Question

so I’m creating a linked list counter for fun, here’s my program.

WHAT I WANT:
I want it to give me the correct count

WHAT I GET:
The following errors:

linkedlist.c:12: warning: assignment from incompatible pointer type
linkedlist.c:14: warning: assignment from incompatible pointer type
linkedlist.c: In function ‘main’:
linkedlist.c:22: warning: assignment from incompatible pointer type
linkedlist.c:23: warning: assignment from incompatible pointer type
linkedlist.c:24: warning: assignment from incompatible pointer type
linkedlist.c:25: warning: assignment from incompatible pointer type

MY CODE
I realize that this isn’t a good way to instantiate a linked list. That doesn’t matter though, what I’m stressing is that my counter works.

I have made some amendments to my code. No, this is not a c++ program, it actually is a c program.

My program now works, but I would like to know why I receive so many warnings about an incompatible pointer type. Any ideas? I’m sure it’s a simple problem.

 #include <stdio.h>
typedef struct {
  int x;
  char *y;
 struct CELL *next;
} CELL;

  int list_length(CELL *head){
    int counter;
    CELL *current;
    if(head->next!=NULL){
    current=head->next;
    for(counter=1;current!=NULL;++counter){
      current=current->next;}}
    else
      return 0;
    return counter;}


main(){
  CELL a,b,c,d,e;
  a.next=&b;
  b.next=&c;
  c.next=&d;
  d.next=&e;
  e.next=NULL;
  int l=list_length(&a);
  printf("The list length is %d \n",l);
}

Answer 1

First, to get this program to compile…

1. CELL isn’t a type, so you need struct CELL in your type declarations
2. A structure requires a semi-colon at the end
3. use char * instead of string (string isn’t a C data type)

As for the algorithm in list_length(), you want something more like this:

int list_length(struct CELL *head)
{
    int counter = 0;
    while (head) {
        head = head->next;
        ++counter;
    }
    return counter;
}

You should also note that your instantiate() function returns a pointer to stack-allocated memory resulting in undefined behavior. Move the code from instantiate() into the main function.

Here is a complete working program:

#include <stdio.h>

struct CELL {
    struct CELL *next;
};

int list_length(struct CELL *head)
{
    int counter = 0;
    while (head) {
        head = head->next;
        ++counter;
    }
    return counter;
}

int main(void)
{
    struct CELL a, b, c, d, e;
    a.next = &b;
    b.next = &c;
    c.next = &d;
    d.next = &e;
    e.next = NULL;
    printf("The list length is %d \n", list_length(&a));
    return 0;
}

To fix the warnings in your latest iteration you need to declare CELL something like this:

typedef struct CELL_ {
    int x;
    char *y;
    struct CELL_ *next;
} CELL;