Bubble Sort in linked lists C - c

Question

any idea why does my program crash after showing the right result? (i.e. i insert 6 2 4 3 he returns 2 3 4 6 but the console crashes right after!)
The main idea is to sort a linked list after the user giving the integers. I tried to solve it like if it was a bubble sort but for some reason it's not 100% operational. sorry for the portuguese comments!
#define ITEM_TYPE int
typedef struct lnode* List;
typedef struct lnode{
ITEM_TYPE info;
List next;
}List_node;
void ordenalista(List lista){
int contador = 0, i;
/* lista é uma lista recebida */
List head = lista; /* termos sempre disponível o início da lista*/
List actual = NULL; /*lista vazia, ou nó */
List temp = NULL;
if(head == NULL || head->next == NULL){ /* caso de lista vazia ou so com 1 elemento*/
printf("A lista esta vazia ou tem apenas um elemento");
return 0;
}
while(head != NULL){ /* contar quantos elementos tem a lista*/
contador++;
head = head->next;
printf("%d \n", contador);
}
for(i=0; i<contador; i++){ /* percorrer todos os elementos da lista*/
while(head->next != NULL){ /* enquanto não chegarmos ao final da lista*/
if(head->info > head->next->info){ /* se o valor do atual for maior que o valor do seguinte*/
temp = head->next;
head->next = head->next->next; /* não precisamos de usar actual->data, apenas precisamos de mudar os ponteiros*/
temp->next = head;
}
}
}
}
void ex213(){
int numero;
List lista;
lista = create_list();
while((scanf("%d",&numero)) == 1){ /* lê da esquerda para a direita. SCANF DÁ 1 SE INSERIR INTEIRO, 0 CASO CONTRÁRIO */
insertwithoutorder(lista, numero);
}
ordenalista(lista);
printlist(lista);
}
void insertwithoutorder(List lista, ITEM_TYPE it){
List no;
List ant, inutil;
no = (List)malloc(sizeof(List_node));
if (no != NULL) {
no->info = it;
searchwithoutorder(lista, it, &ant, &inutil);
/*while(ant->next != NULL){
ant = ant->next;
}*/
no->next = NULL;
ant->next = no;
}
}
void searchwithoutorder(List lista, ITEM_TYPE chave, List *ant, List*actual){
*ant = lista; *actual = lista->next;
while ((*actual) != NULL){
*ant = *actual;
*actual = (*actual)->next;
}
if ((*actual) != NULL && (*actual)->info != chave)
*actual = NULL; /* Se elemento não encontrado*/
}
void printlist(List lista){
List l = lista->next; /* Salta o header */
while (l){
printf("%d ", l->info);
l=l->next;
}
}
int main(){
ex213();
return 0;
}

Answer 1

#include <stdio.h>
#include <stdlib.h>
#define ITEM_TYPE int
typedef struct lnode* List;
typedef struct lnode{
ITEM_TYPE info;
List next;
} List_node;
List create_list(void){
//top node unused
return calloc(1, sizeof(List_node));//note : might NULL != 0
}
List lastNode(List lista){
while (lista->next != NULL){
lista = lista->next;
}
return lista;
}
void insertwithoutorder(List lista, ITEM_TYPE it){
List no, ant;
no = create_list();
if (no != NULL) {
no->info = it;
no->next = NULL;
ant = lastNode(lista);
ant->next = no;
} else {
printf("failed to create a new node.\n");
}
}
void ordenalista(List lista){
List head = lista;
List actual, neighbor, sentinel = NULL;
if(head->next == NULL){
printf("A lista esta vazia\n");
return ;
}
while(head->next != sentinel){
actual = head->next;
neighbor= actual->next;
while(neighbor != sentinel){
if(actual->info > neighbor->info){
ITEM_TYPE temp = actual->info;
actual->info = neighbor->info;
neighbor->info = temp;
}
actual = neighbor;
neighbor= neighbor->next;
}
sentinel = actual;
}
}
void printlist(List lista){
List l = lista->next;
while (l){
printf("%d ", l->info);
l=l->next;
}
puts("");
}
void ex213(void){
int numero;
List lista = create_list();
while((scanf("%d", &numero)) == 1){
insertwithoutorder(lista, numero);
}
//printlist(lista);
ordenalista(lista);
printlist(lista);
//deallocate ?
}
int main(void){
ex213();
return 0;
}

Answer 2

The crash must be occurring in the function (now) named printlist(), as your program does nothing else after that function returns.
I don't see anything inherently wrong with printlist(), but it does depend on the list being in a valid state. In particular, my best guess at why the program fails where it does would be that the next pointer of the last list element contains a junk value instead of the expected NULL. You could verify that by running the program in a debugger.
How, then, might the list be corrupted?
Well, your insertion function looks ok. The searchwithoutorder() function on which it relies doesn't actually do what its name says, but it does do what insertwithoutorder() needs it to do.
That leaves the sort function, ordenalista(), and here I'm a bit flumoxed. I don't see how the version you posted could do any sorting at all. You have a while loop like so: while(head != NULL){...}, whithout any break or goto inside, so when control passes beyond this loop it must be that head == NULL. Then, without modifying the value of head, you go into a loop nest like this:
for(i=0; i<contador; i++) { /* percorrer todos os elementos da lista*/
while(head->next != NULL) { /* enquanto não chegarmos ao final da lista*/
...
}
}
But head is NULL at that point, so dereferecing it produces undefined behavior. If that behavior were not itself a crash, then correctly guessing and dereferencing the pointer you meant is an extremely unlikely alternative. Moreover, you do not modify head in the body of the loop, so it remains NULL. There are other problems with this loop, too.
As if all that weren't enough, there is also no good way for your sort function to change which element is at the head of the list -- at least none that would be effective for the caller.
So basically, I don't believe you have accurately described the problem. It must be either that the failure behavior you observed is different from what you described, or that the code you presented does not correspond to the program whose misbehavior you described.

Answer 3

When asking the user for data, it is useful to let the user know what you are asking for and how to handle/terminate any requests for input. A blinking cursor on a blank line tells the user nothing. (even when it is testing for you, it can help prevent an inadvertent keystroke of the wrong type). For example your ex213 function leaves a blinking cursor on a blank line leaving the user wondering "Is the program hung?" "Did it encounter a race condition?" etc.. When asking the user for linked-list data, a simple additional line or two of guidance can really help. Example:
void ex213 (void)
{
int numero;
List lista = create_list ();
printf ("\nEnter a number to add to the list [CTRL+D] when done:\n\n");
while (printf (" data: ") && (scanf ("%d", &numero)) == 1) {
insertwithoutorder (lista, numero);
}
ordenalista (lista);
printf ("\n\nThe ordered linked-list is:\n\n");
printlist (lista);
printf ("\n");
//deallocate ?
}
Output
$ ./bin/ll_single_sort
Enter a number to add to the list [CTRL+D] when done:
data: 2
data: 8
data: 20
data: 6
data: 9
data: 1
data:
The ordered linked-list is:
1 2 6 8 9 20
Adding that to the answer provided by BLUEPIXY provides the results you were looking for.