c - 列表中的学生 ID 未打印出来

Question

我正在尝试在用户输入学生ID后打印出未排序的学生ID列表。它可以很好地打印一个空列表，但是当有实际的ID要打印时，它就会变成空白。为什么会发生这种情况？我只能得到一个可以正常工作的空列表，但不能得到一个包含任何内容的列表。

#include <stdio.h>  /* printf, scanf                                  */
#include <stdlib.h> /* malloc, exit, free                             */
#include <ctype.h>  /* typedef, tolower                               */

/**********************************************************************/
/*                          Symbolic Constants                        */
/**********************************************************************/

#define MAXIMUM_ID        999999  /* Maximum student id number        */
#define MINIMUM_ID        1       /* Minimum student id number        */
#define LIST_HEADER       MINIMUM_ID - 1
                                  /* Header of the student id list    */
#define LIST_TRAILER      MAXIMUM_ID + 1 
                                  /* Trailer of the student id list   */
#define HEADER_ALLOC_ERR  1       /* Error allocating the list header */
                                  /* memory                           */
#define TRAILER_ALLOC_ERR 2       /* Error allocating the list        */
                                  /* trailer memory                   */
#define ID_ALLOC_ERR      3       /* Error allocating the student id  */
                                  /* memory                           */
#define QUIT              0       /* Program exit value               */

/**********************************************************************/
/*                          Program Structures                        */
/**********************************************************************/
/* A school student id list                                           */
struct student_id
{
   int               student_id;      /* A student's id number        */
   struct student_id *p_next_student; /* Points to next student id    */
};
typedef struct student_id ID;

/**********************************************************************/
/*                          Function Prototypes                       */
/**********************************************************************/
void print_heading();
   /* Print the program heading                                       */
void print_instructions();
   /* Print the program instructions                                  */
char get_response();
   /* Get the response to continue or quit                            */
ID *create_empty_list();
   /* Create an empty list                                            */
void insert_student(ID *p_id_list, int student_id);
   /* Insert a student at the front of the list                       */
void print_list(ID *p_id_list);
   /* Print all the student id's in the list                          */
int get_count(ID *p_id_list);
   /* Get the count of the students on the list                       */
void sort_list(ID *p_student_id);
   /* Sort the students into ascending order by id                    */
void remove_duplicates(ID *p_id_list);
   /* Remove all duplicate id's                                       */

/**********************************************************************/
/*                             Main Function                          */
/**********************************************************************/
int main()
{
   ID  *p_id_list,   /* Points to the student id list                 */
       *p_next_list; /* Points to each student id in the list         */
   int  student_id;  /* A student's id number                         */

   /* Loop processing student id list until the user says to quit     */
   while(print_instructions(), get_response() == 'y')
   {
      /* Create an empty list of student IDs                          */
      p_id_list = create_empty_list();

      /* Loop processing inserted student ids until the user says     */
      /* to quit                                                      */
      while(printf("Enter the next student id (%d to quit): ", QUIT),
            scanf ("%d", &student_id), student_id > LIST_HEADER)
      {
         if(student_id > MAXIMUM_ID)
         {
            printf("    Id rejected - it cannot exceed %d", MAXIMUM_ID);
            printf("\n");
         }
         else
            insert_student(p_id_list, student_id);
      }

      /* Print the list of unsorted student ID's and show the number  */
      /* of student IDs                                               */
      printf("\nThe unsorted student ID list, as entered, is: ");
      print_list(p_id_list);
      printf("\nThe length of the unsorted student ID list is: %d", 
         get_count(p_id_list));

      /* Print the list of sorted student ID's and show the number of */
      /* student IDs                                                  */
      sort_list(p_id_list);
      printf("\n\nThe student id list, sorted ascending by id, is:");
      print_list(p_id_list);
      printf("\nThe length of the sorted student ID list is: %d", 
         get_count(p_id_list));

      /* Delete duplicate student IDs, print the student id list,     */
      /* show the number of student ids, and free the id list         */
      printf("\n");
      remove_duplicates(p_id_list);
      printf("\nThe student id list, with duplicate id's removed,");
      printf("sorted ascending by id, is:");
      print_list(p_id_list); 
      printf("\nThe length of the cleared and sorted student"); 
      printf("ID list is: %d", get_count(p_id_list));

      while(p_id_list != NULL)
      {
         p_next_list = p_id_list->p_next_student;
         free(p_id_list);
         p_id_list = p_next_list;
      }
   }

   /* Print a goodbye message and terminate the program               */
   printf("\n\nThanks for using this program. Have a great day! :D");
   printf("\n\n\n\n\n\n");
   return 0;
}

/**********************************************************************/
/*                   Print the program instructions                   */
/**********************************************************************/
void print_instructions()
{
   printf("\n\n\n  This program maintains a list of elements by their");
   printf(    "\n  id number.  After entry of all id's, it sorts them");
   printf(    "\n  into ascending order, then removes all duplicates.");
   return;
}

