wrong output when code runs problem with linked list? - c

Question

Upon running the code below i get the output
NAME: (null) | GPA: 0.000000 | YEAR: (NULL)
are the linked lists not implemented correctly? I am currently using a makefile and bringing in a test.data file with names and gpa and senior/ect..
Ollie 2.9 freshmen
John 3.2 senior
Julie 2.2 freshmen
Joe 1.8 freshmen
Mary 3.8 senior
Sue 3.4 junior
Jane 2.7 senior
Bob 2.8 senior
Fred 3.2 freshmen
Bill 3.3 junior
#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include "students.h"
Student *top = NULL;
Student *temp, *temp1, *temp2;
// Creates the entire linked list from the file.
// Should call readNext and push
// Returns head of the linked list
Student *buildStudentList()
{
Student *p;
p = readNext();
push(&top, p);
return top; //TODO: Change return
}
//Read a single line from standard input and return a student structure located on the heap
Student *readNext()
{
Student *s =(Student*) malloc(sizeof(Student));
scanf("%s", s -> name);
scanf("%f", &s -> gpa);
scanf("%s", s -> year);
s->next = NULL;
return s; //TODO: Change return
}
//Return a student structure stored on the heap
Student *makeStudent(char *name, float gpa, char *year)
{
Student *s =(Student*) malloc(sizeof(Student));
s -> name = name;
s -> gpa = gpa;
s -> year = year;
s -> next = NULL;
return s; //TODO: Change return
}
//insert a new student node at the head of the linked list
void push(Student **list, Student *student)
{
top = *list;
student -> next = top;
top = student;
}
//Insert a student node in the desired position on the linked list
void insert(Student *list, Student *s, int position)
{
int i;
top = list;
temp = top;
for(i = 1; i < position -1; i++)
{
temp = temp -> next;
}
if(temp == NULL)
{
//blank
}
else
{
s -> next = temp -> next;
temp -> next = s;
}
}
//Displays contents of a single student structure
void display(Student *s){
printf("NAME:%s | GPA: %f | YEAR:%s
", s -> name, s-> gpa, s -> year);
}
//Displays contents of the entire linked list
void displayAll(Student *list)
{
temp = list;
while(temp != NULL)
{
display(temp);
temp = temp -> next;
}
}
//Delete all data allocated on the heap before terminating program
void cleanUp(Student *list)
{
temp1 = list;
temp2 = temp1 -> next;
while(temp1 != NULL)
{
free(temp1);
temp1 = temp2;
}
if(temp2 != NULL)
{
temp2 = temp2 -> next;
}
}
//Main function tests your functions.
int main()
{
printf("Program Started
");
//Construct Linked List from Standard Input
Student *list = buildStudentList();
//Insert a new student in desired position
Student *s = makeStudent("Max",3.0, "senior");
insert(list, s, 3);
//Display entire linked list
displayAll(list);
//Free all heap memory
cleanUp(list);
printf("Program Successful Exit
");
exit(EXIT_SUCCESS);
}

Answer 1

Since you didn't post your struct definition, I had to guess whether (e.g.) name was char *name; or (e.g. char name[100];). Within the code, it used it as a pointer.
So ...
Your readNext and makeStudent don't allocate space for the strings (char * pointers) name and year, so they're probably segfaulting.
insert takes Student *list when it really needs Student **list.
IMO, you should have a separate List type to avoid confusion (that has a single element: Student *head;). So, wherever you have Student *list, you replace it with List *list
When you do that, you don't have to pass down Student ** [a double star] pointer when you mean a list. Using list->head is a lot easier and more descriptive than *list.
Also, be consistent. Some functions take Student **list if they modify the list [they have to]. Others use Student *list, but they should be consistent as well.
No need for the various global scope temp variables. These should be function scoped and use more descriptive names.
Your insert has issues. It will orphan the node it's trying to insert if no position match is found (e.g. insert at position 99 in your example). Usual is to insert at tail or return an error code. Also, it wasn't totally clear what position meant [to me], because of the code you had. It could have been "insert before" or "insert after" the Nth node.
You can't insert a literal newline in a double quoted string. So, use the \n escape sequence (e.g.) printf("hello world\n");
Also, functions that take no arguments should use void (e.g.) instead of int main(), use int main(void).
Here's a cleaned up version of your code, incorporating what I've mentioned above:
#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
//#include "students.h"
typedef struct student Student;
struct student {
Student *next;
float gpa;
char *name;
char *year;
};
typedef struct list {
Student *head;
} List;
//insert a new student node at the head of the linked list
void
push(List *list, Student *student)
{
student->next = list->head;
list->head = student;
}
//Return a student structure stored on the heap
Student *
makeStudent(char *name, float gpa, char *year)
{
Student *s = (Student *) malloc(sizeof(Student));
s->name = strdup(name);
s->gpa = gpa;
s->year = strdup(year);
s->next = NULL;
return s;
}
//Read a single line from standard input and return a student structure located on the heap
Student *
readNext(void)
{
char name[1000];
float gpa;
char year[1000];
Student *s = NULL;
int count = scanf("%s %f %s",name,&gpa,year);
if (count == 3) {
printf("readNext: name='%s' gpa=%g year='%s'\n",name,gpa,year);
s = makeStudent(name,gpa,year);
}
return s;
}
// Creates the entire linked list from the file.
// Should call readNext and push
// Returns head of the linked list
List *
buildStudentList(List *list)
{
Student *p;
while (1) {
p = readNext();
if (p == NULL)
break;
push(list, p);
}
return list;
}
//Insert a student node in the desired position on the linked list
int
insert(List *list, Student *s, int position)
{
Student *cur;
Student *prev;
int i;
int goflg;
//position -= 1;
#if 0
i = 1; // insert before Nth position
#else
i = 0; // insert after Nth position
#endif
prev = NULL;
for (cur = list->head; (cur != NULL) && (i < position);
++i, cur = cur->next) {
prev = cur;
}
// this may not be needed -- usual is to insert at tail if position is not
// found -- this will orphan the node to be inserted
#if 0
goflg = (i == position);
#else
goflg = 1;
#endif
if (goflg) {
s->next = cur;
if (prev != NULL)
prev->next = s;
else
list->head = s;
}
return goflg;
}
//Displays contents of a single student structure
void
display(Student *s)
{
printf("NAME:%s | GPA: %f | YEAR:%s\n", s->name, s->gpa, s->year);
}
//Displays contents of the entire linked list
void
displayAll(List *list)
{
Student *temp = list->head;
while (temp != NULL) {
display(temp);
temp = temp->next;
}
}
//Delete all data allocated on the heap before terminating program
void
cleanUp(List *list)
{
Student *cur;
Student *next;
for (cur = list->head; cur != NULL; cur = next) {
next = cur->next;
free(cur->name);
free(cur->year);
free(cur);
}
list->head = NULL;
}
//Main function tests your functions.
int
main(void)
{
List top = { NULL };
List *list;
printf("Program Started\n");
//Construct Linked List from Standard Input
list = buildStudentList(&top);
//Insert a new student in desired position
Student *s = makeStudent("Max", 3.0, "senior");
insert(list, s, 3);
//Display entire linked list
displayAll(list);
//Free all heap memory
cleanUp(list);
printf("Program Successful Exit\n");
exit(EXIT_SUCCESS);
}