String to number conversion in Kotlin - Time & Space Complexity

Time Complexity: String to number conversion

O(n)

Understanding Time Complexity

When converting a string to a number, the time it takes depends on the length of the string.

We want to know how the work grows as the string gets longer.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fun stringToInt(s: String): Int {
    var result = 0
    for (char in s) {
        val digit = char - '0'
        result = result * 10 + digit
    }
    return result
}
    

This code converts a string of digits into an integer by processing each character one by one.

Identify Repeating Operations

• Primary operation: Looping through each character in the string.
• How many times: Exactly once for each character in the input string.

How Execution Grows With Input

As the string gets longer, the number of steps grows in a straight line with the length.

Input Size (n)	Approx. Operations
10	About 10 steps
100	About 100 steps
1000	About 1000 steps

Pattern observation: Doubling the string length roughly doubles the work done.

Final Time Complexity

Time Complexity: O(n)

This means the time to convert grows directly with the length of the string.

Common Mistake

[X] Wrong: "Converting a string to a number takes the same time no matter how long the string is."

[OK] Correct: Each character must be read and processed, so longer strings take more time.

Interview Connect

Understanding how string length affects conversion time helps you explain efficiency clearly and shows you know how input size matters in real code.

Self-Check

"What if we changed the code to convert a string to a floating-point number with decimals? How would the time complexity change?"

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()