/**********************************************************************/
/*                Get the response to continue or quit                */
/**********************************************************************/
char get_response()
{
   char response[2]; /* Response to continue or quit                  */

   printf("\n");
   do
   {
      printf("\nDo you want to enter another student id list (y or n): ");
      scanf ("%1s", &response[0]);
      response[0] = tolower(response[0]);
   }
   while(response[0] != 'y' && response[0] != 'n');

   return (response[0]);
}

/**********************************************************************/
/*                        Create an empty list                        */
/**********************************************************************/
ID *create_empty_list()
{
   ID *p_new_list; /* List of student ID's                            */

   if((p_new_list = (ID *)malloc(sizeof(ID))) == NULL)
   {
      printf("\n\nError #%d occurred in create_empty_list",
                                                      HEADER_ALLOC_ERR);
       printf(  "\nUnable to allocate memory for the list header.");
       printf(  "\nThe program is aborting.");
       exit  (HEADER_ALLOC_ERR);
   }
   p_new_list->student_id = LIST_HEADER;

   if((p_new_list->p_next_student = (ID *)malloc(sizeof(ID))) == NULL)
   {
      printf("\n\nError #%d occurred in create_empty_list",
                                                     TRAILER_ALLOC_ERR);
      printf(  "\nUnable to allocate memory for the list header.");
       printf(  "\nThe program is aborting.");
       exit  (TRAILER_ALLOC_ERR);
   }
   p_new_list->p_next_student->student_id = LIST_TRAILER;
   p_new_list->p_next_student->p_next_student = NULL;

   return p_new_list;
}

/**********************************************************************/
/*             Insert a student at the front of the list              */
/**********************************************************************/
void insert_student(ID *p_id_list, int insert_id)
{
   ID *p_current_id = p_id_list->p_next_student,
      *p_new_list,  /* Points to the new id list                      */
      *p_previous_id = p_id_list;

   while(

   if((p_new_list = (ID *)malloc(sizeof(ID))) == NULL)
   {
      printf("\n\nError #%d occurred in insert_student", ID_ALLOC_ERR);
       printf(  "\nUnable to allocate memory for a new student id.");
       printf(  "\nThe program is aborting.");
       exit  (ID_ALLOC_ERR);
   }

   p_new_list->student_id     = insert_id;
   p_new_list->p_next_student = p_current_id;
   p_previous_id->p_next_student  = p_new_list;

   return;
}

/**********************************************************************/
/*               Print all the student id's in the list               */
/**********************************************************************/
void print_list(ID *p_id_list)
{
   printf("\nEntering print_list");
   if(get_count(p_id_list) == LIST_HEADER)
      printf("\n   The student id list is empty.");
   else
   {
      while(p_id_list = p_id_list->p_next_student,
            p_id_list->student_id < LIST_TRAILER)
      {
         printf("\nEntering loop");
         printf("\n                   %6d", p_id_list->student_id);
      }
   }

   return;
}

/**********************************************************************/
/*              Get the count of the students on the list             */
/**********************************************************************/
int get_count(ID *p_id_list)
{
   int student_count=0; /* Counts the length of the id list           */

   while(p_id_list->p_next_student->student_id != LIST_TRAILER)
      student_count += 1;

   return(student_count);
}

/**********************************************************************/
/*            Sort the students into ascending order by id            */
/**********************************************************************/
void sort_list(ID *p_student_id)
{
   ID  *p_id_list,     /* Pointer to a student ID                     */
       *p_temp_id;     /* Pointer to a temporary student ID           */
   int student_number; /* Student ID sorting counter                  */

   for (student_number = 0; student_number < get_count(p_student_id);
                                                       student_number++)
   {
      p_id_list = p_student_id;

      while(p_id_list->p_next_student->p_next_student->student_id !=
            LIST_TRAILER)
      {
         if(p_id_list->p_next_student->student_id < 
            p_id_list->p_next_student->p_next_student->student_id)
         {
            p_temp_id                 = 
               p_id_list->p_next_student->p_next_student;
            p_id_list->p_next_student->p_next_student
               = p_temp_id->p_next_student;
            p_temp_id->p_next_student = p_id_list->p_next_student;
            p_id_list->p_next_student    = p_temp_id;
         }
         p_student_id = p_student_id->p_next_student;
      }
   }
}

/**********************************************************************/
/*                      Remove all duplicate id's                     */
/**********************************************************************/
void remove_duplicates(ID *p_id_list)
{
   ID *p_current_id  = p_id_list->p_next_student,  
                                  /* Points to the current id         */
      *p_previous_id = p_id_list; /* Points to the previous id        */

   printf("\n");
   while(p_current_id->student_id != LIST_TRAILER)
   {
      if(p_previous_id->student_id == p_current_id->student_id)
      {
         printf("\nDeleting the duplicate student id: %d",
                                              p_current_id->student_id);
         p_previous_id->p_next_student = p_current_id->p_next_student;
         free(p_current_id);
         p_current_id = p_previous_id->p_next_student;
      }
      else
      {
         p_previous_id = p_previous_id->p_next_student;
         p_current_id  = p_current_id->p_next_student;
      }
   }

   return;
}

我不确定哪个函数有问题，因为它对我来说似乎没问题。

Answer 1

在 get_count 中存在无限循环，因为它一遍又一遍地检查相同的 p_id_list 而不会移动到下一条记录。

while(p_id_list->p_next_student->student_id != LIST_TRAILER)
    student_count += 1;

只需确保每次循环都访问下一条记录即可。例如:

ID *cur = p_id_list->p_next_student;
while (cur->student_id != LIST_TRAILER) {
    student_count += 1;
    cur = cur->p_next_student;
}