Deleting elements in linked list until i find word - c

Question

I have problem with deleting elements from list
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 30
typedef struct elem{
char name[MAX];
int statistic;
int price;
struct elem *next;
struct elem *prev;
} shop;
I created function with search in list for a word defined in main and then if it finds it it keeps deleting 1st element until it finds the word and then it deletes selected word and makes next as first:
void delete_from_list(shop *first, char word[MAX], int* check)
{
shop *tmp= first;
while (tmp!=NULL && strcmp(tmp->name, word) != 0)
{
tmp = tmp->next;
}
if (tmp != NULL && strcmp(tmp->name, word)==0)
{
printf("FOUND!");
}
else
{
*check=1;
}
if (check==0)
{
while (strcmp(first->name, tmp->name)!=0)
{
first=first->next;
free(first->prev);
first->prev=NULL;
}
first=first->next;
free(first->prev);
first->prev=NULL;
}
}
void print_list(shop *first)
{
first=first->next;
if(first->name==NULL)
printf("There is nothing!!!\n");
while(first->next!=NULL){
printf("%20s",first->name);
printf("%20d \t\t",first->statistic);
printf("%d\n",first->price);
first=first->next;
}
}
I print the list and then take word.
main()
{
int check = 0;
print_list(first);
scanf("%s", word);
delete_from_list(first, word, &check);
if (check!=1)
{
print_list(first);
}
else
{
check=0;
}
}
The problem is that linked list doesn't change so probably I should add pointers but I have no idea how.

Answer 1

You have to pass pointer first by reference.
Also the function has a bug. The next element after the found element can be equal to NULL. In this case these statements
first=first->next;
free(first->prev);
^^^^^^^^^^^
first->prev=NULL;
^^^^^^^^^^^
result in undefined behaviour.
The function can be written the following way
void delete_from_list( shop **first, const char *word, int *check )
{
shop *tmp = *first;
while ( tmp!=NULL && strcmp( tmp->name, word ) != 0 )
{
tmp = tmp->next;
}
if ( tmp != NULL)
{
printf("FOUND!");
}
else
{
*check = 1;
}
if ( check == 0 )
{
tmp = tmp->next;
while ( *first != tmp )
{
shop *current = *first;
*first = ( *first )->next;
free( current );
}
if ( *first ) ( *first )->prev = NULL;
}
}
Also it is a bad design that the function relies on the value of check that is set in main. It would be better if the function itself set varaible check. In this case you could split the function into tow functions. The first one would search the target element and the second one wpuld delete all elements that satisfy the criteria if it is required.
I would define the function the following way
int delete_from_list( shop **first, const char *word )
{
shop *tmp = *first;
int deleted = 0;
while ( tmp!=NULL && strcmp( tmp->name, word ) != 0 )
{
tmp = tmp->next;
}
if ( tmp != NULL)
{
deleted = 1;
tmp = tmp->next;
while ( *first != tmp )
{
shop *current = *first;
*first = ( *first )->next;
free( current );
}
if ( *first ) ( *first )->prev = NULL;
}
return deleted;
}
Also function print_list is also wrong. In general parameter first or first->next can be equal to NULL. In this case statements like these
first=first->next;
if(first->name==NULL)
while(first->next!=NULL){
result in undefined behaviour.
And the logic itself of the function is wrong.

Answer 2

First of all you have to adopt the parameter list of your function delete_from_list,
so that it is able to delete the first element. Use shop **first instead of
shop *first. Find the element with name word. Delete the first element of your list as long as first element is not the found element.
void delete_from_list(shop **first, char word[MAX], int* check)
// ^^ pointer to pointer of first element
{
// search for elemnt with name word
shop *found = *first;
while ( found != NULL && strcmp(found->name, word) != 0 )
found = found ->next;
if ( found != NULL )
{
// found is element with name word
printf("FOUND!");
// Delete the first element of your list as long as first elment is not found element
shop *temp = *first;
while ( temp != found )
{
shop *next = temp->next;
free( temp );
temp = next;
}
*first = found; // write back the new first element of your list
(*first)->prev = NULL; // predecessor of first element is NULL
}
else
*check=1;
}