//Tiger是一个自定义对象，里面有实例变量name和weight
    Tiger *t0 = [Tiger tigerWithName:@"zhangsan" weight:20];
    Tiger *t1 = [Tiger tigerWithName:@"lisi" weight:30];
    Tiger *t2 = [Tiger tigerWithName:@"wangwu" weight:40];
    Tiger *t3 = [Tiger tigerWithName:@"zhaoliu" weight:50];
    NSArray *arr = @[t0, t1, t2, t3];
    //总和
    NSNumber *sum = [arr valueForKeyPath:@"@sum.weight"];
    NSLog(@"%@", sum);
    //个数
    NSNumber *count = [arr valueForKeyPath:@"@count.weight"];
    NSLog(@"%@",count);
    //平均数
    NSNumber *avg = [arr valueForKeyPath:@"@avg.weight"];
    //最小值
    NSNumber *min = [arr valueForKeyPath:@"@min.weight"];
    //最大值
    NSNumber *max = [arr valueForKeyPath:@"@max.weight"];
    NSLog(@"%@", avg);
    NSLog(@"%@", min);
    NSLog(@"%@", max);
    
    NSArray *arr2= @[@11, @22, @33, @44];
    //整数数组求和
    NSNumber *sum1 = [arr2 valueForKeyPath:@"@sum.integerValue"];
    NSLog(@"%@", sum1);