Kotlin: Variable 'result' must be initialized - kotlin

Question

The compiler is showing error Kotlin: Variable result must be initialized.
Here is the code.
fun main(args: Array<String>) {
print("Enter two numbers: ")
// nextDouble() reads the next double from the keyboard
var first= readLine()!!.toDouble()
var second = readLine()!!.toInt()
print("Enter an choice(1-4)): ")
val operator = readLine()!!.toInt()
var result: Double
when (operator) {
1 -> result = first + second
2 -> result = first - second
3 -> result = first * second
4 -> result = first / second
else -> {
println("Error.")
}
}
println("The result is :- " +result)
}

Answer 1

The problem is that when you read the value of result here:
println("The result is :- " +result)
result might not be initialized, because here:
var result: Double
when (operator) {
1 -> result = first + second
2 -> result = first - second
3 -> result = first * second
4 -> result = first / second
else -> {
println("Error.")
}
You are NOT assigning a value to result in the else branch. You have several options, one could be to make result nullable. For instance:
var result = when (operator) {
1 -> first + second
2 -> first - second
3 -> first * second
4 -> first / second
else -> null
}
if (result != null) {
println("The result is :- " + result)
} else {
println("Error.")
}
Notice that if the operator is not (1-4) the value of result will be null. Also, in your code you're printing "Error" and then again you're attempting to print the result.
To add something more, you could make the thing a bit nicer if you defined your operator with the when statement as a method reference:
print("Enter an choice(1-4)): ")
val operatorCode = readLine()!!.toInt()
val operator: ((Int) -> Double)? = when (operatorCode) {
1 -> first::plus
2 -> first::minus
3 -> first::times
4 -> first::div
else -> null
}
if (operator != null) {
val result = operator.invoke(second)
println("The result is :- " + result)
} else {
println("Error.")
}

Answer 2

As jrtapsell points out the main issue is that you declare result to be a Double but you since you don't cover every possible value in the when statement, you are accessing an un-initialized variable.
Another option, if you really operators other than 1-4 want it to be an error condition and don't want result to end up with a valid Double is to use Optionals.
fun main(args: Array<String>) {
print("Enter two numbers: ")
// nextDouble() reads the next double from the keyboard
var first= readLine()!!.toDouble()
var second = readLine()!!.toInt()
print("Enter an choice(1-4)): ")
val operator = readLine()!!.toInt()
var result: Double? = null
when (operator) {
1 -> result = first + second
2 -> result = first - second
3 -> result = first * second
4 -> result = first / second
else -> {
println("Error.")
}
}
// only print the result if not null
result?.let {
println("The result is :- " +result)
}
}
In this case result will be null if the operator is not one of 1-4.

Answer 3

What is the error message trying to tell you
Variable 'result' must be initialized.
This means there are paths through your code where you access result before you have given it a value.
This is a problem because it means you have probably missed off handling a condition.
How do you get through your code without setting result
If the user inputs 5 result is not set
How to fix
Either set result in the else branch, or give it a default value in the declaration