linked list adding operation with two structure type

quartaela

hi there i am working on a project that reads some data from a file and storing it into nodes. i actually made some operations like adding,deleting,search ect. in singly linked list with one structure type. now i am trying to use two structure types which are;

typedef struct node {

int birth\_date;
int zipcode;
int phone\_num;
char first\_name\[50\];
char last\_name\[50\];
char city\[50\];
char address\[50\];
char email\_addr\[50\];

struct node\* fn\_next;
struct node\* ln\_next;
struct node\* birdat\_next;

} NODE;

typedef struct {

int count;
NODE\* head;
NODE\* tail;
NODE\* current;

}LIST;

and this is the block which i read and store the data and finally call add_Node function;

while ( !feof(myfile) ) {

    if( !(student = (NODE\*) malloc(sizeof(NODE))) ) {

        printf("ERROR NOT ENOUGH MEMORY!!!\\n");
        exit(100);
    }

    fscanf(myfile,"%s", &(student->first\_name) );
    fscanf(myfile,"%s", &(student->last\_name) );
    fscanf(myfile,"%s", &(student->email\_addr) );
    fscanf(myfile,"%d", &(student->phone\_num) );
    fscanf(myfile,"%s", &(student->address) );
    fscanf(myfile,"%s", &(student->city) );
    fscanf(myfile,"%d", &(student->zipcode) );

    add\_Node(&list,student);

    printf("%s\\n", student->first\_name );

}

}

and of course add_Node function;

void add_Node(LIST** list, NODE* student) {

    if( (\*list)->head == NULL ) {

        student->fn\_next = (\*list)->head;
        (\*list)->head = student;
        (\*list)->tail = student;

        printf("Adding operation is succesfull\\n");

        (\*list)->count++;

        return;
    }

    else {

        (\*list)->current = (\*list)->head;

        while ( (\*list)->current != NULL )

            (\*list)->current = (\*list)->current->fn\_next;

        (\*list)->current->fn\_next = student;
        student->fn\_next = NULL;

        (\*list)->tail = (\*list)->current;

        return;
    }

}

i am taking segmentation faults in add_Node and i don't know how to fix it.

Luc Pattyn

Sure it goes wrong, as this statement

while ( (*list)->current != NULL ) (*list)->current = (*list)->current->fn_next;

is bound to result in (*list)->current being zero/null, hence the following

(*list)->current->fn_next=...

will bomb. Some ideas: 1. if you want to add a node at the end of the list, and your data structures have a tail member, why not use it, rather than scanning the list? 2. try not to have special cases. When done properly, one often does not need them. And when one does, try making the alternate paths as short as possible, they tend to have common things (such as return, and count++ of which you forgot one). :)

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quartaela

i changed my way to implement the linked list with only NODE structure which is the way i know well. but this fault is not acceptable for me, i am angry to myself now because i want implement to list with using LIST structure. anyway, could you explain why i can't print the data with using code like this --> list->head->first_name

Andrew Phillips

This is obviously an assignment but I'll give you some ideas. a. It's better to use an existing container like std::list but if this is C and you don't have one or you have to write your own linked list then ... b. You don't need to store current. I also assume you store tail and count for efficiency as you can work these out too. c. You store tail but then in add_Node() you scan the list for the end anyway. d. It may be simpler just to add new students to the start of the list rather than the end.

NODE \* tmp = head;
head = student;
student->next = tmp;

e. If you step through your code the bvious problem is assigning through a NULL pointer. Why not do it like this:

void add_Node(LIST** list, NODE* student)
{
NODE ** pcurr;
for (pcurr = &head; *pcurr != NULL; pcurr = &((*pcurr)->next) )
; // no loop body
*pcurr = student;
student->next = NULL;
}

Andrew Phillips http://www.hexedit.com andrew @ hexedit.com

quartaela

hi andrew; firstly, i must create my own linked and i know how to create it and i know how to make some simple operations with linked list. but this is the first time i want to use LIST structure(as you can see there is a head,tail,current and count). for example i wanna use like this;

list->head->first_name

and logically isn't it true_?. in main function the code is work well if i use like this

printf("%s\n", student->first_name );

but if use like this it isn't working. so what is the difference_?.

printf("%s\n", list->head->first_name );

Luc Pattyn

I did not tell you to drop the LIST structure; if you want new nodes added at the tail, having a tail pointer is the obvious way of doing it, as it is much faster than searching for the last node each time. And your original code would never reach the print statement as it bombs when you use the null reference that is the resulting of spinning the while loop. So either use the tail pointer and drop the while loop, or fix the while loop. :)

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Andrew Phillips

> printf("%s\n", list->head->first_name ); Yes, this should work. It could be that list or list->head are NULL or uninitialised. Try: printf("%p %p %s\n", list, list->head, list->head->first_name ); which will print out the values of the pointers. Or step through the code and look at the values in the debugger.

Andrew Phillips http://www.hexedit.com andrew @ hexedit.com

Luc Pattyn

Andrew Phillips wrote:

printf("%p %p %s\n", list, list->head, list->head->first_name );

I guess one needs separate print statements here, as if one argument bombs, nothing will be printed at all. :)

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quartaela

i opened another project to implement the list with LIST and NODE structure. but i am facing with an error. i couldn't fix it. the error is; expected '=', ',', ';', 'asm' or '__attribute__' before '*' token and the error is in create_list function which is;

LIST* create_list(void)
{

LIST\* list;

list = (LIST\*) malloc(sizeof(LIST));

if (!list) {

    printf("ERROR NOT ENOUGH MEMORY!!!\\n");
    exit(100);
}

else {

    list->count = 0;
    list->current = NULL;
    list->head = NULL;
    list->tail = NULL;

}

return list;

}

and this is the main function;

LIST* create_list(void);

int main( void ) {

LIST\* list;

list = create\_list();

build\_list(list);

print\_list(list);

return 0;

}

Luc Pattyn

most compilers give file name (and/or class name) and line number; tell your IDE to always show line numbers in editor windows. And try double-clicking the error message, any of these measures should bring you right to the objectionable statement. BTW: Did you provide the definition of NODE and LIST before referencing them in your source code? :)

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quartaela

i can see the location of error when i double clicked :D. this is the line which error appears in create_list.h

LIST* create_list(void)

quartaela

well i solved the problem. and this is the most stupid problem i have ever faced. it takes for hours to see. i only changed the location include "structures.h" with include "create_list.h" :D. so know create_list function can realize the structures :)

Luc Pattyn

:zzz:

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