C- Linked List traversing issue

Question

Ok so I have the following code:

typedef struct node {
char line[3];
struct node* next;
}NODE;

NODE * enq(char b[]);
NODE * deq(NODE *head);
void printQueue (NODE *head);

int main(void)
{
    FILE* fp;
    char* filename = "expressions.txt";
    char buffer[50];
    int len;

    struct node *head = NULL, *tail, *temp, *temp2;

    if((fp=fopen(filename, "r"))==NULL)
    {
        printf("unable to open %s\n", filename);
        exit(1);
    }


    while(fgets(buffer, sizeof(buffer), fp) !=NULL)
    {
        len=strlen(buffer);
        if(buffer[len-1]=='\n')
            buffer[len-1]='\0';

        if (strcmp("=",buffer) ==0)
        {

            printQueue(head);
        }
        else
        {           
            temp = enq(buffer);

            if(head ==NULL)
                head = temp;
            else
                tail->next = temp;
            tail = temp;
        }
    }

    }

    NODE * enq(char b[])
    {
        NODE *temp = malloc(sizeof(NODE));
        strcpy(temp->line, b);
        temp -> next = NULL;    

        return temp;
    }

    NODE *deq(NODE *head)
    {
        NODE *temp = head->next;

        free(head);
        return temp;
    }

    void printQueue (NODE *head)
   {
        char hold[3];
        int sum = 0, place, sign;
        while(head !=NULL)
        {
            strcpy(hold, head->line);
            if(hold[0] < 58 && hold[0]>47)
            {
                place = atoi(hold);
                if (sign == -1)
                {
                   place = place *sign;
                   sign = 1;
                }
                sum = sum + place;
           }
           else if (strcmp(hold, "-")==0)
           {
              sign = -1;
          }
          printf("%s ", hold);
        head=deq(head);
      }
     printf("= %d\n",sum);

     }

This is my input data from the file:

71
-
67
-
71
+
9
-
19
=
27
+
8
+
10
-
94
-
64
=

The expected output is this:

71 - 67 - 71 + 9 - 19 = -77
27 + 8 + 10 - 94 - 64 = -113

Output I am getting:

71 - 67 - 71 + 9 - 19 = -77
64 = 64

It always does the first set of expressions correctly, any expression after that, it skips some of the elements in the linked list, now I THINK I have narrowed it down to the dequeue method, that is not working correctly when I am freeing head. If I don't free and just test the print output, it prints out every single element in the list which is ok for observations but at the end of the day, I need to free each node as I extract it from the list and I am not sure why it is skipping a whole bunch of nodes. I printed the list before it is sent to printQueue and it seems fine, but right after dequeueing, the elements start to disappear. Any insight would be helpful.

Answer 1

This happens because you forget to reset the head pointer after you printed the result of an expression and are starting to parse a new expression (hence, a new linked list).

In your main(), try to replace the snippet

    if (strcmp("=",buffer) ==0)
    {

        printQueue(head);
    }

by

    if (strcmp("=",buffer) ==0)
    {

        printQueue(head);
        head = NULL;
    }

And you should get the expected output.

Please also beware of the memory leak that would happen if your input file does not end by a '=' (this would not call your printQueue() in which you currently free your list nodes).