4) {
      this[[1]] = [2]
    }
  }
}

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

Aword.uppercase()

Bword.length

Cword

Dword.lowercase()

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase() instead of uppercase() for the key.

Using incorrect loop variable names.

The key is the uppercase version of the word, the value is its length, and the loop variable is word.

Practice

1. Which Kotlin method should you use to check if two strings have exactly the same characters and case?
val a = "Hello"
val b = "hello"

easy

A. a.equals(b)

B. a.compareTo(b)

C. a === b

D. a.equals(b, ignoreCase = true)

Solution

1. Step 1: Understand equals() method

   The equals() method checks if two strings are exactly the same including case.

2. Step 2: Compare equals() with other options

   compareTo() returns an order number, === checks reference equality (not content), equals(ignoreCase=true) ignores case, so only equals() matches exact case-sensitive check.

3. Final Answer:

   a.equals(b) -> Option A

4. Quick Check:

   Use equals() for exact string match [OK]

Hint: Use equals() for exact case-sensitive string match [OK]

Common Mistakes:

• Using compareTo() to check equality
• Assuming === checks content equality
• Using equals(ignoreCase=true) when case matters

2. Which of the following is the correct syntax to compare two strings str1 and str2 for equality ignoring case in Kotlin?

easy

A. str1.equals(str2, ignoreCase = true)

B. str1.compareTo(str2, ignoreCase = true)

C. str1.equals(str2)

D. str1 == str2

Solution

1. Step 1: Recall equals() with ignoreCase parameter

   In Kotlin, equals() can take ignoreCase=true to compare strings ignoring case differences.

2. Step 2: Check other options

   A: equals() without ignoreCase is case-sensitive, B: compareTo(ignoreCase=true) returns Int not Boolean, D: == is case-sensitive.

3. Final Answer:

   str1.equals(str2, ignoreCase = true) -> Option A

4. Quick Check:

   Use equals(ignoreCase=true) to ignore case [OK]

Hint: Use equals(ignoreCase=true) to compare ignoring case [OK]

Common Mistakes:

• Using compareTo(ignoreCase=true) directly (returns Int, not Boolean)
• Using == which is case-sensitive
• Forgetting to set ignoreCase=true

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

val s1 = "apple"
val s2 = "banana"
println(s1.compareTo(s2))

medium

A. A positive number

B. A negative number

C. 0

D. Compilation error

Solution

1. Step 1: Understand compareTo() behavior

   compareTo() returns a negative number if the first string is lexicographically less than the second.

2. Step 2: Compare "apple" and "banana" lex order

   "apple" comes before "banana" alphabetically, so compareTo returns a negative number.

3. Final Answer:

   A negative number -> Option B

4. Quick Check:

   "apple" < "banana" means compareTo() < 0 [OK]

Hint: compareTo() < 0 means first string is before second [OK]

Common Mistakes:

• Assuming compareTo() returns boolean
• Confusing positive and negative return values
• Thinking compareTo() returns 0 for different strings

4. The following Kotlin code is intended to check if two strings are equal ignoring case, but it does not compile. What is the error?

val a = "Test"
val b = "test"
if (a.compareTo(b, ignoreCase = true)) {
    println("Equal")
} else {
    println("Not Equal")
}

medium

A. Missing parentheses in if condition

B. ignoreCase is not a valid parameter for compareTo()

C. Strings cannot be compared with compareTo()

D. compareTo() returns Int, cannot be used as Boolean

Solution

1. Step 1: Check compareTo() return type

   compareTo() returns an Int, not a Boolean, so it cannot be used directly in if condition.

2. Step 2: Understand how to check equality with compareTo()

   To check equality ignoring case, compareTo() == 0 should be used, not just compareTo() alone.

3. Final Answer:

   compareTo() returns Int, cannot be used as Boolean -> Option D

4. Quick Check:

   compareTo() returns Int, use == 0 to check equality [OK]

Hint: compareTo() returns Int; compare with 0 for equality [OK]

Common Mistakes:

• Using compareTo() directly as Boolean
• Forgetting to compare result with 0
• Assuming ignoreCase is invalid parameter (it is valid)

5. You want to sort a list of strings ignoring case in Kotlin. Which code snippet correctly sorts the list val fruits = listOf("Banana", "apple", "Cherry") ignoring case?

hard

A. fruits.sortedBy { it }

B. fruits.sortedBy(String.CASE_INSENSITIVE_ORDER)

C. fruits.sortedWith(String.CASE_INSENSITIVE_ORDER)

D. fruits.sorted()

Solution

1. Step 1: Understand sorting ignoring case

   To sort strings ignoring case, use a comparator that compares strings case-insensitively.

2. Step 2: Check options for case-insensitive sorting

   A and D are case-sensitive. C: sortedBy expects key selector, not Comparator (compile error). B uses built-in case-insensitive Comparator correctly.

3. Final Answer:

   fruits.sortedWith(String.CASE_INSENSITIVE_ORDER) -> Option C

4. Quick Check:

   Use sortedWith(String.CASE_INSENSITIVE_ORDER) for case-insensitive sort [OK]

Hint: Use sortedWith(String.CASE_INSENSITIVE_ORDER) to ignore case in sort [OK]

Common Mistakes:

• Using sorted() which is case-sensitive
• Using sortedBy { it } which is case-sensitive
• Confusing sortedBy (selector) with sortedWith (Comparator)