Linked list troubles list not linking to head pointer

Question

I have been having trouble with this linked list specifically it seems like my head pointer is not linking to the rest of my list and I am confused as to why it is not. Where I insert my head pointer by pointer by reference it is not connected to the linked list referenced in main. unless the list is not linked together in the main function and I am missing something.

#include <stdio.h>
#include <stdlib.h>


typedef struct node{
    int number;
    struct node * next;
} Node;

typedef Node * Nodeptr;

void printlist (Node * head){
    Node * n = head;

    while(n != NULL){
        printf("%d\n",n ->number);
        n = n ->next;
    }

}
void sumlist (Node * head){
    Node * n = head;
    int sum;
    while(n != NULL){
        sum = n ->number +sum;
        n = n ->next;
    }
    printf("the total of all numbers in this list is %d",sum);
}
search(head){



}
int main(){
    int i =0;
Nodeptr head=NULL;

if((head = malloc(sizeof(Node))) == NULL)
return 0;

head->number =rand()%50+50;
head ->next = malloc(sizeof(Node));

int n;


Nodeptr newnode = NULL;
for(n=0;n<99;n++)
{

newnode = malloc(sizeof(Nodeptr));  

newnode->number = rand()%50+50;
newnode->next =NULL;
head -> next = newnode;



}


printlist(head);
sumlist(head);


return 0;
}

BTW int sum; --> int sum = 0;

BLUEPIXY
– BLUEPIXY

2017-04-23 17:39:00 +00:00
Commented Apr 23, 2017 at 17:39 — BLUEPIXY
– BLUEPIXY, Commented Apr 23, 2017 at 17:39
It is similar to this

BLUEPIXY
– BLUEPIXY

2017-04-23 17:40:52 +00:00
Commented Apr 23, 2017 at 17:40 — BLUEPIXY
– BLUEPIXY, Commented Apr 23, 2017 at 17:40

granmirupa · Accepted Answer · 2017-04-23 17:35:53Z

2

The error is that you are linking everything as next of head

  head -> next = newnode;

You need to use a pointer that gets updated:

Nodeptr newnode = NULL;
Nodeptr last = head;
for(n=0;n<99;n++)
{
  newnode = malloc(sizeof(Nodeptr));  
  newnode->number = rand()%50+50;
  newnode->next =NULL;
  last -> next = newnode;
  last = last->next;
}

You should also change this:

head ->next = malloc(sizeof(Node)); // otherwise you will lose this element.

into

head ->next = NULL;

edited Apr 23, 2017 at 17:35

answered Apr 23, 2017 at 17:30

granmirupa

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3 Comments

Guillermo Diaz Granados Over a year ago

I seem to be getting a memory leak somewhere.... when I run this program it crashes around the middle of list printing.

granmirupa Over a year ago

@GuillermoDiazGranados as you can see here it prints correctly. The error is not in the insert

Guillermo Diaz Granados Over a year ago

Ah ok I just ran my code in that and it works fine and asked my friend to run it and it ran fine. huh ok then. Thank you for your help!

aghast · Accepted Answer · 2017-04-23 17:36:07Z

You execute these steps in a loop:

newnode = malloc(sizeof(Nodeptr));  
newnode->number = rand()%50+50;
newnode->next =NULL;
head -> next = newnode;

You are setting the newnode->next to point to null, and head->next to point to newnode.

This means, each time through the loop your head gets a new next, and that's it.

Effectively, each time you pass through the loop you drop the previous newnode on the floor, and link to a new one. At the end, you'll have head pointing to 1 node, and you'll have 98 nodes dropped on the floor that you can't reach.

You need to either maintain a "tail" pointer, or a copy of "head", and set head or tail or something to the most recent value of newnode. Then, you can set tail->next = newnode; tail = newnode; which will continually extend your list, rather than overwriting the same head->next each time.

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