num a = 10;
  a = 12.5;
  print(a);
  print(a.runtimeType);

  int b = 20;
  //  b = 20.5;
  print(b);
  print(b.runtimeType);

  double c = 10.5;
  //  c = 30;
  print(c);
  print(c.runtimeType);
  print("\n");

12.5
double
20
int
10.5
double

  print(b + c);
  print(b - c);
  print(b * c);
  print(b / c);
  print(b ~/ c);
  print(b % c);

30.5
9.5
210.0
1.9047619047619047
1
9.5

  //NaN
  print(0.0 / 0.0);
  //isEven 是否是偶数
  print(b.isEven);
  //isOdd 是否是奇数
  print(b.isOdd);
  int d = 11;
  print(d.isEven);
  print(d.isOdd);

  int e = -100;
  //绝对值
  print(e.abs());

  double f = 10.5;
  print(f.round());//最靠近f的整数
  print(f.floor());//不大于f的最大整数
  print(f.ceil());//不小于f的最小整数

  print(f.toInt());
  print(f.toString());
  print(d.toDouble());

NaN
true
false
false
true
100
11
10
11
10
10.5
11.0