Show list in descending order - c

Question

I have this program that does not throw error, but the viewl procedure only shows the first item in the list. How do I for the viewl procedure that printing but in the reverse order to what does the procedure viewr?
#include <stdio.h>
#include <conio.h>
#include <windows.h>
#include <stdlib.h>
struct list
{
int info;
struct list *nxt,*prv;
}*HEAD=NULL,*AUX=NULL,*P=NULL,*F=NULL,*QD=NULL,*FD=NULL;
main function (ok)
int main()
{
void insertr(void);
void extractr(void);
void viewr(void);
void viewl(void);
void deleten();
void deletep();
char opc;
do
{
system("cls");
printf("___________________________________________________\n");
printf("_______¡¡¡DOUBLE-LINKED CIRCULAR LIST MENU!!!______\n");
printf("___________________________________________________\n");
printf("_________________SELECT AN OPTION__________________\n");
printf("___________________________________________________\n");
printf("___________________________________________________\n");
printf("__________1) INSERT________________________________\n");
printf("__________2) VIEW ASCENDING________________________\n");
printf("__________3) VIEW DESCENDING_______________________\n");
printf("__________4) ENTERING AND ELIMINATE NEXT___________\n");
printf("__________5) ENTERING AND ELIMINATE PREVIOUS_______\n");
printf("__________6) EXIT__________________________________\n");
printf("___________________________________________________\n");
opc=getch();
switch(opc)
{
case '1':
insertr();
break;
case '2':
viewr();
break;
case '3':
viewl();
break;
case '4':
deleten();
break;
case '5':
deletep();
break;
}
}
while(opc!='6');
getch();
return 0;
}
insert right process
void insertr(void)
{
P=HEAD;/* very first execution of this method P=NULL */
AUX=(struct list *)malloc(sizeof(struct list));
system("cls");
printf("ENTER AN ENTIRE NUMBER: ");
scanf("%d",&AUX->info);
AUX->nxt=HEAD;
AUX->prv=HEAD;
F=AUX;
if(HEAD==NULL)
{
HEAD=AUX;
P = AUX;/*first execution of this method P is no longer NULL but P is pointing to AUX */
}
else
{
while (P->nxt!=HEAD)
{
P=P->nxt;
}
}
P->nxt=AUX;
AUX->prv=P;
HEAD->prv=AUX;
}
void deleten()
{
int x;
system("cls");
printf("ENTER A NUMBER TO ELIMINATE THE FOLLOWING: ");
scanf("%d",&x);
FD=HEAD;
QD=HEAD;
while(FD->info!=x&&FD->nxt!=HEAD)
{
FD=FD->nxt;
}
QD=FD->nxt;
if(FD->nxt==HEAD&&FD->info!=x)
{
printf("\nENTERED NUMBER IS NOT LISTED");
}
else
{
if(FD->info==x)
{
FD->nxt=QD->nxt;
(QD->nxt)->prv=FD;
printf("\nDELETED %d",QD->info);
free(QD);
}
}
getch();
}
deletea process (ok)
void deletep()
{
int x;
system("cls");
printf("ENTER A NUMBER TO REMOVE THE PREVIOUS ");
scanf("%d",&x);
FD=HEAD;
QD=HEAD;
while (FD->info!=x&&FD->nxt!=HEAD)
{
FD=FD->nxt;
}
QD=FD->prv;
if(FD->nxt==HEAD&&FD->info!=x)
{
printf("\nENTERED NUMBER IS NOT LISTED");
}
else
{
if(FD->info==x)
{
FD->prv=QD->prv;
(QD->prv)->nxt=FD;
printf("\nDELETED %d",QD->info);
free(QD);
}
}
getch();
}
viewr process (ok):
void viewr(void)
{
system("cls");
if(HEAD==NULL)
{
printf("EMPTY LIST");
getchar();
return;
}
AUX=HEAD;
printf("LIST:\n\n");
while(AUX->nxt!=HEAD)
{
printf("-> %d\n",AUX->info);
AUX=AUX->nxt;
}
if(AUX->nxt==HEAD)
{
printf("-> %d\n",AUX->info);
}
getch();
}
this function fails:
void viewl(void)
{
system("cls");
if(HEAD==NULL)
{
printf("EMPTY LIST");
getchar();
return;
}
AUX=F;
printf("LIST:\n\n");
do
{
printf("-> %d\n",AUX->info);
AUX=AUX->prv;
}
while(AUX->nxt!=HEAD);
getch();
}

