insert node in a linked list runtime error

Question

the code is supposed to add a node after the previous node argument, given the city name and the head pointer. I get a runtime error, however, when I run the code, why?

 city* addCity(city *head, city *previous, string cityName )
    {
        city* add = new city;
        add->name=cityName;
        add->next = NULL;
        city* tmp = new city;
        tmp = head;


        if(tmp==NULL){
            tmp = add;
        }

        while(tmp != NULL && tmp != previous){
            tmp = head;
            tmp = tmp->next;
        }


    if(tmp == previous){
        add->next = previous->next;
        tmp->next = add;
        head = tmp;
        return head;
        } 


        }

city* tmp = new city; tmp = head; This will cause a memory leak in your program. — Josh Weinstein
– Josh Weinstein, Commented Oct 9, 2017 at 4:59
It's a singly linked list. I am running the code on an online platform, i'll send you a screenshot of the error shortly — Mahmoud Almansouri
– Mahmoud Almansouri, Commented Oct 9, 2017 at 5:00
Yeah, I am not sure if I can post images since I am a new user. — Mahmoud Almansouri
– Mahmoud Almansouri, Commented Oct 9, 2017 at 5:07
U can make this code lot more easier. No need to do all these things. Wait I'll share the code — Sushin Pv
– Sushin Pv, Commented Oct 9, 2017 at 5:30

Saurav Sahu · Accepted Answer · 2017-10-09 05:12:29Z

1

  while(tmp != NULL && tmp != previous){
        tmp = head;
        tmp = tmp->next;
  }

This will run infinite times as tmp is reset to head in each iteration. tmp is just toggling in a cyclic way between values head and head->next in this loop.

answered Oct 9, 2017 at 5:12

Saurav Sahu

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1 Comment

CiaPan Over a year ago

Not necessarily infinite. At the beginning, the list is probably empty, head==NULL, so the tmp variable is assigned the add value before the above loop. Then the loop will enter the first iteration (due to tmp != NULL) and most probably crash on accessing tmp->next after tmp=head (i.e.: NULL) assignment.

CiaPan · Accepted Answer · 2017-10-24 06:45:17Z

0

Too much complication.

You have two general cases: either you're given the previous node in the list, so you don't have to seek it, or previous == NULL and you insert the new node at the front of the list (which also covers the case of an empty list).

city* addCity(city *head, city *previous, string cityName)
{
    city *newcity = new city;
    newcity->name = cityName;

    if (previous != NULL)
    {
        newcity->next = previous->next;
        previous->next = newcity;
    }
    else
    {
        newcity->next = head;
        head = newcity;
    }

    return head;
}

Note the return statement – your function has no access to the original head of the list, so the only way to inform the code outside that a new node appeared at the front of the list is returning a pointer to it. Of course, if a new node is inserted somewhere inside the list or appended at the end, the head doesn't change and the returned value equals the input head.

edited Oct 24, 2017 at 6:45

answered Oct 9, 2017 at 7:08

CiaPan

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1 Comment

Mahmoud Almansouri Over a year ago

Thanks a lot! this worked. I am kinda new to data structures, do you have suggestions as to how become more familiar with it?

Seeker · Accepted Answer · 2017-10-09 06:50:26Z

Your code has a lot of issues.

Firstly,

city* tmp = new city;
tmp = head;

Your are creating a new city for tmp but immediately assigning it to head. Assuming you just need a *tmp, so you can change city *tmp = new city; to city *tmp = head;

Then,

if(tmp==NULL){
    tmp = add;
}

tmp == NULL means the list is empty. So you should add the new city to head and simply return from there. So, your code should be-

if(tmp==NULL){
    head = add;
    return head;
}

Continuing,

while(tmp != NULL && tmp != previous){
    tmp = head;
    tmp = tmp->next;
}

This is an infinite loop if tmp is not NULL. Because your are doing the same things in each iteration. It should be like-

tmp = head;

while(tmp != NULL || tmp != previous){
    tmp = tmp->next;
}

Forwarding,

if(tmp == previous){
    add->next = previous->next;
    tmp->next = add;
    head = tmp;
    return head;
}

I don't know why you are assigning tmp to head. Just removing head = tmp; should solve a lot of issues.

Sushin Pv · Accepted Answer · 2017-10-09 09:15:25Z

0

If you have the previous node that contain the next the node and the String you can directly add the new node to that.

Try this

city* addCity(city *head, city *previous, string cityName ){
    city* add = new city;
    add->name=cityName;
    if (head == NULL || previous == NULL){
        add->next = head ;
        return add;
    }
    add->next = previous->next;
    previous->next = add;
    return head;
}

This is enough too achieve the same ans.

For the first node to create check the following example

void Cites(){
    city *head = new city();
    head = addCity(head, head ,"First city name");
    //Do the rest of the codes
}

Edited: Now this code will work even to add node in the first. by passing previous as NULL

edited Oct 9, 2017 at 9:15

answered Oct 9, 2017 at 5:56

Sushin Pv

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18 Comments

Mahmoud Almansouri Over a year ago

For the first code, you would get "nothing in path". The second one a run time error :/.

Mahmoud Almansouri Over a year ago

For the updated code, if you add a member in the beginning of the list you would end up with a runtime error. But it works if the new city was in the middle of the linked list.

Mahmoud Almansouri Over a year ago

== CURRENT PATH == Los Angeles -> Phoenix -> Denver -> Dallas -> Atlanta -> New York -> NULL === Runtime error

Mahmoud Almansouri Over a year ago

I am using moodle to submit the code and instead of the runtime error message we're supposed to update the linked list and put a city before LosAngeles in this case

Sushin Pv Over a year ago

@MahmoudAlmansouri You need to insert before or after the previous node ?

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