C - Popping last item from linked lists - c

Question

I am learning linked lists and they are causing me a lot of troubles.
I am calling the function with this call:
pop(&list);
ANd here's the code:
void pop(NODE** first) {
if(*first != NULL && first!= NULL){
NODE* ptr = *first;
while(ptr->next->next != NULL){
ptr = ptr->next;
}
free(ptr->next);
ptr->next = NULL;
}
It's also causing memory leak error even if I call it single time..
When calling this function multiple times, there are more memory leak errors.
Thanks in advance, Mimpopo.
EDIT: Definition of NODE
typedef struct node {
int data;
struct node *next;
} NODE;
The full CODE:
#include <stdio.h>
#include <stdlib.h>
typedef struct node {
int data;
struct node *next;
} NODE;
NODE* insert(NODE *first, int n){
// create new node
NODE* new = (NODE*)malloc(sizeof(NODE));
new->data = n;
new->next = NULL;
// if first is NULL, this will be the first
if(first == NULL)
return new;
// otherwise, place it correctly
NODE* ptr = first;
// check inserting at the begining
if(ptr->data > new->data){
new->next = ptr;
return new;
}
// insert in the middle
while(ptr->next != NULL){
if(ptr->next->data > n && ptr->data < n){
new->next = ptr->next;
ptr->next = new;
break;
}
ptr = ptr->next;
}
// insert at the end of list
if(ptr->next == NULL){
ptr->next = new;
}
return first;
}
void traverse(NODE *first){
NODE* ptr = first;
while(ptr != NULL){
printf("%d\n", ptr->data);
ptr = ptr->next;
}
}
NODE* search(NODE *first, int n){
NODE* ptr = first;
while(ptr != NULL){
if(ptr->data == n){
printf("FOUND %d\n!", n);
return ptr;
}
ptr = ptr->next;
}
}
int main(){
NODE* first = NULL;
NODE* this = NULL;
first = insert(first, 7);
first = insert(first, 10);
first = insert(first, 11);
first = insert(first, 1);
first = insert(first, 3);
first = insert(first, 5);
first = insert(first, 22);
first = insert(first, 23);
first = insert(first, 24);
first = insert(first, 125);
pop(&first);
}

Answer 1

I have not looked through all your code but as for the function then it can be written the following way
void pop( NODE ** first )
{
if ( *first != NULL )
{
NODE *prev = NULL;
NODE *current = *first;
while ( current->next )
{
prev = current;
current = current->next;
}
if ( prev != NULL ) prev->next = current->next;
else ( *first = NULL );
free( current );
}
}
As for your function implementation then it contains many errors. For example in this statement
if(*first != NULL && first!= NULL){
you shall swap the first and the second comparisons. That is the condition shall look like
if(first != NULL && *first!= NULL){
In this statement
while(ptr->next->next != NULL){
You have to be sure that ptr->nextis not equal to NULL.
Also you do not check whether the deleted node is the first node of the list.
Take into account that function insert is also wrong. You consider only one condition in this code snippet
while(ptr->next != NULL){
if(ptr->next->data > n && ptr->data < n){
new->next = ptr->next;
ptr->next = new;
break;
}
ptr = ptr->next;
}
However it can be that for example
ptr->next->data >= n && ptr->data < n
or
ptr->next->data > n && ptr->data <= n