Thread: Poiter to a pointer

What is the problem with my display function....

Code:

/*PROGRAM TO IMPLEMENT DIFFERENT OPERATIONS ON LINKED LISTS
*/
#include<stdio.h>
#include<stdlib.h>
#define draw for(i = 0 ;i<70;++i)printf(" ");
int flag_error = 0;

struct list
        {
                int num;
                struct list *link;
        };
typedef struct list node;

void display(node **p)
{
        node **temp = p;
        while(*temp != NULL)
        {
                printf("%d ->",*temp->num);

        } 

}
main()
{
         node * head;
        //MENU//
        //while(1)
        {       
                flag_error = 0;
                int i;
                system("clear");
                draw
                printf("-----------------------------\n\n\n");
                draw
                printf("------LIST MANIPULATION------\n\n");
                draw
                printf("      1 : CREATE LIST \n");
                draw
                printf("      2 : ADD ELEMENT \n");
 printf("      3 : DELETE ELEMENT \n");
                draw
                printf("      4 : DISPLAY LIST \n");
                draw
                printf("      5 : EXIT\n");
                printf("-----------------------------\n\n\n");
                draw
                printf("      ENTER YOUR CHOICE\n");


                {
                        int choice;
                        scanf("%d",&choice);
                        switch( choice )
                        {
                                case 1: draw
                                        printf("--------ENTER ELEMENT--------\n");
                                        {
                                                int num;
                                                scanf("%d",&num);
                                                create(&head);
                                        }
                                        printf("--------LIST IS--------\n");
                                        display(&head);
                                        break;
                                case 2: draw
                                        printf("--------ENTER ELEMENT TO ADD--------\n");
                                        {
                                                int num;
                                                scanf("%d",&num);
                                                draw
                                                printf("--------1 : ENTER AFTER AN ELEMENT-------- \n");
                                                draw
                                                printf("--------2 : ENTER BEFORE AN ELEMENT--------\n");
                                                draw
                                                printf("--------3 : ENTER AT A GIVEN POSITION--------\n");
                                                draw
                                                printf("----------------\n");
                                                {
                                                        int ch;
                                                        scanf("%d",&ch);
                                                        draw
                                                        switch(ch)
                                                                {
                                                                        case 1: 
                                                                        case 2:
                                                                                printf("--------ENTER THE ELEMENT--------\n");
                                                                                break;
                                                                        case 3:printf("--------ENTER THE POSITION--------\n");
                                                                }
                                                        {       
                                                                int no;
                                                                scanf("%d",&no);
                                                                add(&head,num,ch,no);
                                                        }
                                                }
                                                draw
                                                if( flag_error != 1)
                                                {

                                                        printf("--------NUMBER ADDED TO LIST--------\n");
        draw
                                                        printf("--------LIST IS--------");
                                                        display(&head);
                                                }
                                                else
                                                printf("--------LIST IS FULL ...--------");

                                        }
                                        break;
                                case 3 :draw
                                        printf("--------DELETE FROM WHICH POSITION\n--------\n");
                                        {
                                                int pos;
                                                scanf("%d",&pos);
                                                delete(&head,pos);
                                        }
                                        draw
                                        if(flag_error != 1)
                                        {

                                                printf("--------ELEMENT DELETED--------\n");
      draw
                                                printf("--------LIST IS:");
                                                display(&head);
                                                printf("--------");
                                        }
                                        else
                                        printf("--------LIST IS EMPTY--------\n");
                                        break;
                                case 4 :display( &head );
                                        break;
                                case 5:return 0 ;
                                default:draw
                                        printf("--------INVALID CHOICE--------\n");
                        }
                }
                draw
                printf("--------CONTINUE Y?N?--------\n");
                {
                        char ch ;
                        scanf(" %c",&ch);
                        if( ch != 'y' || ch != 'Y' )
                       return 0;
                }
        }
}

when I compile,the compiler says "error: request for member ‘num’ in something not a structure or union" about the statement " printf("%d ->",*temp->num);"

No, what I initially meant was:

Code:

void display(node **p)
{
        node *temp = p;
        while(temp != NULL)
        {
                printf("%d ->",(*temp)->num);
                temp = (*temp)->link;
        } 
}

But I'm going to change my suggestion slightly. You aren't modifying *p in the function (i.e. you aren't trying to change the value outside the function), so there is no need to pass a double pointer. You can just do:

Code:

void display (node *p)
{
    while (p != NULL) {
        printf("%d ->", p->num);
        p = p->next;
    }
}

That only changes the local copy of p, so you don't have to worry about anything outside that function changing. It also avoids your awkward dereferencing issue and allows you to drop the temp variable.

In your eg there is no point to pass pointer to pointer.
Since you are not going to modify what pointer points.

Originally Posted by anduril462

No, what I initially meant was:

Code:

void display(node **p)
{
        node *temp = p;
        while(temp != NULL)
        {
                printf("%d ->",(*temp)->num);
                temp = (*temp)->link;
        } 
}

But I'm going to change my suggestion slightly. You aren't modifying *p in the function (i.e. you aren't trying to change the value outside the function), so there is no need to pass a double pointer. You can just do:

Code:

void display (node *p)
{
    while (p != NULL) {
        printf("%d ->", p->num);
        p = p->next;
    }
}

That only changes the local copy of p, so you don't have to worry about anything outside that function changing. It also avoids your awkward dereferencing issue and allows you to drop the temp variable.

Ok.....my aim is to clarify my mis concepts about pointer to pointer ...In the other way it can be done in a simple manner...But my doubt is that
data type of p is node**
you wrote node *temp =p;
why it is node *temp;
data type of temp is node*
you assigns node** type variable to node* type variable.....please clarify...

Yes, it was a typo. Thanks Bayint!

Originally Posted by linuxlover

Ok.....my aim is to clarify my mis concepts about pointer to pointer ...In the other way it can be done in a simple manner...But my doubt is that
data type of p is node**
you wrote node *temp =p;
why it is node *temp;
data type of temp is node*
you assigns node** type variable to node* type variable.....please clarify...

Frankly I think your biggest misconception is that you seem to think you need them when most often you don't. I've been programming in C for about 6 years now and I'd bet real money you could count the number of places I've used pointer to pointer on the fingers of one hand.

You're doing way more work than you need to....

Go back to my original explaination in the other thread...
after the others have taken you to the same place...
does it now make sense to you?

Thanks to common Tater...