A rudimentary mathematical expression evaluator in Kotlin.

Calculator.kt

import java.util.*
import javax.script.ScriptEngine
import javax.script.ScriptEngineManager
import javax.script.ScriptException

// Instance variables
val engine: ScriptEngine = ScriptEngineManager().getEngineByName("JavaScript")
val jsMathMap = hashMapOf("sin" to "Math.sin", "cos" to "Math.cos", "tan" to "Math.tan", "pi" to "Math.PI")

fun main(args: Array<String>) {
    val scanner = Scanner(System.`in`)
    val expr = scanner.nextLine()
    val modifiedExpr = mathToJs(expr)
    println("$modifiedExpr = ${eval(modifiedExpr)}")
}

/**
 * Evaluates a mathematical expression.
 */
fun eval(expr: String): Number {
    return try {
        engine.eval(expr) as Number
    } catch (ex: ScriptException) {
        Double.NaN
    }
}

/**
 * JavaScript contains a lot of math utilities, but requires
 * a reference to methods in the Math class. This method uses
 * the "jsMathMap" to convert standard mathematical notation
 * to valid Javascript.
 */
fun mathToJs(expr: String): String {
    var ret: String = expr
    jsMathMap.forEach { old, new ->
        run {
            ret = ret.replace(old, new)
        }
    }
    return ret
}