Passing lists to functions

First of all i would like to greet the forum,i just registered.Hope someday to be able to help here too,like you.

My assignment is about implementing a linked stack and a linked queue and my program should be able to enqueue-dequeue/push-pop,show and retrieve data from them (seperate operations for each).i have created all this using global variables to avoid double and triple pointers, but professor wants it with pointers.we are learning C this semester together with data structures,so i have zero experience with pointers.it took me days to write these 360 lines and i want it to be as it must be.i have noone to cooperate with,since i am the only one that had managed to do it...

my problem is passing lists to functions.this is the small part of my program ( enqueue a node to a linked queue) that i am working on to solve the problem,but i do not know what goes wrong with the pointers:

Code:

---

#include <stdio.h>
#include <malloc.h>

struct node
{
 int info;
 struct node *next;
};

void enqueue(struct node **fro,struct node **rea,struct node **tmp,int *qs);
struct node newnode(int value);


main()
{     
  int value,qsize=0;
  struct node *front=NULL,*rear=NULL,*temp;
  do{
  scanf("&#37;d",&value);
  *temp = newnode(value);
  enqueue(&front,&rear,&temp,&qsize);
  } while (value!=0);
}

struct node newnode(int item)
{
 struct node *tmp;
 tmp=(struct node *)malloc(sizeof(struct node));
 tmp->info=item;
 tmp->next=NULL;
 return *tmp;
}
 
 
void enqueue(struct node **fro,struct node **rea,struct node **tmpe,int *qs)
{
  if(rea == NULL)
  {
  rea = tmpe;
  fro= rea;
  *qs++;
  }
  else
  {
  (*rea)->next = *tmpe;
  rea = tmpe;
  *qs++;
  }
}

---

Thanks in advance.I would really appreciate it ,if you could suggest me a book or give me a link that could help me.

Since no one else is touching this, here's a rewrite. Study the changes carefully, especially those involving pointers.

Code:

---

#include <stdio.h>
#include <malloc.h>

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

Node *newnode( int value);
void enqueue( Node **fro, Node **rea, Node *tmpe, int *qs);

int main()
{
    int value, qsize = 0;
    Node *front = NULL, *rear = NULL, *temp;

    while (1)
    {
        scanf( "%d", &value);
        if (value == 0) break;
        temp = newnode( value);
        enqueue( &front, &rear, temp, &qsize);
    }

    for (temp = front; temp; temp = temp->next)
        printf( "%d ", temp->info);
    printf( "\n");

  system("pause");
}

Node *newnode( int item)
{
    Node *tmp;
    tmp = (Node*) malloc( sizeof( Node));
    tmp->info = item;
    tmp->next = NULL;
    return tmp;
}

void enqueue( Node **fro, Node **rea, Node *tmpe, int *qs)
{
    if (*rea == NULL)
    {
        *rea = tmpe;
        *fro = *rea;
        *qs++;
    }
    else
    {
        (*rea)->next = tmpe;
        *rea = tmpe;
        *qs++;
    }
}

---

looking at it for days,never thought of using a pointer with a function.also the use of pointers in enqueue function make sense now.thank you very much for your time,you indirectly helped the whole class.i will go apply this to all the code.

here's my 2c and btw what does system("pause") do as I couldn't get it to compile on my machine.

Code:

---

#include <stdio.h>
#include <malloc.h>

struct node
{
    int info;
    struct node *next;
};

struct node *enqueue(struct node *fro, struct node *tmpe, int *qs);
struct node *newnode(int value);

int main(void)
{     
    int value, qs=0;
    struct node *front=NULL,*rear=NULL,*temp=NULL;

    do {
        scanf("&#37;d",&value);
        temp = newnode(value);
        front = enqueue(front, temp, &qs);
    } while (value != 0);

    for (front=front->next; front; front=front->next)
        printf("front->info is %d\n", front->info);
}

struct node *newnode(int item)
{
    struct node *tmp;
    tmp=(struct node *)malloc(sizeof(struct node));
    tmp->info=item;
    tmp->next=NULL;
    return tmp;
}
 
struct node *enqueue(struct node *fro, struct node *tmpe, int *qs)
{
    struct node *x ;
    if (fro) {
        x = fro;
        fro = tmpe;
        tmpe = x;
        fro->next = tmpe;
    }
    else
        fro = tmpe;
    (*qs)++;
    return fro;
}

---

A few problems:
* You entered the terminating 0 into the queue.
* You've ignored the rear pointer.
* You've created a stack, not a queue (i.e., following the "front" pointer prints out the input backwards).

system("pause") is a magical command that only compiles on my machine, apparently. What could it possibly do?. I assume you're joking. However, I should have removed it before posting.

Quote:

---

Originally Posted by nucleon

A few problems:
* You entered the terminating 0 into the queue.

---

that's easily undone.

Quote:

---

Originally Posted by nucleon

* You've ignored the rear pointer.

---

didn't ignore it but there's no use for it so I took it out.

Quote:

---

Originally Posted by nucleon

* You've created a stack, not a queue (i.e., following the "front" pointer prints out the input backwards).

---

correct it's a stack as the OP wanted a queue and a stack so now he's got it all covered. :D

Quote:

---

Originally Posted by nucleon

system("pause") is a magical command that only compiles on my machine, apparently. What could it possibly do?. I assume you're joking. However, I should have removed it before posting.

