FILE reading and sorting a link list - c

Question

It seems my FILE reading function is slightly off. Instead of printing all the data (First Name: Last Name: priority: Reading Level:) per entry when I call my print function it stops after two and just has the skeleton of what needs to be printed there for the last three entries. And because of that, when I then call my sort function based off priority level in each struct link list, I can't tell if it's printing correctly.
#include <stdlib.h>
#include <stdbool.h>
#include <stdio.h>
#define NAME 25
#define TITLE 50
typedef struct Book{
int bookID;
char* title;
bool checkedOut;
struct Book* next;
}book;
typedef struct Student{
char* firstName;
char* lastName;
int priority;
int readingLevel;
book* backPack;
bookids* wishlist;
struct Student* next;
}student;
student* buildStudentList(char* studentFile, student* head)
{
FILE *cfptr=fopen(studentFile, "r");
if(cfptr==NULL){
printf("\nFIle could not be opened\n");
return 0;
}
if(head==NULL){
head=malloc(sizeof(student));
head->next=NULL;
head->firstName=malloc(sizeof(student)*35);
head->lastName=malloc(sizeof(student)*35);
fscanf(cfptr,"%s %s %d %d", head->firstName,head->lastName,
&head->priority, &head->readingLevel);
}
student* current=head;
while(current->next!=NULL){
current=current->next;
}
current->next=malloc(sizeof(student));
current=current->next;
current->firstName=malloc(sizeof(student)*35);
current->lastName=malloc(sizeof(student)*35);
while( fscanf(cfptr, "%s %s %d %d", current->firstName, current->lastName,
&current->priority, &current->readingLevel)!=EOF){
student* current=head;
while(current->next!=NULL){
current=current->next;
}
current->next=malloc(sizeof(student));
current=current->next;
current->firstName=malloc(sizeof(student)*35);
current->lastName=malloc(sizeof(student)*35);
}
return head;
}
void createPriorityQueue(student* head)
{
if(head==NULL ||head->next==NULL){
return;
}
student* curr; student* largest; student* largestPrev; student* prev;
curr=head;
largest=head;
prev=head;
largestPrev=head;
while(curr!=NULL){
if(&curr->priority>&largest->priority){
largestPrev=prev;
largest=curr;
}
prev=curr;
curr=curr->next;
}
student* tmp;
if(largest!=head)
{
largestPrev->next=head;
tmp=head->next;
head->next=tmp;
}
}

