Why is C not creating my Linked List head? [duplicate] - c

Question

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How do I modify a pointer that has been passed into a function in C?
(7 answers)
Closed 3 years ago.
I am currently trying to implement a Linked List in C. However, my function to create the head is not working apparently, since my other function to add a new node throws a dereferencing null pointer exception. Also, the size variable that keeps the amount of nodes is not being increased. Here is my full code.
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <math.h>
#include <time.h>
typedef struct node {
int val;
struct node* next;
} Node;
void printView(int);
bool terminate();
void createHead(Node*, int);
void addNext(Node*, int);
int main()
{
srand(time(NULL));
int size = 0;
Node* head = NULL;
bool created = false;
bool end = false;
while (!end)
{
printView(size);
int choice;
scanf_s("%d", &choice);
switch (choice)
{
case 1:
{
if (!created)
{
createHead(head, size);
created = true;
}
else
printf("The head has already been created \n");
break;
}
case 2:
{
if (created)
addNext(head, size);
else
printf("The head needs to be created first \n");
break;
}
case 0:
{
bool t = terminate();
if (t)
end = true;
break;
}
}
}
return 0;
}
void printView(int size)
{
printf("Welcome to Linked Lists! - %d Nodes in List\n", size);
printf(" Type 1 to create a head \n");
printf(" Type 2 to create a new node \n");
printf(" Type 0 to exit \n");
}
bool terminate() //Exit
{
int save;
printf("Would you like to save your Linked List? \n(Enter 1 to save or 0 for not to save) \n");
scanf_s("%d", &save);
if (save == 1)
{
printf("The Linked List has been saved. It will show up next time you start the program \n");
}
else if (save == 0)
printf("Goodbye! \n");
else
{
printf("Please type a valid option \n");
return false;
}
return true;
}
void createHead(Node* head, int size)
{
head = malloc(sizeof(Node));
if (head == NULL) {
printf("Failed to create head, aborting operation \n");
return;
}
printf("Type a value for the new node: \n(It must be an integer / Type 0 to assign a random number) \n");
int value; scanf_s("%d", &value);
if (value == 0)
{
value = rand() % 11;
head->val = value;
printf("Value set to: %d \n", value);
}
else
head->val = value;
head->next = NULL;
size++;
}
void addNext(Node* node, int size)
{
Node* current = node;
while (current->next != NULL)
current = current->next;
current->next = malloc(sizeof(Node));
if (current->next == NULL)
{
printf("Failed to create new node, aborting operation \n");
return;
}
printf("Type a value for the new node: \n(It must be an integer / Type 0 to assign a random number) \n");
int value; scanf_s("%d", &value);
if (value == 0)
{
value = rand() % 11;
current->val = value;
printf("Value set to: %d \n", value);
}
else
current->val = value;
current->next = NULL;
size++;
}

Answer 1

The function create_head only modifies its arguments, not the variables in the calling function main. You should change the prototype to bool createHead(Node **headp, int *sizep) and update the values indirectly`.
There are other problems:
some include files are missing
the same problem in add_next() prevents size from getting updated in main.
passing the address of head to addNext removes the need for a separate function to create the initial list node.
it would be safer to define a List structure with a head and a size fields and pass that to the different functions.
Here is a modified version:
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
typedef struct node {
int val;
struct node *next;
} Node;
void printView(int size);
bool terminate(void);
bool createHead(Node **headp, int *sizep);
bool addNext(Node **headp, int *sizep);
int main() {
srand(time(NULL));
int size = 0;
Node *head = NULL;
bool created = false;
bool end = false;
while (!end) {
printView(size);
int choice;
scanf_s("%d", &choice);
switch (choice) {
case 1:
if (!created) {
created = createHead(&head, &size);
} else {
printf("The head has already been created \n");
}
break;
case 2:
if (created)
addNext(&head, &size);
else
printf("The head needs to be created first \n");
break;
case 0:
end = terminate();
break;
}
}
}
return 0;
}
void printView(int size) {
printf("Welcome to Linked Lists! - %d Nodes in List\n", size);
printf(" Type 1 to create a head \n");
printf(" Type 2 to create a new node \n");
printf(" Type 0 to exit \n");
}
bool terminate(void) {
int save = 0;
printf("Would you like to save your Linked List? \n"
"(Enter 1 to save or 0 for not to save) \n");
scanf_s("%d", &save);
if (save == 1) {
// XXX: save the list
printf("The Linked List has been saved. It will show up next time you start the program \n");
} else if (save == 0) {
printf("Goodbye! \n");
} else {
printf("Please type a valid option \n");
return false;
}
return true;
}
bool createHead(Node **headp, int *sizep) {
Node *head = malloc(sizeof(Node));
if (head == NULL) {
printf("Failed to create head, aborting operation \n");
*headp = head;
*sizep = 0;
return false;
}
printf("Type a value for the new node: \n"
"(It must be an integer / Type 0 to assign a random number) \n");
int value;
scanf_s("%d", &value);
if (value == 0) {
value = rand() % 11;
printf("Value set to: %d \n", value);
}
head->val = value;
head->next = NULL;
*headp = head;
*sizep = 1;
return true;
}
bool addNext(Node **headp, int *sizep) {
Node *new_node = malloc(sizeof(Node));
if (new_node == NULL) {
printf("Failed to create new node, aborting operation \n");
return false;
}
printf("Type a value for the new node: \n(It must be an integer / Type 0 to assign a random number) \n");
int value;
scanf_s("%d", &value);
if (value == 0) {
value = rand() % 11;
printf("Value set to: %d \n", value);
}
current->val = value;
current->next = NULL;
if (*headp == NULL) {
*headp = new_node;
} else {
Node *current = *headp;
while (current->next != NULL)
current = current->next;
current->next = new_node;
}
*sizep += 1;
return true;
}