How to keep track of the initial head pointer in C? [closed] - c

Question

Closed. This question needs details or clarity. It is not currently accepting answers.
Want to improve this question? Add details and clarify the problem by editing this post.
Closed 4 years ago.
Improve this question
I am trying to append at the end of the list and return head at the end of this function.
This is a singly linked list. So, I have lost my head after traversing to the end.
file.c
#define NAME_LEN 30
struct equipment{
char type[NAME_LEN+1];
char description[NAME_LEN+1];
int quantity;
struct equipment *next;
};
struct equipment *append_to_list(struct equipment *list){
char type[NAME_LEN + 1], description[NAME_LEN + 1];
int quantity;
printf("Enter equipment type: ");
fgets(type, NAME_LEN, stdin);
printf("Enter description of the equipment: ");
fgets(description, NAME_LEN, stdin);
printf("Enter quantity: ");
scanf("%d", &quantity);
struct equipment *temp = (struct equipment *)malloc(sizeof(struct equipment));
strcpy(temp->type, type);
strcpy(temp->description, description);
temp->quantity = quantity;
temp->next = NULL;
bool doesExist = false;
if ( list == NULL ){
list = temp;
}
else{
while ( list->next != NULL ){
if ( list == temp ){
printf("This equipment is already in the list\n");
}
list = list->next;
}
list->next = temp;
}
// return head of this list here;
}
In this int main function, e_list should point to head of the linked list after calling append_to_list(e_list).
int main(void)
{
struct equipment *e_list = NULL;
e_list = append_to_list(e_list);
}
How do I create a reference and traverse that dummy head? That way I never lose my original head.

Answer 1

You need a temporary variable, in the else add
struct equipment *tmp;
tmp = list;
And replace all occurrences of list in the else. At the end return you list variable.

Answer 2

Since you are already putting the newly created structure temp at the end. Your head will not change between different calls in main because the head is at the opposite side of the end.
You need a static variable to keep track of head. Here is the tested code:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define NAME_LEN 30
struct equipment
{
char type[NAME_LEN + 1];
char description[NAME_LEN + 1];
int quantity;
struct equipment *next;
};
struct equipment *
append_to_list (struct equipment *list)
{
char type[NAME_LEN + 1], description[NAME_LEN + 1];
int quantity;
printf ("Enter equipment type: ");
fgets (type, NAME_LEN, stdin);
printf ("Enter description of the equipment: ");
fgets (description, NAME_LEN, stdin);
printf ("Enter quantity: ");
scanf ("%d", &quantity);
struct equipment *temp =
(struct equipment *) malloc (sizeof (struct equipment));
strcpy (temp->type, type);
strcpy (temp->description, description);
temp->quantity = quantity;
temp->next = NULL;
bool doesExist = false;
static struct equipment * head = NULL; //New variable
if (list == NULL)
{
list = temp;
head = list;
}
else
{
while (list->next != NULL)
{
if (list == temp)
{
printf ("This equipment is already in the list\n");
}
list = list->next;
}
list->next = temp;
}
return head; //return head here
}
int main()
{
struct equipment *e_list = NULL;
e_list = append_to_list(e_list);
e_list = append_to_list(e_list);
e_list = append_to_list(e_list);
return 0;
}

Answer 3

You really could do a few things here.
Firstly though, your code is rather unsafe. Your buffers are far too small, don't use scanf to convert an integer (it doesn't check for failure), and don't use strcpy (especially when your buffers are tiny). Also, the calls to printf are a bit pointless if you're just using a literal string - puts() should be considered instead - and don't cast the return from malloc(). This is maybe a bit safer.
#include <bsd/bsd.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#define NAME_LEN 32
#define nputs(STR) fputs((STR), stdout)
struct equipment {
char type[NAME_LEN];
char description[NAME_LEN];
int quantity;
struct equipment *next;
};
struct equipment *
append_to_list(struct equipment *list)
{
char type[BUFSIZ], description[BUFSIZ], quantity_string[BUFSIZ];
int quantity;
nputs("Enter equipment type: ");
fgets(type, BUFSIZ, stdin);
nputs("Enter description of the equipment: ");
fgets(description, BUFSIZ, stdin);
nputs("Enter quantity: ");
fgets(quantity_string, BUFSIZ, stdin);
char *endptr;
quantity = strtol(quantity_string, &endptr, 10);
if (quantity_string == endptr) {
fprintf(stderr, "Error: invalid integer input '%s'\n", quantity_string);
exit(1);
}
struct equipment *temp = malloc(sizeof *temp);
strlcpy(temp->type, type, NAME_LEN);
strlcpy(temp->description, description, NAME_LEN);
temp->quantity = quantity;
temp->next = NULL;
bool doesExist = false;
if (list == NULL) {
list = temp;
} else {
while (list->next != NULL) {
if (list == temp)
puts("This equipment is already in the list");
list = list->next;
}
list->next = temp;
}
// return head of this list here;
}
As to keeping track of the head, you could use a different function to initialize the tree than to append to it, thus giving you a slightly unique root object. You could add a field to each struct that stores the location of the root, or you could make a separate struct entirely that holds the root and perhaps some metadata about it. There are plenty of options. I agree with a few of the comments above, it isn't entirely clear what you're asking for here.