Answer 1

There are a few bugs.
The malloc for allocation of strings should be malloc(35) and not malloc(sizeof(student) * 35). It won't crash but it's wasteful.
Your build function was way too complicated and had several bugs--I had to recode it.
Your priority queue function was comparing addresses of priority instead of values [what we want].
Anyway, here's your code with some annotations for the bugs and a recoded create function [please pardon the gratuitous style cleanup]:
#include <stdlib.h>
#include <stdbool.h>
#include <stdio.h>
#define NAME 25
#define TITLE 50
typedef struct Book {
int bookID;
char *title;
bool checkedOut;
struct Book *next;
} book;
typedef struct Student {
char *firstName;
char *lastName;
int priority;
int readingLevel;
book *backPack;
bookids *wishlist;
struct Student *next;
} student;
// NOTE/BUG: this function is way to complicated -- see below for a simpler one
student *
buildStudentList(char *studentFile, student * head)
{
FILE *cfptr = fopen(studentFile, "r");
if (cfptr == NULL) {
printf("\nFIle could not be opened\n");
return 0;
}
if (head == NULL) {
head = malloc(sizeof(student));
head->next = NULL;
// NOTE/BUG: we want just enough to store strings -- this is true for all the
// other mallocs that allocate names
#if 0
head->firstName = malloc(sizeof(student) * 35);
head->lastName = malloc(sizeof(student) * 35);
#else
head->firstName = malloc(35);
head->lastName = malloc(35);
#endif
fscanf(cfptr, "%s %s %d %d",
head->firstName, head->lastName, &head->priority,
&head->readingLevel);
}
student *current = head;
while (current->next != NULL) {
current = current->next;
}
current->next = malloc(sizeof(student));
current = current->next;
current->firstName = malloc(sizeof(student) * 35);
current->lastName = malloc(sizeof(student) * 35);
while (fscanf(cfptr, "%s %s %d %d",
current->firstName, current->lastName, &current->priority,
&current->readingLevel) != EOF) {
student *current = head;
while (current->next != NULL) {
current = current->next;
}
current->next = malloc(sizeof(student));
current = current->next;
current->firstName = malloc(sizeof(student) * 35);
current->lastName = malloc(sizeof(student) * 35);
}
// NOTE/BUG: close input file
#if 1
fclose(cfptr);
#endif
return head;
}
// simplified version
student *
buildStudentList(char *studentFile, student * head)
{
FILE *cfptr = fopen(studentFile, "r");
student *current;
student *tail;
char firstName[5000];
char lastName[5000];
if (cfptr == NULL) {
printf("\nFIle could not be opened\n");
return 0;
}
// find last element in list
tail = NULL;
for (current = head; current != NULL; current = current->next)
tail = current;
while (1) {
// allocate new node
current = malloc(sizeof(student));
// initialize the next pointer (in a singly linked list, this is nulled)
current->next = NULL;
// get data for new node
if (fscanf(cfptr, "%s %s %d %d",
firstName, lastName, &current->priority,
&current->readingLevel) == EOF) {
free(current);
break;
}
// allocate space for strings
current->firstName = strdup(firstName);
current->lastName = strdup(lastName);
// add to empty list
if (head == NULL)
head = current;
// add to tail of list
else
tail->next = current;
// set this as new tail of list
tail = current;
}
fclose(cfptr);
return head;
}
void
createPriorityQueue(student * head)
{
if (head == NULL || head->next == NULL) {
return;
}
student *curr;
student *largest;
student *largestPrev;
student *prev;
curr = head;
largest = head;
prev = head;
largestPrev = head;
while (curr != NULL) {
// NOTE/BUG: this compares _addresses_ but we want to compare values
#if 0
if (&curr->priority > &largest->priority) {
#else
if (curr->priority > largest->priority) {
#endif
largestPrev = prev;
largest = curr;
}
prev = curr;
curr = curr->next;
}
student *tmp;
if (largest != head) {
largestPrev->next = head;
tmp = head->next;
head->next = tmp;
}
}
UPDATE:
how do I make the createpriority function sort in the opposite order, its form highest to lowest, i need lowest to highest?
Not sure exactly which order you want now or what changes you've already made. In your original code, you were only doing a single pass on the list instead of as many passes as there were elements.
I've fixed the original code to do the correct number of passes, using simple insertion/selection. This is slow. You could speed it up by doing a mergesort on the linked list, but that's more involved.
Anyway, here's the code. I didn't test it or even compile it but it should get you closer:
student *
createPriorityQueue(student *head)
{
student *curr;
student *bestCurr;
student *bestPrev;
student *prev;
student *tail;
student *rtn;
// initialize sorted list
rtn = NULL;
tail = NULL;
// process all elements of unsorted list
while (head != NULL) {
// assume head of unsorted list is best priority in unsorted list
bestCurr = head;
bestPrev = NULL;
// find best in unsorted list
prev = NULL;
for (curr = head; curr != NULL; curr = curr->next) {
#ifdef SORT_LOW_TO_HIGH
if (curr->priority < bestCurr->priority) {
bestCurr = curr;
bestPrev = prev;
}
#else
if (curr->priority > bestCurr->priority) {
bestCurr = curr;
bestPrev = prev;
}
#endif
prev = curr;
}
// remove element from unordered list
if (bestPrev != NULL)
bestPrev->next = bestCurr->next;
else
head = bestCurr->next;
// add the next best priority to the end of the ordered list
if (tail != NULL)
tail->next = bestCurr;
else
rtn = bestCurr;
// set new tail of ordered list
tail = bestCurr;
tail->next = NULL;
}
return rtn;
}