Singly linked list program - c

Question

So here I have created a C program to create and display a singly linked list.
#include <stdio.h>
#include <conio.h>
int main()
{
struct node
{
int data;
struct node *next;
};
struct node *head = malloc(sizeof(struct node));
struct node *temp = malloc(sizeof(struct node));
struct node *newnode;
int choice=1,count=0;
while(choice==1){
newnode=(struct node *)malloc(sizeof(struct node));
printf("Enter data");
scanf("%d",&newnode->data);
newnode->next=0;
if(head==0)
{
head=temp=newnode;
}
else
{
temp->next=newnode;
temp=newnode;
}
printf("\nDo you want to continue\n");
scanf("%d",&choice);
}
temp=head;
while(temp!=NULL){
printf("%d",temp->data);
temp=temp->next;
count++;
}
getch();
}
I'm not getting the desired output i.e. the linked list I am creating is not being displayed. Can someone please help me out and tell me where I went wrong?

Answer 1

Some comments on your code:
head should not start as a node instance: you don't have any value yet. It should start as NULL, which is also in line with the if condition you have in the loop. NULL represents an empty list, which is what you want to start with. Two problems are the consequence of this:
Since it isn't NULL at the start, the (head==0) condition is never true, and the loop never updates head. All the work done remains detached from this head node.
head references a node that is not initialised. Its value is undefined, and worse, its next reference is undefined. This leads to undefined behaviour during the print loop, where this next reference is used.
temp starts as a node instance, but as in the loop, temp is assigned temp->next, the code loses a reference to the original temp node. And each time temp progresses to the next, also the references to all other created nodes are lost.
temp is not a very useful name, as this variable has the same life time as head. In practice it represents the tail of the list in the first loop, so call it tail. The first point holds also for this tail: it should start as NULL
count is never used. Moreover, it should not be incremented during a print-job: that is mixing two jobs. Either count the nodes or print them, but don't do it together. Anyway, as this code has no use for this count, I'd just drop it.
It is better practice to use NULL when related to a pointer data type, not 0.
Separate functionality: now you have literally everything happening in one function: creating nodes, getting input, outputting nodes,... This is not maintainable. Separate different domains of functionality in separate functions.
If the only reason for include conio.h is to make that final getch call, presumably because you want to use to press enter at the end of the program, then just leave it out.
Here is some suggested code:
#include <stdio.h>
#include <stdlib.h>
// Define struct outside of main
struct node
{
int data;
struct node *next;
};
// Define some functions that perform common actions for a list
struct node *createNode(int data)
{
struct node *newnode = malloc(sizeof(struct node));
// Initialise the struct, so avoiding undefined behaviour
newnode->data = data;
newnode->next = NULL;
return newnode;
}
struct node *appendNode(struct node *tail, int data)
{
struct node *newnode = createNode(data);
if (tail != NULL) {
tail->next = newnode;
}
return newnode;
}
void printList(struct node *head)
{
while (head != NULL) {
printf("%d ", head->data); // Space between outputs
head = head->next; // head is a local variable, so we can change it
}
printf("\n"); // Terminate line
}
int main()
{
struct node *head = NULL; // Start with empty list
struct node *tail = NULL; // Better name. In beginning we don't have a tail
int choice = 1, data = 0;
while (choice == 1) {
// First get the data, and only then create the node
printf("Enter data: ");
scanf("%d", &data);
tail = appendNode(tail, data);
if (head == NULL) // Use NULL constant
{
head = tail;
}
printf("Do you want to continue? (0=no, 1=yes): ");
scanf("%d", &choice);
}
printList(head);
return 0;
}