Problem using strcmp and strlen in a Linked List

This is a discussion on Problem using strcmp and strlen in a Linked List within the C Programming forums, part of the General Programming Boards category; Hello everyone, I have created a program that counts, prints, adds and deletes nodes in a linked list. I am ...

  1. #1
    Registered User matrixx333's Avatar
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    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.

  2. #2
    cas
    cas is offline
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    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.

  3. #3
    Registered User matrixx333's Avatar
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    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!

  4. #4
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by matrixx333
    Below is the code I implemented to get it out of there.
    What happens if the string does not contain a newline? You would then have removed a character that you would like to keep. So, you should check if that character is a newline. Personally, I would use strchr() instead of strlen().
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  5. #5
    Registered User matrixx333's Avatar
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    I can see your point laserlight. Thank you for your input.

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

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