Cannot store elements in a linked list - c

Question

My goal is to create a linked list and store elements inside that list.
struct node
{
int a;
struct node *b;
} p,*temp,*head;
void create ( struct node *temp)
{
char c;
temp = malloc (sizeof(struct node));
printf("enter data\n");
scanf(" %d",&temp->a);
printf("do you want to insert another node y/n\n");
scanf("%s", &c);
if (c=='y')
{
create(temp->b);
}
else if ( c=='n')
{
temp->b= NULL;
temp=&p;
return;
}
}
void traverse ( struct node *head)
{
while(head != NULL)
{
printf("%d ",head->a);
head=head->b;
}
}
int main ()
{
int i,j,k,l,m,n;
do{
if(i==1)
{
printf("enter data\n");
scanf("%d",&p.a);
create (p.b);
}
else if ( i==2)
traverse(temp);
}
while(i!=3);
printf("%d",temp->a);
}
I can't recover the elements once i've stored them. When I try to traverse the list, it only gives me the first element of the list and nothing else.

Answer 1

In main
do {
if(i==1)
{
...
}
else if ( i==2)
traverse(temp);
}
while(i!=3);
must be something like
do {
if (scanf("%d", &i) != 1)
break;
if(i==1)
{
...
}
else if ( i==2)
traverse(temp);
}
while(i!=3);
to know what the user want (i not initialized in your code)
in create
scanf("%s", &c);
is wrong because c is a char rather than a string
Do not mix read of int and char because you will read newline and space when reading a character, so read a string for c, for instance
char c[2];
...
scanf("%1s", &c);
if (*c == 'y')
...
else if (c == 'n')
...
the return in else branch is useless, and in case the answer is not 'y' or 'n' you do nothing so you do not set temps, probably you have to just check if 'y' and all other answers must be considered to be 'n', or you need to ask again for the choice
in create you assign the local variable temps, that has no effect on p.b in main, you need to get a node** for instance
in main temp is used but never set elsewhere, and the variables j,k,l,m,n are useless. You also ask for the data in main while you also do in create, must not be done in main. The way you manage your variables do not allow you to modify/print the list
I encourage you to not use global variables the must you can, and to not use the same name for a global and local variable like you do for temp and head because that do not help the reader of your code
A proposal solving the problems :
#include <stdlib.h>
#include <stdio.h>
struct node
{
int a;
struct node * b;
};
/* flush input up to the end of the line */
void flush()
{
int c;
while ((c = getchar()) != '\n') {
if (c == EOF)
exit(-1);
}
}
void create (struct node ** l)
{
/* go to the end of the list */
while (*l != NULL)
l = &(*l)->b;
for (;;) {
char c[2];
int v;
printf("enter data\n");
if (scanf("%d", &v) != 1) {
puts("invalid value");
flush();
}
else {
*l = malloc (sizeof(struct node));
(*l)->a = v;
(*l)->b = NULL;
l = &(*l)->b;
for (;;) {
printf("do you want to insert another node y/n\n");
scanf("%1s", c);
if (*c == 'y')
break;
else if (*c == 'n')
return;
}
}
}
}
void traverse ( struct node *head)
{
while(head != NULL)
{
printf("%d ",head->a);
head = head->b;
}
putchar('\n');
}
int main ()
{
int i;
struct node *head = NULL;
for (;;) {
puts("enter choice : 1 to create new node, 2 to print list, 3 to exit");
if (scanf("%d", &i) != 1)
flush();
switch(i) {
case 1:
create(&head);
break;
case 2:
traverse(head);
break;
case 3:
return 0;
default:
break;
}
}
}
Compilation and execution :
/tmp % gcc -pedantic -Wextra -Wall t.c
/tmp % ./a.out
enter choice : 1 to create new node, 2 to print list, 3 to exit
2
enter choice : 1 to create new node, 2 to print list, 3 to exit
1
enter data
11
do you want to insert another node y/n
y
enter data
22
do you want to insert another node y/n
n
enter choice : 1 to create new node, 2 to print list, 3 to exit
2
11 22
enter choice : 1 to create new node, 2 to print list, 3 to exit
1
enter data
3
do you want to insert another node y/n
n
enter choice : 1 to create new node, 2 to print list, 3 to exit
2
11 22 3
enter choice : 1 to create new node, 2 to print list, 3 to exit
4
enter choice : 1 to create new node, 2 to print list, 3 to exit
3
I encourage you to add the free of the list

