Error implementing queue using linked list in C

Question

I'm implementing a queue using linked lists in C. Here's my structure -

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

I'm facing problem while executing push(). Here's my push() definition -

void push(node *head,int n)
{
    if (head==NULL)
    {
        head=(node *)(malloc((sizeof(node))));
        head->data=n;
        head->next=NULL;
        printf("=>%d\n",head->data);
    }
    else
    {
        node *ptr;
        ptr=head;
        while(ptr->next!=NULL)
        {
            ptr=ptr->next;
        }
        ptr->next=(node *)(malloc((sizeof(node))));
        ptr=ptr->next;
        ptr->data=n;
        ptr->next=NULL;
    }
    return;
}

and here's my main() function -

int main()
{
    int choice,n;
    node *head;
    head=NULL;
    while(1)
    {
        printf("Enter your choice -\n1. Push\n2. Pop\n3. Exit\n");
        scanf("%d",&choice);
        switch(choice)
        {
            case 1:
                printf("Enter element to push: ");
                scanf("%d",&n);
                push(head,n);
                if (head==NULL)//To check if head is NULL after returning from push()
                {
                    printf("Caught here!\n");
                }
                break;
            case 2:
                pop(head);
                break;
            case 3:
                return 0;
        }
    }
}

Now the problem is that after push() exits in case 1, head becomes NULL again, ie, the Caught here! statement does get executed. How is it possible?

You are modifying a local copy of the list in push(). To modify the one in main() you need a double-pointer. push() should be void push(node **head, int n) and wherever you use head in push() you should be using *head instead, and you should pass push(&head, n) when you call it. — leif
– leif, Commented Feb 10, 2014 at 16:32
head in main function and head in pop function are two different variables. — ajay
– ajay, Commented Feb 10, 2014 at 16:54

HelloWorld123456789 · Accepted Answer · 2014-02-10 16:39:09Z

5

Since you are calling by value and you are modifying the value (in this case node *head), the value is not retained in main(). So either

Pass pointer to node *head

push(&head,n); in main()

and modify

void push(node **head,int n)
Return head

node* push(node *head,int n)

and in main():

head=push(head,n);

edited Feb 10, 2014 at 16:39

answered Feb 10, 2014 at 16:32

HelloWorld123456789

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Comments

Soumen · Accepted Answer · 2014-02-10 18:16:41Z

0

Just adding to the accepted answer, another option would be to declare head variable as global. Then you don't need to pass head as a parameter to push or pop.

answered Feb 10, 2014 at 18:16

Soumen

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1 Comment

nsane Over a year ago

Yes, I did keep it as a backup option. But I wanted to know why the above code wasn't working. Any ways good suggestion.

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