Kotlin How to Convert String to Int Easily

In Kotlin, you can convert a string to an integer using toInt() like this: val number = "123".toInt().

Examples

Input"123"

Output123

Input"0"

Output0

Input"-45"

Output-45

How to Think About It

To convert a string to an integer in Kotlin, think of the string as a number written in text form. You use the toInt() function to tell Kotlin to read that text and turn it into a number you can use for math or other operations.

Algorithm

1

Get the input string that represents a number.

2

Use the <code>toInt()</code> function on the string to convert it to an integer.

3

Store or use the resulting integer value.

Code

kotlin

fun main() {
    val strNumber = "123"
    val number = strNumber.toInt()
    println(number)
}

Output

123

Dry Run

Let's trace converting the string "123" to an integer.

1

Start with string

strNumber = "123"

2

Convert string to int

number = strNumber.toInt() // number = 123

3

Print the integer

Output: 123

Step	Value
strNumber	"123"
number	123

Why This Works

Step 1: Using toInt()

The toInt() function reads the string characters and converts them into an integer number.

Step 2: Valid string format

The string must contain only digits (and optionally a leading minus sign) to convert successfully.

Step 3: Result is an integer

After conversion, you get a real number type Int that you can use in calculations.

Alternative Approaches

toIntOrNull()

kotlin

fun main() {
    val strNumber = "abc"
    val number = strNumber.toIntOrNull()
    println(number) // prints null
}

This method returns null instead of throwing an error if the string is not a valid number.

Integer.parseInt()

kotlin

fun main() {
    val strNumber = "456"
    val number = Integer.parseInt(strNumber)
    println(number)
}

This is a Java method available in Kotlin but less idiomatic than <code>toInt()</code>.

Complexity: O(n) time, O(1) space

Time Complexity

The toInt() function reads each character once, so it takes time proportional to the string length.

Space Complexity

Conversion uses a fixed amount of extra space, so it is constant space.

Which Approach is Fastest?

toInt() and Integer.parseInt() have similar speed; toIntOrNull() adds safety but with minimal overhead.

Approach	Time	Space	Best For
toInt()	O(n)	O(1)	Simple, valid strings
toIntOrNull()	O(n)	O(1)	Safe conversion with error handling
Integer.parseInt()	O(n)	O(1)	Java interoperability