Thread: Problem using strcmp and strlen in a Linked List

Hello everyone,

I have created a program that counts, prints, adds and deletes nodes in a linked list. I am having a problem with the delete function of the program. The struct contains a name value (ex. Paul).

When I call the fuction del_node, I am using strcmp to see if the value returned is 0, if it is, the function should mark the current pointer as "del_node" and free the space allocated for that node at the end of the function call, relinking the pointers as needed.

It seems that strcmp in the del_node function isn't comparing my two pointers correctly. Below is the code:

Code:

#include <stdio.h>
#include <string.h>
#define DEBUG

typedef struct linked_list LIST;

int print_menu(void);
void print_nodes(LIST *head, int *reval);
int count_nodes(LIST *head);
LIST *add_node(LIST *head, int *reval);
LIST *del_node(LIST *head);

struct linked_list {
	struct linked_list *next;
	char name[30];
};		

enum reval {SUCCESS, FAIL}; 

int main(void)
{
	char line[100];
	int i, count = 0;
	int option;
	int reval = SUCCESS;

	LIST *first_node = NULL;
	LIST *node_2 = NULL;
	LIST *head = NULL; // Points to first node in the list
		
	first_node = malloc(sizeof(LIST));
	if (first_node == NULL) {
		printf("Out of memory.\n");
		return 1;
	}		

	node_2 = malloc(sizeof(LIST));
	if (node_2 == NULL) {
		printf("Out of memory.\n");
		return 1;
	}

	strcpy(first_node->name, "Paul");
	strcpy(node_2->name, "Susan");

	head = first_node;
	first_node->next = node_2;
	node_2->next = NULL;

	while (1) {
		option = print_menu();
			
		switch (option) {
			case 1:
				count = count_nodes(head);
				printf("\nNumber of nodes in list: %d\n", count);
				break;
			case 2:
				print_nodes(head, &reval);
				break;
			case 3:
				head = add_node(head, &reval);
				break;
			case 4:
				head = del_node(head);
				break;
			default:
				printf("Incorrect menu option.\n");
				break;
		}
				
		printf("\nWould you like to continue? (Y)es or (N)o? ");
		fgets(line, sizeof(line), stdin);
		printf("\n");
		
		if ((line[0] == 'y') || (line[0] == 'Y'))
			continue;
		else
			break;
	}	
		
	if (count != 0) {
		free(first_node);
		free(node_2);
	}
		
	return reval;
} 

// print_menu() definition
int print_menu(void)
{
	char line[100];
	int option;
	
	printf("Linked Lists\n");
	printf("------------\n");
	printf("1. Count number of links\n");
	printf("2. Print names in list\n");
	printf("3. Add a name to the list\n");
	printf("4. Delete a name from the list.\n\n");
	printf("What would you like to do: ");
		
	fgets(line, sizeof(line), stdin);
	sscanf(line, "%d", &option);
	
	return option;
}

// node_count() definition
void print_nodes(LIST *head, int *reval)
{
	LIST *current;
		
	if (head == NULL) {
		printf("\nThere are no names in the list.");
		*reval = FAIL;
	}
		
	current = head;
	printf("\n");
	while (current != NULL) {
		printf("Name: %s\n", current->name);
		current = current->next;
	}
}

int count_nodes(LIST *head)
{
	LIST *current;
	int count = 0;
		
	current = head;
	while (current != NULL) {
		count++;
		current = current->next;
	}
		
		return count;
}

// add_node() definition
LIST *add_node(LIST *head, int *reval)
{
	LIST *current = NULL, *before = NULL, *after = NULL, *new_node = NULL;
	char line[100];
	int count = 0;
	int value;
		
	new_node = malloc(sizeof(LIST));
	if (new_node == NULL) {
		printf("Out of memory.\n");
		*reval = FAIL;
	}
		
	current = head;
		
	printf("Enter a name: ");
	fgets(line, sizeof(line), stdin);
	sscanf(line, "%s", new_node->name);
		
