Creating a Calculator App in Android: A Beginner’s Guide with Kotlin

				
					class MainActivity : AppCompatActivity() {

    private lateinit var displayTextView: TextView
    private var firstNumber = 0.0
    private var currentOperation = "="

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        displayTextView = findViewById(R.id.displayTextView)

        // Set up click listeners for number buttons
        setNumberButtonListeners()

        // Set up click listeners for operation buttons
        setOperationButtonListeners()
    }

    private fun setNumberButtonListeners() {
        val numberButtons = listOf(
            R.id.btn0, R.id.btn1, R.id.btn2, R.id.btn3, R.id.btn4,
            R.id.btn5, R.id.btn6, R.id.btn7, R.id.btn8, R.id.btn9
        )

        numberButtons.forEach { buttonId ->
            findViewById<Button>(buttonId).setOnClickListener {
                val number = (it as Button).text.toString()
                appendToDisplay(number)
            }
        }
    }

    private fun setOperationButtonListeners() {
        val operationButtons = mapOf(
            R.id.btnAdd to "+",
            R.id.btnSubtract to "-",
            R.id.btnMultiply to "*",
            R.id.btnDivide to "/"
        )

        operationButtons.forEach { (buttonId, operation) ->
            findViewById<Button>(buttonId).setOnClickListener {
                performOperation(operation)
            }
        }

        findViewById<Button>(R.id.btnEquals).setOnClickListener {
            calculateResult()
        }

        findViewById<Button>(R.id.btnClear).setOnClickListener {
            clearDisplay()
        }
    }

    private fun appendToDisplay(number: String) {
        if (displayTextView.text == "0") {
            displayTextView.text = number
        } else {
            displayTextView.append(number)
        }
    }

    private fun performOperation(operation: String) {
        firstNumber = displayTextView.text.toString().toDouble()
        currentOperation = operation
        clearDisplay()
    }

    private fun calculateResult() {
        val secondNumber = displayTextView.text.toString().toDouble()
        val result = when (currentOperation) {
            "+" -> firstNumber + secondNumber
            "-" -> firstNumber - secondNumber
            "*" -> firstNumber * secondNumber
            "/" -> firstNumber / secondNumber
            else -> secondNumber
        }
        displayTextView.text = result.toString()
    }

    private fun clearDisplay() {
        displayTextView.text = "0"
    }
}