Thread: [Dev-C++] C++ Linked Lists and Add After

My teacher has given us the following code, within this code I have added the function "void add_after" and am trying to get it to work I do not get an error with this program. It will run but it will not add the word after. I think once you review the code it should be self explanatory:

<code>
#include <iostream>
#include <cstdlib>
#include <cstring>
using namespace std;
 
const int MAX_WORD_LENGTH = 80;
 
/* definition of a node */
struct Node;
typedef Node *Node_ptr;
 
struct Node
{
            char word[MAX_WORD_LENGTH];
            Node_ptr ptr_to_next_node;
};
 
/* Function to assign a linked list to "a_node" */
void assign_list(Node_ptr &a_list);
 
/* Function to assign a new dynamic node variable to "a_node" */
void assign_new_node(Node_ptr &a_node);
 
/* Function to print the strings in the list "a_node" */
void print_list(Node_ptr a_node);


void add_after(Node_ptr &list, char a_word[], char word_after[]);
//This function inserts a node containing "word_after" in the linked list "list", after the first occurrence of a node containing "a_word". If "list" does not contain such a node, the function leaves it unchanged.

 
//*****************************************************
void main()
{
	

	
			char word_a[MAX_WORD_LENGTH];
			char aword[MAX_WORD_LENGTH];
			Node_ptr my_list = NULL;
            assign_list(my_list);
            cout << "\nTHE LIST IS NOW:\n";
            print_list(my_list);
		   
			cout << "After which word would you like to add a word?:";
			cin >> word_a;
			cout << "What word would you like to add after that word?:";
			cin >> aword;
			add_after(my_list,aword, word_a);

			cout << "The list is now:"<< endl;

			 print_list(my_list);

			 




}			
//*****************************************************
 
/* DEFINITION OF FUNCTION "assign_list" */
void assign_list(Node_ptr &a_list)
{
            Node_ptr current_node, last_node;
           
            assign_new_node(a_list);
            cout << "Enter first word (or '.' to end list): ";
            cin >> a_list->word;
            if (!strcmp(".",a_list->word))
            {
                        delete a_list;
                        a_list = NULL;
            }
            current_node = a_list;
           
            while (current_node != NULL)
            {
                        assign_new_node(last_node);
                        cout << "Enter next word (or '.' to end list): ";
                        cin >> last_node->word;
                        if (!strcmp(".",last_node->word))
                        {
                                    delete last_node;
                                    last_node = NULL;
                        }
                        current_node->ptr_to_next_node = last_node;
                        current_node = last_node;
            }
}          
/* END OF FUNCTION DEFINITION */
 
/* DEFINITION OF FUNCTION "assign_new_node" */
void assign_new_node(Node_ptr &a_node)
{
            a_node = new Node;
            if (a_node == NULL)
            {
                        cout << "sorry - no more memory\n";
                        exit(1);
            }
}
//*****************************************************
 
/* DEFINITION OF FUNCTION "print_list" */
void print_list(Node_ptr a_node)
{
            while (a_node != NULL)
            {
                        cout << a_node->word << " ";
                        a_node = a_node->ptr_to_next_node;
            }
}
//*****************************************************

void add_after(Node_ptr &list, char a_word[], char word_after[])
{
	
	
	Node_ptr currNode;
	Node_ptr newNode;
	Node_ptr nextNode;

	currNode = list;
	while(currNode != NULL)
	{
		if(currNode -> word == a_word)
		{
			assign_new_node(newNode);
			nextNode = currNode ->ptr_to_next_node;
			currNode ->ptr_to_next_node = newNode;
			newNode ->ptr_to_next_node = nextNode;
			strncpy(newNode->word, a_word, MAX_WORD_LENGTH);

		}
		else
			currNode = currNode -> ptr_to_next_node;
			
	}

}
</code>

Dne 24.4.2011 20:23, Jordan Burnam napsal(a):
> if(currNode -> word == a_word)

You cannot compare strings with ==, you'll have to use strcmp() function 
like it is already used inside the assign_list() function.

Now I get an infinite loop when using the compare string functions for the if statement in add_after any ideas?

Sent from my iPhone

On Apr 24, 2011, at 4:51 PM, Michal Molhanec <mol...@se...> wrote:

> Dne 24.4.2011 20:23, Jordan Burnam napsal(a):
>> if(currNode -> word == a_word)
> 
> You cannot compare strings with ==, you'll have to use strcmp() function like it is already used inside the assign_list() function.
> 

Note that your function is using the same word twice in the code - it
isn't bothering about the other word.

And also - you claim it should insert the new word after the "first" match
- but if the function doesn't return after a match, it will insert the new
word after every match.

/pwm

On Sun, 24 Apr 2011, jordanburnam1990 wrote:

> Now I get an infinite loop when using the compare string functions for the if statement in add_after any ideas?
>
> Sent from my iPhone
>
> On Apr 24, 2011, at 4:51 PM, Michal Molhanec <mol...@se...> wrote:
>
> > Dne 24.4.2011 20:23, Jordan Burnam napsal(a):
> >> if(currNode -> word == a_word)
> >
> > You cannot compare strings with ==, you'll have to use strcmp() function like it is already used inside the assign_list() function.
> >
>
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There are two additional errors:
1) As already pointed out by Per, you are using a_word instead of
word_after in the condition, so the line:
if(currNode -> word == a_word)
should be:
if (strcmp(currNode -> word, word_after) == 0)

2) The infinite loop is because when you found the word you do not
change the currNode so the
while(currNode != NULL)
never becames true. Using break is correct solution you just have to put 
it right before the else part:
if (strcmp(currNode -> word, word_after) == 0)
{
assign_new_node(newNode);
nextNode = currNode ->ptr_to_next_node;
currNode ->ptr_to_next_node = newNode;
newNode ->ptr_to_next_node = nextNode;
strncpy(newNode->word, a_word, MAX_WORD_LENGTH);
break; // HERE
}
else
currNode = currNode -> ptr_to_next_node;
}
(You can as well do currNode = NULL; instead of the break or use return 
in this case.)


Dne 25.4.2011 2:10, jordanburnam1990 napsal(a):
> Now I get an infinite loop when using the compare string functions
> for the if statement in add_after any ideas?
>
> Sent from my iPhone
>
> On Apr 24, 2011, at 4:51 PM, Michal Molhanec<mol...@se...>
> wrote:
>
>> Dne 24.4.2011 20:23, Jordan Burnam napsal(a):
>>> if(currNode ->  word == a_word)
>>
>> You cannot compare strings with ==, you'll have to use strcmp()
>> function like it is already used inside the assign_list()
>> function.
>>
>
> I saw why I had the infinite loop cause it traversed through the
> dynamic memory till it found the end which can take a while. So I
> added a break statement after the if statement and that has solved
> it. But now it adds the name to the wrong place in the list each
> time. I know it's a simple logic error but I just can't seem to pin
> point it.

On 4/24/2011 2:23 PM, Jordan Burnam wrote:
> if(currNode -> word == a_word)
you can't compare strings this way
man or google strcmp and friends