String to number conversion in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code converting string to integer?

Consider the following Kotlin code snippet that converts a string to an integer. What will be printed?

Kotlin

fun main() {
    val numberString = "1234"
    val number = numberString.toInt()
    println(number + 10)
}

A NumberFormatException

B 1244

C 123410

D 1234

💡 Hint

Remember that toInt() converts the string to an integer number, so arithmetic operations work as expected.

The string "1234" is converted to the integer 1234. Adding 10 results in 1244, which is printed.

❓ Predict Output

intermediate

What happens when converting a non-numeric string to Int in Kotlin?

What will happen when this Kotlin code runs?

Kotlin

fun main() {
    val invalidNumber = "abc"
    val number = invalidNumber.toInt()
    println(number)
}

A NumberFormatException

B 0

C abc

D null

💡 Hint

Think about what happens if you try to convert a string that does not represent a number.

The string "abc" cannot be converted to an integer, so Kotlin throws a NumberFormatException.

❓ Predict Output

advanced

What is the output of safe string to number conversion using toIntOrNull()?

What will this Kotlin program print?

Kotlin

fun main() {
    val input = "42a"
    val number = input.toIntOrNull() ?: -1
    println(number)
}

A 42

B null

C NumberFormatException

D -1

💡 Hint

toIntOrNull() returns null if conversion fails, so the Elvis operator ?: provides a default.

Since "42a" is not a valid integer, toIntOrNull() returns null, so the default -1 is printed.

🧠 Conceptual

advanced

Which Kotlin function converts a string to a Double safely without throwing an exception?

Choose the Kotlin function that converts a string to a Double and returns null if the string is not a valid number.

A toDoubleOrNull()

B toIntOrNull()

C toFloat()

D toDouble()

💡 Hint

Look for the function that ends with 'OrNull' and works with Double type.

toDoubleOrNull() tries to convert the string to Double and returns null if it fails, avoiding exceptions.

❓ Predict Output

expert

What is the output of this Kotlin code converting hexadecimal string to integer?

What will be printed by this Kotlin program?

Kotlin

fun main() {
    val hexString = "1A"
    val number = hexString.toInt(16)
    println(number)
}

A NumberFormatException

B 1A

C 26

D 16

💡 Hint

toInt() can take a radix parameter to convert from different bases like hexadecimal.

The string "1A" in base 16 equals decimal 26, so 26 is printed.

Practice

1. Which Kotlin function converts a String to an Int?

easy

A. toChar()

B. toString()

C. toDouble()

D. toInt()

Solution

1. Step 1: Understand the conversion functions

   Kotlin provides toInt() to convert strings to integers, toDouble() for doubles, and toString() converts other types to strings.

2. Step 2: Identify the correct function for String to Int

   Since the question asks for converting a String to an Int, toInt() is the correct function.

3. Final Answer:

   toInt() -> Option D

4. Quick Check:

   String to Int = toInt() [OK]

Hint: Use toInt() to convert string numbers to integers [OK]

Common Mistakes:

• Confusing toInt() with toString()
• Using toDouble() when integer is needed
• Trying to convert non-numeric strings without error handling

2. Which of the following Kotlin code snippets correctly converts the string "123" to an integer?

easy

A. val num = "123".toInteger()

B. val num = toInt("123")

C. val num = "123".toInt()

D. val num = Int("123")

Solution

1. Step 1: Check Kotlin syntax for string to int conversion

   The correct syntax is calling toInt() on the string object, like "123".toInt().

2. Step 2: Analyze each option

   val num = "123".toInt() uses the correct method call. Options B and C are invalid function calls in Kotlin. val num = "123".toInteger() uses a non-existent method toInteger().

3. Final Answer:

   val num = "123".toInt() -> Option C

4. Quick Check:

   Correct syntax = "123".toInt() [OK]

Hint: Call toInt() directly on the string value [OK]

Common Mistakes:

• Trying to call toInt() as a standalone function
• Using Java-like Integer.parseInt() syntax
• Using non-existent methods like toInteger()

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

val s = "45"
val n = s.toInt() + 10
println(n)

medium

A. 55

B. 4510

C. 45

D. Error

Solution

1. Step 1: Convert string to integer and add

   The string "45" is converted to integer 45 using toInt(). Then 10 is added, resulting in 55.

2. Step 2: Print the result

   The println(n) outputs the integer 55, not string concatenation.

3. Final Answer:

   55 -> Option A

4. Quick Check:

   45 + 10 = 55 [OK]

Hint: toInt() converts string to number, so + means addition [OK]

Common Mistakes:

• Thinking + concatenates strings here
• Expecting output '4510' instead of 55
• Ignoring toInt() conversion effect

4. Identify the error in this Kotlin code snippet:

val s = "abc"
val n = s.toInt()
println(n)

medium

A. No error, prints 0

B. Runtime exception due to invalid number format

C. Compilation error: toInt() not found

D. Prints "abc"

Solution

1. Step 1: Understand toInt() behavior on invalid strings

   Calling toInt() on a string that is not a valid number throws a NumberFormatException at runtime.

2. Step 2: Analyze the given string "abc"

   Since "abc" cannot be converted to an integer, the program will crash with a runtime exception.

3. Final Answer:

   Runtime exception due to invalid number format -> Option B

4. Quick Check:

   Invalid string toInt() causes runtime error [OK]

Hint: Invalid strings cause runtime errors with toInt() [OK]

Common Mistakes:

• Expecting toInt() to return 0 for invalid strings
• Thinking toInt() throws compile-time error
• Ignoring exception possibility

5. You have a list of strings representing numbers: listOf("10", "20", "abc", "30"). Which Kotlin code safely converts valid strings to integers and ignores invalid ones?

hard

A. list.mapNotNull { it.toIntOrNull() }

B. list.filter { it.toInt() > 0 }

C. list.map { it.toInt() }

D. list.filterNotNull().map { it.toInt() }

Solution

1. Step 1: Understand safe conversion with toIntOrNull()

   toIntOrNull() returns null if conversion fails, avoiding exceptions.

2. Step 2: Use mapNotNull to convert and skip nulls

   mapNotNull { it.toIntOrNull() } converts valid strings and removes nulls from invalid ones.

3. Step 3: Analyze other options

   list.map { it.toInt() } throws exception on "abc". list.filter { it.toInt() > 0 } tries to convert without safety and filters by value, causing error. list.filterNotNull().map { it.toInt() } filters non-null strings but does not handle invalid numbers.

4. Final Answer:

   list.mapNotNull { it.toIntOrNull() } -> Option A

5. Quick Check:

   Safe conversion uses toIntOrNull() + mapNotNull [OK]

Hint: Use toIntOrNull() with mapNotNull() to skip invalid strings [OK]

Common Mistakes:

• Using toInt() directly causing exceptions
• Filtering strings before conversion incorrectly
• Not handling null results from toIntOrNull()