Why does my program is terminating abruptly? - c

Question

#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
static int COUNT = 1;
typedef struct node NODE;
struct node
{
int data;
NODE *next;
};
NODE *START = NULL;
NODE *create_node() {
NODE *n = malloc(sizeof(NODE));
n->data = 0;
n->next = NULL;
return n;
}
void insert_at_beginning() {
NODE *n, *temp;
n = create_node();
printf("Enter a number: ");
scanf("%d",&n->data);
if(START == NULL) {
START = n;
} else {
temp = START;
START = n;
n->next = temp;
}
COUNT++;
printf("Successfully Inserted!\n");
}
void insert_at_a_position() {
int position, count;
NODE *n, *temp, *t;
printf("Enter position at which you want to insert: ");
scanf("%d", &position);
if(COUNT<position) {
printf("Out of Bound! Please try again.\n");
} else
{
count = 1;
n = create_node();
printf("Enter a number: ");
scanf("%d",&n->data);
temp = START;
while(count != position) {
count++; //To stoping the variable at given position
t = temp; //Getting Previous node where we will set link to n
temp = temp->next; //getting next element whose link will be attached to n to form complete linked list
}
n->next = temp;
t->next = n;
COUNT++;
printf("Successfully Inserted!\n");
}
}
void insert_at_end() {
NODE *n, *temp;
temp = START;
n = create_node();
printf("Enter a number: ");
scanf("%d", &n->data);
while(temp!=NULL) {
printf("I am here!");
temp = temp->next;
}
temp->next = n;
COUNT++;
printf("Successfully Inserted!\n");
}
void display() {
NODE *temp;
temp = START;
while (temp!=NULL)
{
printf("%d", temp->data);
temp = temp->next;
}
}
int main() {
int ch;
printf("1.Insert at beginning\n2.Insert at mid\n3.Insert at end\n4.Delete from beginning\n5.Delete from position\n6.Delete from end\n7.Display");
printf("\nEnter your choice: ");
scanf("%d", &ch);
while (ch!=0)
{
switch(ch) {
case 1:
insert_at_beginning();
break;
case 2:
insert_at_a_position();
break;
case 3:
insert_at_end();
break;
case 4:
display();
break;
default:
printf("Wrong Choice!!");
}
printf("Enter Your Choice: ");
scanf("%d",&ch);
}
return 0;
}
insert_at_beginning and insert_at_end is working perfectly but insert_at_end and display function is showing problem
In display function: Program goes in infinite loop
In insert_at_end: Program goes through while loop (i.e., printing "I am here" node number of time) but then it terminates abruptly without assigning the value in the given position.

Answer 1

For starters it is a bad idea to declare the initial pointer to nodes as a global variable and when function depends on global variables.
Also it is unclear why the static variable COUNT is initialized by 1 instead of 0 when initially the list is empty.
static int COUNT = 1;
It should be initialized by zero
static int COUNT = 0;
insert_at_beginning and insert_at_end is working perfectly
You are wrong. The function insert_at_end is invalid.
void insert_at_end() {
NODE *n, *temp;
temp = START;
n = create_node();
printf("Enter a number: ");
scanf("%d", &n->data);
while(temp!=NULL) {
printf("I am here!");
temp = temp->next;
}
temp->next = n;
COUNT++;
printf("Successfully Inserted!\n");
}
For example the function can be called when the list is empty that is when the pointer START is equal to NULL. In this case the pointer START is not changed in the function.
Moreover even if the pointer START is not equal to NULL then after this loop
while(temp!=NULL) {
printf("I am here!");
temp = temp->next;
}
the pointer temp is equal to NULL. So the next statement
temp->next = n;
invokes undefined behavior.
The function can be written at least the following way
void insert_at_end() {
NODE *n;
n = create_node();
printf("Enter a number: ");
scanf("%d", &n->data);
if ( START == NULL )
{
START = n;
}
else
{
NODE *temp = START;
while ( temp->next !=NULL )
{
printf("I am here!");
temp = temp->next;
}
temp->next = n;
COUNT++;
printf("Successfully Inserted!\n");
}
}
As for the function insert_at_a_position then there is a confusion relative to the global variable COUNT. As I pointed out initially when the list is empty COUNT is equal to 1. So a valid position can be less than COUNT. Take into account that the user can enter for example the value of position equal to 0.
So for example this if statement
if(COUNT<position) {
printf("Out of Bound! Please try again.\n");
} else
should be rewritten like
if ( !( position < COUNT ) ) {
printf("Out of Bound! Please try again.\n");
} else
Also when the user will enter 0 then this loop
count = 1;
//...
while(count != position) {
//...
can invoke undefined behavior.
And again if the list is empty that is START is equal to NULL then START is not changed in the function.
Also if the user entered the position equal to 1 then in this case the while loop will not executed. On this case the pointer t has indeterminate value because it was not initialized outside the loop. So this statement
t->next = n;
again invoke undefined behavior.
The function can be defined the following way
void insert_at_a_position() {
int position;
printf("Enter position at which you want to insert: ");
scanf("%d", &position);
if( !( position < COUNT ) ) {
printf("Out of Bound! Please try again.\n");
} else
{
NODE *n = create_node();
printf("Enter a number: ");
scanf("%d",&n->data);
NODE *temp = START;
NODE *prev = START;
while( position-- != 0 )
{
prev = temp;
temp = temp->next;
}
n->next = temp;
if ( prev == NULL ) START = n;
else prev->next = n;
COUNT++;
printf("Successfully Inserted!\n");
}
}

Answer 2

You are dereferencing a NULL pointer.
while(temp!=NULL) {
printf("I am here!");
temp = temp->next;
}
temp->next = n; // temp is NULL here!!
You can change the loop condition to:
while (temp->next != NULL) {
But before you do that, you should check that temp is not NULL:
temp = START;
if (temp == NULL) {
insert_at_beginning();
return;
}
Aside: it is more standard to just write:
if (!temp)
than
if (temp == NULL)