Answer 1

The code had several logical errors, other variables, etc. Here it is working without problems
#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
#include<windows.h>
//THIS IS TO CREATE THE LIST
struct list{
int info;
struct list *nxt,*prv;
}*HEAD=NULL,*AUX=NULL,*P=NULL,*FD=NULL,*QD=NULL,*Fd=NULL,*Qd=NULL;
int main(){
void insertr(void);
void viewr(void);
void viewl(void);
void deleten();
void deletep();
char opt;
do{
system("cls");
printf("___________________________________________________\n");
printf("_______¡¡¡DOUBLE-LINKED CIRCULAR LIST MENU!!!______\n");
printf("___________________________________________________\n");
printf("_________________SELECT AN OPTION__________________\n");
printf("___________________________________________________\n");
printf("___________________________________________________\n");
printf("__________1) INSERT________________________________\n");
printf("__________2) VIEW ASCENDING________________________\n");
printf("__________3) VIEW DESCENDING_______________________\n");
printf("__________4) ENTERING AND ELIMINATE NEXT___________\n");
printf("__________5) ENTERING AND ELIMINATE PREVIOUS_______\n");
printf("__________6) EXIT__________________________________\n");
printf("___________________________________________________\n");
opt=getch();
switch(opt) {
case '1':
insertr();
break;
case '2':
viewr();
break;
case '3':
viewl();
break;
case '4':
deleten();
break;
case'5':
deletep();
break;
}
}while(opt!='6');
return 0;
}
void insertr(void){
P=HEAD; /* very first execution of this method P=NULL */
AUX=(struct list *)malloc(sizeof(struct list));
system("cls");
printf("ENTER AN ENTIRE NUMBER: ");
scanf("%d",&AUX->info);
AUX->nxt=HEAD;
AUX->prv=HEAD; // The pointer to the previous node is null (HEAD is null)
if(HEAD==NULL){ //I ask if HEAD is equal to Null
HEAD=AUX;
P=AUX; /* The first execution of this method P is no longer NULL but P is pointing to AUX */
}else{ //THIS YES
while(P->nxt!=HEAD){
P=P->nxt;
}
}
P->nxt=AUX; // The next pointer of P takes the value of AUX
AUX->prv=P;
HEAD->prv=AUX;
}
void deleten(){
int x;
system("cls");
printf("ENTER A NUMBER TO ELIMINATE THE FOLLOWING: ");
scanf("%d",&x);
FD=HEAD;
QD=HEAD;
while(FD->info!=x&&FD->nxt!=HEAD){ // if what I am going to eliminate is different from the number that was entered and if there is another
FD=FD->nxt; //step to the next node
}
QD=FD->nxt;
if(FD->nxt==HEAD&&FD->info==x){
printf("\nIT IS THE HEADBOARD! CAN NOT BE ELIMINATED");
}else
if(FD->nxt==HEAD&&FD->info!=x){
printf("\nTHE NUMBER ENTERED IS NOT IN THE LIST");
}else{
FD->nxt=QD->nxt;
(QD->nxt)->prv=FD;
printf("\nDELETED %d",QD->info);
free(QD); //free space in memory occupied by QD
}
getch();
}
void deletep()
{
int xd;
system("cls");
printf("IENTER A NUMBER TO REMOVE THE PREVIOUS: ");
scanf("%d",&xd);
Fd=HEAD;
Qd=HEAD;
while(Fd->info!=xd){
Fd=Fd->prv;
} Qd=Fd->prv;
if(Fd==HEAD){
printf("\nIT'S THE HEAD! CAN NOT BE ELIMINATED");
}else{
if(Qd==HEAD){
HEAD=HEAD->nxt;
Fd->prv=Qd->prv;
(Fd->prv)->nxt=Fd;
printf("\nDELETED %d",Qd->info);
}else{
Fd->prv=Qd->prv;
(Qd->prv)->nxt=Fd;
printf("\nDELETED %d",Qd->info);
}
free(Qd);
}
getch();
}
void viewr(void)
{
system("cls");
if(HEAD==NULL){
printf("LISTA VACIA");
getchar();
return;
}
AUX=HEAD;
printf("LISTA:\n\n");
while(AUX->nxt!=HEAD){
printf("->%d\n",AUX->info);
AUX=AUX->nxt;
}
if(AUX->nxt==HEAD){
printf("->%d\n",AUX->info);
}
getch();
}
void viewl(void){
system("cls");
if(HEAD==NULL){
printf("EMPTY LIST");
getchar();
return;
} AUX=HEAD->prv;
printf("LIST:\n\n");
do{
printf("->%d\n",AUX->info);
AUX=AUX->prv;
}while(AUX->nxt!=HEAD);
getch();
}

Answer 2

in your code:
do
{
printf("-> %d\n",AUX->info);
AUX=AUX->prv;
}
while(AUX->nxt!=HEAD);
It can be happening something like this.
As you are going to the previous node and checking if next is head, next will be head if you built your list to be double-linked and circular.
try like this:
do
{
AUX=AUX->prv;
printf("-> %d\n",AUX->info);
}
while(AUX!=HEAD);
and you should avoid using all variables as global.