Question

I’m trying to sort names into alphabetical order inside a linked list but am getting a run time error. what have I done wrong here?

#include <iostream>
#include <string>
using namespace std;

struct node{
    string name;
    node *next;
};

node *A;

void addnode(node *&listpointer,string newname){
    node *temp;
    temp = new node;
    if (listpointer == NULL){
        temp->name = newname;
        temp->next = listpointer;
        listpointer = temp;
    }else{
        node *add;
        add = new node;
        while (true){
            if(listpointer->name > newname){
                add->name = newname;
                add->next = listpointer->next;
                break;
            }
            listpointer = listpointer->next;
        }
    }
}

int main(){

    A = NULL;
    string name1 = "bob";
    string name2 = "tod";
    string name3 = "thomas";
    string name4 = "kate";
    string name5 = "alex";
    string name6 = "jimmy";
    addnode(A,name1);
    addnode(A,name2);
    addnode(A,name3);
    addnode(A,name4);
    addnode(A,name5);
    addnode(A,name6);

    while(true){
        if(A == NULL){break;}
        cout<< "name is: " << A->name << endl;
        A = A->next;
    }

    return 0;

}

Answer 1

I believe the mistake is that you think:

if (listpointer == NULL){
    temp->name = newname;
    temp->next = listpointer;
    listpointer = temp;
}

guarantees that listpointer won’t ever be NULL later. However this isn’t the case, for example:

void addnode(node *&listpointer,string newname){
   node *temp;
    temp = new node;
    if (listpointer == NULL){
        temp->name = newname;
        temp->next = listpointer;
        listpointer = temp;
    }else{
        node *add;
        add = new node;
        while (true){
            if( (listpointer) == NULL){
            std:cout << "oops (listpointer) == NULL)";
            }

            if(listpointer->name > newname){
                add->name = newname;
                add->next = listpointer->next;
                break;
            }
            listpointer = listpointer->next;
        }
    }
}

Will print out “oops” then segfault as lispointer is NULL and using -> on a NULL will cause a segfault. This is because in the while (true) loop listpointer eventually reaches the end and gets set to NULL. You then get the segfault.

I think a better idea would be to do something like:

bool has_inserted;
while ( listpointer != NULL){
  if(listpointer->name > newname){
     add->name = newname;
     add->next = listpointer->next;
     has_inserted = true;
     break;
  }
  listpointer = listpointer->next;
}
if(has_inserted == false){
//insert at end of list
}

Also this code leaks memory as you don’t delete the things you created with new. You may want to run this (and other code) with valgrind to see what I mean.