Delete specific element linked list - c

Question

Can you help me to understand why this function doesn't delete a specific element in the linked list? What am I doing wrong?
typedef struct str_node{
int data;
struct str_node *next;
}node;
...
node *head;
head = malloc(sizeof(node));
...
void delete_spec(node *a){
int num;
node *tmp;
tmp = a;
printf("Insert number to eliminate: ");
scanf("%d",&num);
while(tmp!=NULL){
if(tmp->data == num){
tmp = tmp->next->next;
}
tmp = tmp->next;
}
}

Answer 1

For starters it is unclear what the allocation is doing here
node *head;
head = malloc(sizeof(node));
The pointer head shall be initially set to NULL
node *head = NULL;
and new nodes shall be inserted in the list by the function that inserts new values in the list.
The function that deletes nodes from the list shall not issue any prompt. It is the caller of the function that will ask the user to specify the value that will be deleted from the list and then call the function passing the specified value. So the function should have two parameters: pointer to the pointer to the head node and the integer value that shall be deleted from the list.
The function can be defined the following way
void delete_spec( node **head, int data )
{
while ( *head != NULL )
{
if ( ( *head )->data == data )
{
node *tmp = *head;
*head = ( *head )->next;
free( tmp );
}
else
{
head = &( *head )->next;
}
}
}
Here is a demonstrative program.
#include <stdio.h>
#include <stdlib.h>
typedef struct str_node
{
int data;
struct str_node *next;
} node;
int push_front( node **head, int data )
{
node *new_node = malloc( sizeof( node ) );
int success = new_node != NULL;
if ( success )
{
new_node->data = data;
new_node->next = *head;
*head = new_node;
}
return success;
}
void delete_spec( node **head, int data )
{
while ( *head != NULL )
{
if ( ( *head )->data == data )
{
node *tmp = *head;
*head = ( *head )->next;
free( tmp );
}
else
{
head = &( *head )->next;
}
}
}
void display( node *head )
{
for ( ; head != NULL; head = head->next )
{
printf( "%d -> ", head->data );
}
puts( "null" );
}
int main(void)
{
node *head = NULL;
int a[] = { 1, 3, 5, 7, 1, 2, 3, 1 };
const size_t N = sizeof( a ) / sizeof( *a );
for ( size_t i = 0; i < N; i++ )
{
push_front( &head, a[i] );
}
display( head );
delete_spec( &head, 1 );
display( head );
return 0;
}
Its output is
1 -> 3 -> 2 -> 1 -> 7 -> 5 -> 3 -> 1 -> null
3 -> 2 -> 7 -> 5 -> 3 -> null

Answer 2

public class LL5 {
Node head;
int size;
LL5(){
this.size = 0;
}
class Node{
String data;
Node next;
Node(String data){
this.data = data;
this.next = null;
size++;
}
}
public void push(String data) {
Node newNode = new Node(data);
if(head == null) {
head = newNode;
return;
}
newNode.next = head;
head = newNode;
}
public void deleteelement(String data) {
Node cur = head;
Node x = cur.next;
Node y = x.next;
if(data == head.data) {
head = head.next;
}
else if(x.data == data) {
head.next = y;
}
else{
while(y.data != data) {
x = x.next;
y = y.next;
}
x.next = y.next;
}
}
public void printList() {
if(head == null) {
System.out.println("list is empty");
return;
}
Node cn = head;
while(cn != null) {
System.out.print(cn.data + " -> ");
cn = cn.next;
}
System.out.print("null");
}
public static void main(String[] args) {
LL5 obj = new LL5();
obj.push("1");
obj.push("2");
obj.push("3");
obj.push("4");
obj.push("5");
obj.push("6");
obj.push("7");
obj.push("8");
obj.push("9");
obj.push("10");
System.out.println("before delete");
obj.printList();
System.out.println();
System.out.println("After delete");
obj.deleteelement("10");
obj.printList();
}
}

Answer 3

delete_spec does not modify in anyway the input list. also: it does not free any memory.
in order to actually delete a node you must:
1. free its memory.
2. modify the list so the "next" pointers are updated. in order to update the list, you must provide the delete function the address of the head so it can modify also head.
something like this:
void delete_spec(node **a){
int num;
node *tmp;
if (a == NULL || *a == NULL) return;
tmp = *a;
printf("Insert number to eliminate: ");
scanf("%d",&num);
if (tmp->data == num)
{
*a = (*a)->next;
free(tmp);
return;
}
while(tmp->next!=NULL){
if(tmp->next->data == num){
node* tmp2 = tmp->next;
tmp->next = tmp->next->next;
free(tmp2);
}
tmp = tmp->next;
}
}