Searching Double Linked lists

**AmazingRando** · 09-23-2003

Assuming I have these two structures for a double linked list

Code:

typedef struct node_d
{
char city[40];
char state[3];
char zipcode[6];
double longitude;//double
double latitude;//double
long population;//long
struct node_d *previous;
struct node_d *next;
}Node;

typedef struct
{
struct node_d *psHead;
struct node_d *psTail;
struct node_d *current_node;
int node_count;
}Header;

I am trying to implement a search.
I figured this would work, but doesn't. Can anyone see any flaws in it ? I am confused on how to fix this problem

Code:

int Search_List(Header *psList, int z)
{  
	int x; 
	char answer[40], compare[40];
	
	printf("\n\nWhat City or Zipcode would you like to search for?");
	scanf("%s",&answer);
	if (isdigit(answer[0]) != 0)
	{
		//search for zip
		psList->current_node = psList->psHead;
		for(x=0;x<psList->node_count;x++)
		{
			strcpy(compare, psList->current_node->zipcode);/////////it seems to crash right here
			if(strcmp(answer, compare)== 0)
			{
				system("cls");
				printf("\nThe Zipcode was found with %d comparisons",x+1);
				printf("\nZipcode = %s   City = %s   State = %s",psList->psHead->zipcode, psList->psHead->city, psList->psHead->state);
				printf("\nPopulation = %d  Longitude = %-.2f  latitude = %-.2f\n",psList->psHead->population, psList->psHead->longitude, psList->psHead->latitude);
				x=z;
			}
			else
			{
				psList->current_node = psList->current_node->next;
				
			}
		}
		printf("\nRecord was not found\n");
	}

it seems to crash right at this command
strcpy(compare, psList->current_node->zipcode);
but I don't know why. Is there a problem with the way strcpy is called?
Any help would be appreciated

**quzah** · 09-23-2003

I'd redo the loop personally. Just use a pointer and increment it. It's much easier to read.

Code:

Node *ptr;
for( ptr = psList->head; ptr; ptr = ptr->next )
{
    if( strcmp( answer, ptr->zipcode ) == 0 )
    {
        ...match...
    }
    ...else continue so no need for else...
}

Make sure your zipcode string is null terminated if you're treating it as a string. Make sure the buffer you copy it into also is null terminated. (Either memset it, or do it when you declare it). Also, you may want to use 'strncpy' instead, and specify to copy 5 bytes instead of just a standard strncpy which doesn't care if it runs off the end of your buffer.

Quzah.

**AmazingRando** · 09-23-2003

Thanks again quzah, but I changed it to this
and it is still crashing right at the if statement
It does not even go through the for loop once.

Code:

int Search_List(Header *psList,Node *ptr, int z)
{  
	int x; 
	char answer[40], compare[40];
	
	printf("\n\nWhat City or Zipcode would you like to search for?");
	scanf("%s",&answer);
	if (isdigit(answer[0]) != 0)
	{
		//search for zip
		psList->current_node = psList->psHead;
		
		for( ptr = psList->psHead; ptr; ptr = ptr->next )
		{
			
			
		if( strcmp( answer,ptr->zipcode) == 0 )///////crashes here
			{
				
				system("cls");
				printf("\nThe Zipcode was found with %d comparisons",x+1);
				printf("\nZipcode = %s   City = %s   State = %s",psList->current_node->zipcode, psList->current_node->city, psList->current_node->state);
				printf("\nPopulation = %d  Longitude = %-.2f  latitude = %-.2f\n",psList->current_node->population, psList->current_node->longitude, psList->current_node->latitude);
        
			}
   

		}
return 0;
}
}

**quzah** · 09-23-2003

I'd debug your list first if I was you. Make a function that just goes through the list and prints the entire thing to the screen. If that works, you know the list is fine. If it doesn't, you'll know where you problem is.

From there, it's only a minor modification to get the same function to now search for something in the list.

Quzah.

**AmazingRando** · 09-23-2003

Am I mistaken or should this print?

printf("Zipcode is %s",ptr->zipcode);

the statement comes in main before my search function. But it crashes at this statment

**Hammer** · 09-23-2003

Originally posted by AmazingRando
Am I mistaken or should this print?

printf("Zipcode is %s",ptr->zipcode);

the statement comes in main before my search function. But it crashes at this statment

As long as ptr is pointing to a valid struct, it should print. As it's crashing, I expect it isn't pointing to the right place. Did you assign ptr a value?

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