需要对动态的指针数组进行排序,并在其中包含可能的NULL指针

Question

我试图弄清楚如何对结构中的指针数组进行排序,其中包含可能的空指针.但我无法理解它,并在排序后不断崩溃.

我有两个结构,CAR并且CARLIST: CARLIST有一个指针数组CARS.而我无法做到对.

谢谢你的帮助...

typedef struct Car {
    int parked_Total_Minutes;
    char rz[10];
} CAR;

typedef struct CarList {
    CAR **p_cars;
    unsigned int count;
    unsigned int size;
} CARLIST;

int Compare_ParkedTime(const void *a, const void *b) {
     if (a == NULL)
          return -1;

     if (b == NULL)
          return 1;

     CAR *aa = *(CAR* const *)a;
     CAR *bb = *(CAR* const *)b;

     return  (bb->parked_Total_Minutes < aa->parked_Total_Minutes) - (aa->parked_Total_Minutes < bb->parked_Total_Minutes);
}

int main() {
    ....
    CARLIST *p_AllCars = (CARLIST *)malloc(sizeof(CARLIST));
    p_AllCars->count = 0;
    p_AllCars->size = 10;
    p_AllCars->p_cars = malloc(p_AllCars->size * sizeof(CAR *));

    for(int i = 0; i < p_AllCars->size; i++)
         p_AllCars->p_cars[i] = NULL;

    ... other logic generating cars ...

    qsort((void*)p_AllCars->p_cars, p_AllCars->size, sizeof(CAR*), Compare_ParkedTime);

    ...
}

Answer 1

比较函数在要排序的数组中获取指针.您应该首先从该数组中读取指针,然后进行测试NULL.

如果两个指针都是,NULL并且可能使空指针大于其他值,则应返回0 .

以下是比较函数的更正版本:

int Compare_ParkedTime(const void *a, const void *b) {
    /* read the pointer values */
    CAR *aa = *(CAR * const *)a;
    CAR *bb = *(CAR * const *)b;

    /* sort NULL pointers to the end of the array */
    if (aa == NULL)
        return (bb != NULL);
    if (bb == NULL)
        return -1;

    /* sort by increasing value of parked_Total_Minutes. swap aa and bb for decreasing order */
    return (bb->parked_Total_Minutes < aa->parked_Total_Minutes) -
           (aa->parked_Total_Minutes < bb->parked_Total_Minutes);
}