Thread: Problem with linked list

Hey all,

I'm having a trouble with this code, which supposed to get names of students and their grades, determine fail students and passed, sort the names in lexicographical order and then prints two lists of failed and passed students... if you have any clue about fixing it I'll be more than glad! thank you.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


#define MAX_NAME_SIZE 30


typedef struct student_node
{
	char name[MAX_NAME_SIZE];
	double grade;
	struct Student* next;
} Student;


void free_mem(Student *head)
{
	if (head == NULL)
		return;


	free_mem(head->next);
	free(head);
}


Student *read_students()
{
	char name[MAX_NAME_SIZE];
	double grade;
	
	Student *head = NULL, *nstd = NULL;


	printf("Enter name:\n");
	scanf("%s",&name);
	
	while (strcmp(name, "exit")!= 0)
	{
		printf("Enter grade:\n");
		scanf("%lf", &grade);
		nstd = (Student*) malloc (sizeof(Student));
		
		if (nstd == NULL)
		{
			printf("Fatal error: memory allocation failed!\n");            
				return 0;
		}


		strcpy(nstd->name, name);
		nstd->grade = grade;
		nstd->next = head;
		head = nstd;


		printf("Enter Name:\n");
		scanf("%s",name);
	}
		return head;
}


void print_list(Student *pHead)
{
	Student* list;
	
	for (list = pHead; list != NULL; list = list->next)
		printf("\n%s %g\n", list->name, list->grade);
	
	return;
}


void split_list(Student *pHead, Student** pass, Student** fail)
{
	Student* list= pHead;
	Student *passed_student = NULL;
	Student *failed_student = NULL;
	Student* buffer=pHead;


	while(list!=NULL)
	{
		buffer=list->next;


		if (list->grade >= 60)
		{
			list->next = *pass;
			*pass = list;
		}


		if (list->grade < 60)
		{
			list->next = *fail;
			*fail = list;
		}


		list = buffer;
	}
}


Student *sort_list(Student **pHead)
{
	Student* buffer=NULL, *list=*pHead, *last=NULL, *tmp=*pHead, *first=*pHead;
	if(first==NULL)
		return;
	while(first->next!=last)
	{
		for(list=first;list->next!=last;list=list->next)
		{
			if((strcmp(list->name, list->next->name)>0) && (list == first))
			{
					buffer=list->next;
					first->next=buffer->next;
					buffer->next=list;
					first=buffer;
					list=buffer;		
			}
			
			else
				{
					buffer=list->next;
					tmp->next=buffer;
					list->next=buffer->next;
					buffer->next=list;
					list=buffer;
				}


			tmp=list;
		}


		last=tmp->next;
	}
	if(strcmp(first->name, first->next->name)>0)
			{
				buffer=list->next;
				first->next=buffer->next;
				buffer->next=list;
				first=buffer;
				list=buffer;
			}
	*pHead=first;
	return 0;
}






int main()
{
	Student *head = read_students();
	Student *pass_list=NULL, *fail_list=NULL;
	
	split_list(head, &pass_list, &fail_list);


	sort_list(&pass_list);
	sort_list(&fail_list);


	printf("The Student that passed the test are\n");
	print_list(pass_list);


	printf("The Student that failed the test are\n");
	print_list(fail_list);


	free_mem(head);
	free_mem(pass_list);
	free_mem(fail_list);


	return 0;
}

the problem is in the sort_list function....

Please be specific. What exactly is the problem? How do you know it is in the sort_list function? It could be anywhere, given the number of warnings your code generates. You have some garbage characters at the end of line 47 that caused a few problems. Removing those, here's what I get when I compile your code (if you don't see something similar, turn up the warnings on your compiler):

Code:

$ gcc -Wall -g    llist.c   -o llist
llist.c: In function ‘free_mem’:
llist.c:23: warning: passing argument 1 of ‘free_mem’ from incompatible pointer type
llist.c:17: note: expected ‘struct Student *’ but argument is of type ‘struct Student *’
llist.c: In function ‘read_students’:
llist.c:37: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char (*)[30]’
llist.c:54: warning: assignment from incompatible pointer type
llist.c: In function ‘print_list’:
llist.c:69: warning: assignment from incompatible pointer type
llist.c: In function ‘split_list’:
llist.c:86: warning: assignment from incompatible pointer type
llist.c:91: warning: assignment from incompatible pointer type
llist.c:98: warning: assignment from incompatible pointer type
llist.c:80: warning: unused variable ‘failed_student’
llist.c:79: warning: unused variable ‘passed_student’
llist.c: In function ‘sort_list’:
llist.c:112: warning: ‘return’ with no value, in function returning non-void
llist.c:113: warning: comparison of distinct pointer types lacks a cast
llist.c:115: warning: comparison of distinct pointer types lacks a cast
llist.c:115: warning: assignment from incompatible pointer type
llist.c:117: error: dereferencing pointer to incomplete type
llist.c:119: warning: assignment from incompatible pointer type
llist.c:121: warning: assignment from incompatible pointer type
llist.c:128: warning: assignment from incompatible pointer type
llist.c:129: warning: assignment from incompatible pointer type
llist.c:131: warning: assignment from incompatible pointer type
llist.c:140: warning: assignment from incompatible pointer type
llist.c:142: error: dereferencing pointer to incomplete type
llist.c:144: warning: assignment from incompatible pointer type
llist.c:146: warning: assignment from incompatible pointer type

Quite a few errors there. Fix all of them:

• Line 23,17: Your struct declaration at the top is a little funky. Put the typedef on a line by itself above the struct and use the typedef'ed type in your struct for the next pointer. See below.
• Line 37: Don't use & with char arrays, the name of the array is a pointer to the first element.
• Line 112: Just a return by itself returns garbage unless the function returns void. At the bottom of sort_list you return 0 (NULL since the function returns a pointer), and you never use the return value when you call it, so make it return void.
• Most/all of the rest of the errors seem to be related to the funky typedef.

Code:

typedef struct student_node      Student;
struct student_node {
    char name[MAX_NAME_SIZE];
    double grade;
    Student *next;
};

first of all thanks for helping.

Ok, this code basically have few stages. The first stage gets names and grades. Second stage splits the input to two lists, first list is the students who failed and second list is the students who passed. Then, the code supposed to sort the names of the students in lexicographical order and print the students who failed and students who passed. The code does work until the "bubble-sort". So the obvious guess is that the sort_list is the one making the problem crash.

That's exactly what I did. The programs crashes after I add the last part which is the sorting.

Do you have any idea how to write the Student *sort_list(Student **pHead) function using the same "function name" in the insertion sort you mentioned?