Linked list problem with while loop, should I use continue?

Question

I have to print out every unique integer in a linked list, it turned out the it's not printing anything.I tried using continue and remove the else but by doing that I only get the first unique item in the linked list. My question is that does continue end the loop or just skip the current iteration? If it only skips the current one, say i have a linked list that looks like 1->5, why am I only getting 1 printed here? Thanks in advance.

 struct node {
    int data; 
    struct node* next; 
 }


void printList(struct node *head)  // print out unique items of the list
{

    struct node *ptr = head;
    struct node *prev = NULL;

    while(ptr != NULL)
    {

        if(prev->data == ptr->data)
        {
            prev = ptr;
            ptr = ptr->next;

        }else{
        printf("%d", ptr->data);
        prev = ptr;
        ptr = ptr->next;

        }

    }
}

break breaks out of the loop, continue ... continues the loop. That's very basic C knowledge covered in every C text book. — Jabberwocky
– Jabberwocky, Commented Feb 20, 2019 at 20:56
Are you getting any errors when you run it? It looks like you should to me, since you'll be dereferencing a null pointer the first time you evaluate prev->data. — Harry Cutts
– Harry Cutts, Commented Feb 20, 2019 at 20:56
Aside: the code could be much simplifed by a slight reordering and the removal of the duplication of prev = ptr; ptr = ptr->next; which are needed whether or not there was a match found. — Weather Vane
– Weather Vane, Commented Feb 20, 2019 at 20:58
struct node { int data; struct node* next; }; here's the struct node — Alex H
– Alex H, Commented Feb 20, 2019 at 21:00

John Szakmeister · Accepted Answer · 2019-02-20 21:25:34Z

1

The code as written generates a segmentation fault because you're referencing a NULL pointer. In this line:

if(prev->data == ptr->data)

Since prev is NULL to start, prev->data will dereference the NULL pointer and crash the program. That doesn't explain why you saw it not printing the last element, but the code as written is definitely broken.

Try this:

void printList(struct node *head)  // print out unique items of the list
{

    struct node *ptr = head;
    struct node *prev = NULL;

    while (ptr != NULL)
    {
        // Check if prev is NULL, and if not, that prev->data and ptr->data
        // don't match.
        if (! prev || ptr->data != prev->data)
        {
            printf("%d\n", ptr->data);
        }

        prev = ptr;
        ptr = ptr->next;
    }
}

edited Feb 20, 2019 at 21:25

answered Feb 20, 2019 at 21:09

John Szakmeister

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2 Comments

Alex H Over a year ago

Thank you! It worked. Could you explain what does !prev stand for?

John Szakmeister Over a year ago

!prev is a short way of saying prev == NULL. :-)

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