# Implement Reverse Polish Notation with Kotlin

Implemented a calculator by Reverse Polish Notation with Kotlin.

I defined some operators as token. The tokens are numbers and the four arithmetic operators. I thought numbers and operators should be distinguished, so defined Token class as sealed class, and defined Number and Operator as subclasses of Token class. Number is a data class that has a value of BigDecimal. Operator has subclasses such as Plus which represents the four arithmetic operators. The subclasses of Operator has a function that takes two parameters of Number and returns the result of calculation.

sealed class Token {

data class Number(val value: BigDecimal) : Token() {
override fun toString(): String = value.toPlainString()
}

sealed class Operator : Token() {

abstract fun execute(lhs: Number, rhs: Number): Number

object Plus : Operator() {
override fun execute(lhs: Number, rhs: Number) = Number(value = lhs.value.plus(rhs.value))
}

object Minus : Operator() {
override fun execute(lhs: Number, rhs: Number) = Number(value = lhs.value.minus(rhs.value))
}

object Multiply : Operator() {
override fun execute(lhs: Number, rhs: Number)  = Number(value = lhs.value.multiply(rhs.value))
}

object Divide : Operator() {
override fun execute(lhs: Number, rhs: Number)  = Number(value = lhs.value.divide(rhs.value))
}
}
}


Here implements a function to execute calculation, which receives an array of Token and returns Number. When a caliulation is completed, the Stack has only one Number, else throws exception. The function runs the array and pushes Number to the Stack if an element of the array is Number. Or if an element of the array is Operator, the function pops two Numbers to execute an operation then pushed the result to the Stack.

fun calculate(tokens: Array<Token>): Token.Number {
val stack = Stack<Token.Number>()

for (token in tokens) {
when (token) {
is Token.Number -> {
// if it is Number, push it to the Stack
stack.push(token)
}
is Token.Operator -> {
// if it is Operator, execute the operation
val rhs = stack.pop() // rhs is at the first of the Stack
val lhs = stack.pop() // lhs is at the second of the Stack
stack.push(token.execute(lhs = lhs, rhs = rhs)) // push the result to the Stack
}
}
}

if (stack.size == 1) {
return stack.pop()
} else {
throw IllegalArgumentException("illegal token combination")
}
}


Here it works.

val tokens = arrayOf<Token>(
Token.Number(value = BigDecimal.valueOf(1.0)),
Token.Number(value = BigDecimal.valueOf(2.0)),
Token.Operator.Plus,
Token.Number(value = BigDecimal.valueOf(3.0)),
Token.Number(value = BigDecimal.valueOf(4.0)),
Token.Operator.Multiply,
Token.Operator.Divide,
)

val result = calculate(tokens)

println(result) // 0.25