I set head to be something but it is always null - c

Question

I am simply just trying to print out my doubly linked list. However, even though I clearly set my head and tail to something, it is always NULL when I print it in main. I am unsure on what the issue is. I've tried to simplify the code as much as I can.
In the function grade_word_gen you can clearly see I set head to something and I set tail to something.
test.txt
*A*
Great
Fantastic
Lovely
*B*
Bad
Not Good
Terrible
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct grade {
struct grade_word *A_head;
struct grade_word *A_tail;
struct grade_word *B_head;
struct grade_word *B_tail;
};
struct grade_word {
char *word;
struct grade_word *next;
struct grade_word *prev;
};
struct grade *create_grade() {
struct grade *new_grade = malloc(sizeof(struct grade));
// Check grade was allocated correctly
if (new_grade == NULL) {
fprintf(stderr, "ERROR: Could not allocate memory for grade\n");
exit(1);
}
// Initialise all variables
new_grade->A_head = NULL;
new_grade->A_tail = NULL;
new_grade->B_head = NULL;
new_grade->B_tail = NULL;
return new_grade;
}
struct grade_word *create_grade_word(char *word) {
struct grade_word *new = malloc(sizeof(struct grade_word));
if (new == NULL) {
fprintf(stderr, "ERROR: Unable to allocate memory for grade_words\n");
exit(1);
}
// Initialise vairables
int len = strlen(word);
new->word = malloc(sizeof(char) * (len + 1));
strcpy(new->word, word);
new->next = NULL;
new->prev = NULL;
return new;
}
void grade_word_gen(struct grade *grade_data) {
FILE *fp = fopen("test.txt", "r");
char grade;
char buf[100 + 1];
struct grade_word *new_node;
struct grade_word *head;
struct grade_word *tail;
while (fgets(buf, 100, fp) != NULL) {
if (buf[0] == '*' && buf[2] == '*') {
grade = buf[1];
} else {
new_node = create_grade_word(buf);
// Set next, prev, head, tail pointers
if (grade == 'A') {
head = grade_data->A_head;
tail = grade_data->A_tail;
} else {
head = grade_data->B_head;
tail = grade_data->B_tail;
}
// If first item set the head
if (head == NULL) {
head = new_node;
//printf("head: %s\n", head->word);
// Otherwise just add on to the list
} else {
new_node->prev = tail;
tail->next = new_node;
}
tail = new_node;
}
// Reset buffer
strcpy(buf, "\0");
}
}
void print_grade_list(struct grade_word *list, char grade) {
if (list == NULL) {
printf("Grade %c is empty, so not grade words can be printed\n", grade);
return;
}
printf("Grade: %c\n", grade);
while (list != NULL) {
printf("%s\n", list->word);
list = list->next;
}
}
int main(void) {
struct grade *new_grade = create_grade();
grade_word_gen(new_grade);
print_grade_list(new_grade->A_head, 'A');
print_grade_list(new_grade->B_head, 'B');
}
My output is always Grade %c is empty, so not grade words can be printed. I don't understand why my head is always null, even though I do set it.

Answer 1

You never assign anything to A_head except in the initialization part where you assign NULL. Consequently A_head will remain at NULL:
The problem is here:
if (grade == 'A') {
head = grade_data->A_head; // Here you make head equal A_head
tail = grade_data->A_tail;
} else {
head = grade_data->B_head;
tail = grade_data->B_tail;
}
// If first item set the head
if (head == NULL) {
head = new_node; // Here you try to save new_node.
// But you save it into head and that will not
// change A_head
You need to have code like:
grade_data->A_head = new_node;
so that you actually change A_head.
An alternative way so that you can share the code between case A and B is double pointers. Like:
// Make a double pointer
struct grade_word **head;
. . .
// Make it point to either the A or B head pointer
head = &grade_data->A_head; // or head = &grade_data->B_head;
. . .
// Change A or B head pointer using head
*head = new_node;