How to make flutter calculator [closed]

import 'package:flutter/material.dart';

void main() => runApp(MaterialApp(home: Calc()));

class Calc extends StatefulWidget {
  @override
  _CalcState createState() => _CalcState();
}

class _CalcState extends State<Calc> {
  String d = '0', o = '';
  double n1 = 0, n2 = 0;

  void p(String v) {
    setState(() {
      if (v == 'C') {
        d = '0'; o = ''; n1 = 0; n2 = 0;
      } else if (v == '=') {
        n2 = double.parse(d);
        if (o == '+') d = (n1 + n2).toString();
        if (o == '-') d = (n1 - n2).toString();
        if (o == '×') d = (n1 * n2).toString();
        if (o == '÷') d = (n1 / n2).toString();
        o = '';
      } else if (['+', '-', '×', '÷'].contains(v)) {
        n1 = double.parse(d);
        o = v;
        d = '0';
      } else {
        d = d == '0' ? v : d + v;
      }
    });
  }

  Widget b(String t) => Expanded(
    child: Padding(
      padding: EdgeInsets.all(4),
      child: ElevatedButton(
        onPressed: () => p(t),
        child: Text(t, style: TextStyle(fontSize: 24)),
        style: ElevatedButton.styleFrom(padding: EdgeInsets.all(20)),
      ),
    ),
  );

  @override
  Widget build(BuildContext context) => Scaffold(
    appBar: AppBar(title: Text('Калькулятор')),
    body: Column(
      children: [
        Expanded(
          child: Container(
            alignment: Alignment.bottomRight,
            padding: EdgeInsets.all(24),
            child: Text(d, style: TextStyle(fontSize: 48)),
          ),
        ),
        Row(children: ['7', '8', '9', '÷'].map(b).toList()),
        Row(children: ['4', '5', '6', '×'].map(b).toList()),
        Row(children: ['1', '2', '3', '-'].map(b).toList()),
        Row(children: ['C', '0', '=', '+'].map(b).toList()),
      ],
    ),
  );
}