I cannot create a linked list in C - c

Question

Basically for my assignment I need to implement code to do Huffman coding. For that I need to take input as a string and then create a list of characters and their frequencies. I need to create a new node when there is a new character. I have tried doing it in C with no result. When I try to print my linked list I simply cannot get any output. I believe I have failed in creating the list from the start.
My C code is below:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
struct node {
char character;
int frequency;
struct node *next;
}head;
struct node *insert(int frequency, char character) {
struct node *newNode = malloc(sizeof(struct node));
newNode->character = frequency;
newNode->character = character;
return newNode;
}
void create_freq_list(struct node *initial, int str_size, char str[]) {
int i;
struct node *temp;
bool has_node;
for (i = 0; i < str_size; i++) {
temp = initial;
has_node = false;
while (temp->next != NULL) {
if (temp->character == str[i]) {
has_node = true;
temp->frequency++;
}
temp = temp->next;
}
if (has_node == false) {
while (temp->next != NULL) {
temp = temp->next;
if (temp->next == NULL) {
temp->next = insert(0, str[i]);
}
}
}
}
}
int main() {
struct node *temp;
char str[100];
gets_s(str, 100);
create_freq_list(&head, 100, str);
temp = &head;
while (temp->next != NULL) {
printf("'%c' : %d", temp->character, temp->frequency);
temp = temp->next;
}
getch();
exit(0);
}

Answer 1

There are multiple issues in your code:
Your handling of the head node is incorrect: head should be defined as a node * and you should pass its address to create_freq_list().
There is a typo in the insert() function: newNode->character = frequency;
You should not iterate on characters of the string beyond the null terminator.
The output loop is incorrect: it should iterate while (head), not while (head->next). As coded, the initial node is output but meaningless and the last node is ignored.
Here is a modified version:
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct node {
char character;
int frequency;
struct node *next;
};
struct node *insert(int frequency, char character) {
struct node *newNode = malloc(sizeof(struct node));
if (newNode != NULL) {
newNode->frequency = frequency;
newNode->character = character;
}
return newNode;
}
void create_freq_list(struct node **headp, const char str[]) {
for (int i = 0; str[i]; i++) {
struct node **tailp = *headp;
struct node *temp;
while ((temp = *tailp) != NULL) {
if (temp->character == str[i]) {
temp->frequency++;
break;
}
tailp = &temp->next;
}
if (temp == NULL) {
*tailp = insert(1, str[i]);
}
}
}
int main() {
struct node *head = NULL;
char str[100];
gets_s(str, 100);
create_freq_list(&head, str);
for (struct node *temp = head; temp != NULL; temp = temp->next) {
printf("'%c': %d\n", temp->character, temp->frequency);
}
getch();
return 0;
}
Note that it is much simpler to use an array with 256 elements to compute the character frequencies.

Answer 2

May I propose a variant, using macros of the excellent <sys/queue.h>
This is not a standard include, but every well packaged system should have it.
Well, it's probably less pedagogical than coding linked-list by hands, but it's more secure ;-)
Have a look at man queue (or man LIST_INIT) to see features.
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/queue.h>
typedef LIST_HEAD(listhead, entry) nodes_t;
typedef struct entry {
char character;
int frequency;
LIST_ENTRY(entry) entries;
} node_t;
void insert(nodes_t *list, char character) {
node_t *newNode = malloc(sizeof(node_t));
if (newNode != NULL) {
newNode->frequency = 1;
newNode->character = character;
LIST_INSERT_HEAD(list, newNode, entries);
}
}
node_t *get_node(nodes_t *list, char character) {
node_t *n;
LIST_FOREACH(n, list, entries)
if (n->character == character)
return n;
return NULL;
}
void create_freq_list(nodes_t *list, const char str[]) {
node_t *n;
for (int i = 0; str[i]; i++) {
n = get_node(list, str[i]);
if (n == NULL)
insert(list, str[i]);
else
n->frequency++;
}
}
void print_list(nodes_t *list) {
node_t *n;
LIST_FOREACH(n, list, entries)
printf("'%c': %d\n", n->character, n->frequency);
}
int main(int argc, char* argv[]) {
nodes_t list;
if (argc != 2) {
fprintf(stderr, "Usage: %s <string>\n", argv[0]);
return -1;
}
LIST_INIT(&list);
create_freq_list(&list, argv[1]);
print_list(&list);
return 0;
}