Answer 2

There are several issues here:
struct node
{
int a;
struct node *b;
} p,*temp,*head;
Why do you declare global variables and use it as function parameters ? Global variables are available globally, there is no need to pass them into functions. On the other hand, global variables should be avoided and used with care, so it would be better to create local variables (e.g. in main function) and pass them as parameters into next functions.
void create ( struct node *temp)
{
char c;
temp = malloc (sizeof(struct node));
printf("enter data\n");
scanf(" %d",&temp->a);
printf("do you want to insert another node y/n\n");
scanf("%s", &c);
if (c=='y')
{
create(temp->b);
}
else if ( c=='n')
{
temp->b= NULL;
temp=&p;
return;
}
}
This function looks wrong. temp function parameter is actually an internal function's variable, that is not an in/out parameter. In that case you can assign to the temp variable, but it will not for a list. Also temp function parameter shadows temp global variable. It is also a good think to return status of operation, usually '0' means no error, any other error value.
Another thing is to keep things as simple as possible. This will allow for more reuseability and goes with single responsibility principle. If function actually performs two tasks it should be split into two functions.
One more thing, you allocate memory dynamically but never free the memory. This will lead into memory losses.
A possible implementation of your list could be:
#include <stdio.h>
#include <stdlib.h>
typedef struct _node_t
{
int a;
struct _node_t * next;
} node_t;
static node_t * head = NULL;
static node_t * tail = NULL;
node_t * create(void)
{
node_t * temp = malloc(sizeof(node_t));
if (NULL == temp)
{
return NULL;
}
printf("Enter data\n");
scanf("%d", & temp->a);
return temp;
}
void append(node_t * data)
{
if (NULL == head)
{
head = tail = data;
}
else
{
tail->next = data;
tail = tail->next;
}
tail->next = NULL;
return;
}
int add_data(void)
{
node_t * data = NULL;
char answer = 'y';
data = create();
if (NULL == data)
{
return 1;
}
append(data);
return 0;
}
void traverse(void)
{
node_t * current = NULL;
for (current = head; current != NULL; current = current->next)
{
printf("%d ", current->a);
}
printf("\n");
return;
}
void cleanup(void)
{
node_t * current = head;
while (NULL != current)
{
head = head->next;
free(current);
current = head;
}
return;
}
int main(int argc, char ** argv)
{
int option = 3;
do
{
printf("Enter option:\n 1 - add data\n 2 - traverse list\n 3 - exit\n\n");
scanf("%i", & option);
switch (option)
{
case 1:
if (0 != add_data())
{
printf("ERROR:: Cannot allocate memory.\n");
cleanup();
return 1;
}
break;
case 2:
traverse();
break;
default:
if (option > 3)
{
printf("ERROR:: Improper option, try again.\n");
}
break;
}
}
while (option != 3);
cleanup();
return 0;
}

Answer 3

I tried making it as simple as possible and keeping your logic there.
#include <stdio.h>
#include <stdlib.h>
struct node
{
int data;
struct node *next;
};
// create a new node
struct node* create(int data)
{
struct node *n = malloc(sizeof(struct node));
n->data = data;
n->next = NULL; // by default its not NULL, so we must set it
return n;
}
void traverse(struct node *head)
{
struct node *tmp = head;
while(tmp != NULL)
{
printf("%d\t",tmp->data);
tmp = tmp->next;
}
}
void cleanup(struct node *head)
{
struct node *cur = head;
struct node *next;
while(cur != NULL)
{
next = cur->next;
free(cur);
cur = next;
}
}
int main ()
{
int data;
struct node *head, *tmp;
// head node is always created
printf("enter data\n");
scanf("%d",&data);
head = tmp = create(data);
if(head == NULL) return -1;
// now we loop until we don't want to create any more nodes
while(1)
{
char another;
printf("do you want to insert another node y/n\n");
scanf(" %c", &another); // ignore all the previous whitespace
if(another == 'y')
{
printf("enter data\n");
scanf("%d",&data);
tmp->next = create(data);
tmp = tmp->next;
}
else break;
/*
// anything but 'y' breaks the loop, but it can be set to continue if neither 'y' nor 'n' was read:
else if(another == 'n') break;
else continue;
*/
}
traverse(head);
cleanup(head);
}
EDIT: as #Diodacus stated, i added cleanup and checking if malloc returned NULL