	while (1) {
		
		value = strcmp(new_node->name, current->name);
				
		if (current == NULL)
			break;				

		if ((value < 0) && (count == 0)) {
			new_node->next = current;
			head = new_node;
			break;
		}
		else if ((value < 0) && (count != 0)) {
			new_node->next = current;
			before->next = new_node;
			break;
		}
		else if (value > 0) {
			if (current->next == NULL) {
					new_node->next = NULL;
					current->next = new_node;
					break;
			}
			before = current;
			current = current->next;
			count++;						
		}
		else if (value == 0)
			break;
	}
		
	return head;
}

// del_node() definition
LIST *del_node(LIST *head)
{
	LIST *current = NULL, *del_node = NULL, *before = NULL, *after = NULL;
	char name[100];
	int count = 0;
	int value;
		
	current = head;
		
	printf("Enter a name to remove from the list: ");
	fgets(name, sizeof(name), stdin);
		
	#ifdef DEBUG
	int i;
		
	printf("\n");	
	for (i=0; i<strlen(name); i++)
		printf("name[%d]: %c\n", i, name[i]);			

	printf("string length of name is: %d\n", strlen(name));
	printf("string length of current->name is: %d\n\n", strlen(current->name));
		
	printf("current->name[0] is: %c\n", current->name[0]);
	printf("current->name[1] is: %c\n", current->name[1]);
	printf("current->name[2] is: %c\n", current->name[2]);
	printf("current->name[3] is: %c\n", current->name[3]);
	printf("current->name[4] is: %c\n", current->name[4]);
		
	printf("\nBegin for loop\n");
	for (i=0; i<strlen(current->name); i++);
		printf("current->name[%d]: %c\n", i, current->name[i]);
	#endif
		
	value = strcmp(name, current->name);					
	printf("value is: %d\n", value);
		
	while (current != NULL) {		
				
		if (value == 0) {
			if (count == 0) { // check to see if first_node contains the name
				head = current->next;
				del_node = current;
				break;
			}						
			del_node = current;
			after = current->next;						
			before->next = after;
			break;
		}
		else {
			count++;
			before = current;
			current = current->next;
		}
	}
		
	if (del_node == NULL)
		printf("\nName not found.\n");
		
	free(del_node);		
		
	return head;
}

The output of the debug section is:

Code:

Enter a name to remove from the list: Paul

name[0]: P
name[1]: a
name[2]: u
name[3]: l
name[4]:

string length of name is: 5
string length of current->name is: 4

current->name[0] is: P
current->name[1] is: a
current->name[2] is: u
current->name[3] is: l
current->name[4] is:

Begin for loop
current->name[4]:
value is: 10

Name not found.

From what I can see, the value returned from strlen(current->name) isn't the same as strlen(name). Does this have to do with the fact that current->name is on the heap and name is in the stack?

If I print each element from current->name array individually, it prints successfully, but when I try to use the "for" loop, it only prints the last character (current->name[4]). I'm pretty sure this is the reason strcmp is failing....

Any help would be appreciated. Thank you.

First, #include <stdlib.h>.

fgets() stores the newline that it reads. As such when you type in "Paul" you are giving five characters: 'P', 'a', 'u', 'l', and '\n'. The easiest way to remove the newline is probably to use strchr() to find it, and then replace it with a 0 (not '0').

As for your loop, observe:

Code:

for (i=0; i<strlen(current->name); i++);

The loop body is empty (just a semicolon, which is called a null statement). Thus the statement you meant to execute inside of the loop only gets called once, at the end.

Ahhhh, thank you cas!

fgets always gets me tripped up with that newline it puts in there.

Below is the code I implemented to get it out of there.

Code:

printf("Enter a name to remove from the list: ");
fgets(name, sizeof(name), stdin);		
name[strlen(name) - 1] = '\0';

And I see where I made the null typo at the end of the for loop.....I'm always making typo's

Thanks again!

I can see your point laserlight. Thank you for your input.

As you can see I am still learning, so any advise is appreciated.