String comparison (equals, compareTo) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code comparing strings with equals()?

Consider the following Kotlin code snippet. What will it print?

Kotlin

fun main() {
    val a = "Hello"
    val b = "hello"
    println(a.equals(b))
    println(a.equals(b, ignoreCase = true))
}

A true\ntrue

B false\ntrue

C false\nfalse

D true\nfalse

💡 Hint

Remember that equals() is case-sensitive by default unless ignoreCase is true.

The first equals() call compares "Hello" and "hello" exactly, so it returns false. The second call ignores case, so it returns true.

❓ Predict Output

intermediate

What does compareTo() return when comparing "apple" and "banana"?

What is the output of this Kotlin code?

Kotlin

fun main() {
    val result = "apple".compareTo("banana")
    println(result)
}

A A negative number less than -1

B 0

C 1

D -1

💡 Hint

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

"apple" is lexicographically less than "banana" because the first characters 'a' (97) and 'b' (98) differ, so compareTo returns a negative number less than -1.

🔧 Debug

advanced

What error does this Kotlin code produce?

Examine this Kotlin code and identify the error it causes.

Kotlin

fun main() {
    val s1: String = null
    val s2 = "test"
    println(s1.equals(s2))
}

A Prints false without error

B RuntimeException: NullPointerException

C Compilation error: Unresolved reference: equals

D Compilation error: Null can not be a value of a non-null type String

💡 Hint

In Kotlin, variables must be declared nullable to hold null.

The variable s1 is declared as non-nullable String and assigned null, which is not allowed. This causes a compilation error.

❓ Predict Output

advanced

What is the output of this Kotlin code using compareTo with ignoreCase?

What will this Kotlin program print?

Kotlin

fun main() {
    val s1 = "Kotlin"
    val s2 = "kotlin"
    println(s1.compareTo(s2, ignoreCase = true))
    println(s1.compareTo(s2))
}

A -32\n0

B 0\n32

C 0\n-32

D 32\n0

💡 Hint

compareTo returns 0 if strings are equal ignoring case, otherwise a positive or negative number.

With ignoreCase=true, "Kotlin" and "kotlin" are equal, so 0 is printed. Without ignoreCase, the difference of 'K' and 'k' ASCII codes is -32.

🧠 Conceptual

expert

How many items are in the resulting map after this Kotlin code?

What is the size of the map produced by this Kotlin code?

Kotlin

fun main() {
    val keys = listOf("a", "A", "b", "B")
    val map = keys.associateWith { it.lowercase() }
    val filteredMap = map.filterKeys { it.equals(it.lowercase(), ignoreCase = true) }
    println(filteredMap.size)
}

A 4

B 2

C 3

D 1

💡 Hint

Check which keys equal their lowercase version ignoring case.

Keys "a", "A", "b", and "B" all equal their lowercase ignoring case, so all 4 keys pass the filter. Therefore, filteredMap.size is 4.

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)