---

jokes apart, your source didn't compile so I took a stab at the program. :cool:

Quote:

---

Originally Posted by nucleon

system("pause") is a magical command that only compiles on my machine, apparently.

---

Yes, that is quite possible, since you did not #include <stdlib.h>, and <malloc.h> should be removed in favour of <stdlib.h> anyway.

from the little i know,system("PAUSE") works only on windows systems (DOS command) and on C.

and itCbitC thanks for your time too.i see you are passing addresses than the whole pointers.i tried to do that too but still had no idea about putting a pointer with a function.thanks guys.

Thank you Griever for clarifying the system("pause") call. I didn't know that it was a DOS/Windows thing.
I was baffled by its inclusion though in a way it made me give your program a try.

I didn't realize this place was full of a bunch of unix snobs.
Goodbye.

Quote:

---

Originally Posted by nucleon

I didn't realize this place was full of a bunch of unix snobs.
Goodbye.

---

Eh? Chill out. I'd describe this place as a pretty good mix. We are merely pointing out that a particular piece of code is nonstandard and won't work everywhere. Given that this board is C/C++ centric, not centric to any particular platform, this is not an unreasonable thing to do.

Some people (not pointing fingers) might seem rude from time to time. Maybe they are. Who cares. If you can't stand it, go ahead and leave. Otherwise take a pill and relax a little okay?

Quote:

---

Originally Posted by nucleon

I didn't realize this place was full of a bunch of unix snobs.

---

Perhaps you failed to notice that I said nothing about running the program, and that itCbitC's problem was about compiling the program. The system() function itself is in the standard library, under the <stdlib.h> header, but when adding that function call, you forgot to #include <stdlib.h> so you were relying on that header being included by another, which is not guaranteed. It is as simple as that.

Quote:

---

Originally Posted by nucleon

I didn't realize this place was full of a bunch of unix snobs.
Goodbye.

---

I sincerely apologize if I inadvertently came across as rude or a Unix snob. I was merely stating that I couldn't compile your program.

Cool beans!!!
:cool:

i have a question. LIFO is a stack right so what is a que because i was told that u can work at any end but the end u chose u have to continue using that end throughout the code.

Quote:

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Originally Posted by frassunit

LIFO is a stack right so what is a que because i was told that u can work at any end but the end u chose u have to continue using that end throughout the code.

---

A queue is a FIFO data structure.

hi again guys.half of the code is working now,but still struggling with pointers.my next problem now is passing the *front and *rear
second time in a function.

i want to use the enqueue function in a other function (for example,showqueue, a function that dequeues-prints-enqueues).

Code:

---


void showqueue(int qsiz,struct node **fro,struct node **rea)
{
 int q,va;
 struct node *tmpee;
 for (q=0;q<qsiz;q++)
 {
  dequeue(&va,&fro,&rea,&qsiz);
  if (q==0)
  printf("[%d]<--front\n",va);
  else if (q==qsiz)
  printf("[%d]<--rear",va);
  else
  printf("[%d]\n",va);
  tmpee=newnode(va);
  enqueue(&fro,&rea,tmpee,&qsiz);
 }

}

---

Any ideas?should i give up with all this?totally wrong?

How does your dequeue() function look like and with each function call you are increasing the level of indirection.
You might be better off just using a single level of indirection ie struct node *fro instead of struct ***node fro

i did not posted it to keep the code size small, since this is clearly a pointer syntax question but ok here it is.it works without problems now :

Code:

---


int dequeue(int *it,struct node **frr,struct node **rre,int *qsz)
{
 struct node *tmpe; 
 int dfl=0;
 if (*frr!=*rre)
 {
  *it=(*frr)->info;
  tmpe=*frr;           
  *frr=(*frr)->next; 
  free(tmpe);
  *qsz--;
  dfl=1;
 }
 else if ((*frr!=NULL) && (*frr==*rre))
 {
  *it=(*frr)->info;
  tmpe=*frr;           
  *frr=NULL;
  *rre=NULL;
  free(tmpe);
  *qsz=0;
  dfl=1;
 }
 return dfl;
}

---

i am allowed only to use enqueue-dequeue to show the items of the queue.so is it possible to increase that level of indirection one more level further?

Why do you want add another level of indirection if all's well with the program??

nope there is a problem when i call enqueue and dequeue function in the showqueue.the **fro must pass to the two functions somehow and i cannot think of anything.

maybe you will understand my question from this:

1st level: main(): *front (pointer for structure)
2nd level showqueue: **fro (pointer of pointer of structure in order to be used in the function showqueue)
3rd level enqueue (called in showqueue): ??? (pointer of pointer of pointer of structure in order to be passed from showqueue to enqueue)

tried *&fro,&fro,&*fro in showqueue to pass the double pointer to enqueue but nothing worked.
did you understand where is the problem?

Quote:

---

Originally Posted by Griever

nope there is a problem when i call enqueue and dequeue function in the showqueue.the **fro must pass to the two functions somehow and i cannot think of anything.

maybe you will understand my question from this:

1st level: main(): *front (pointer for structure)

---

From main() call enqueue() using struct node *fro.

Quote:

---

Originally Posted by Griever

2nd level showqueue: **fro (pointer of pointer of structure in order to be used in the function showqueue)

---

You can again pass struct node *fro to showqueue; no need to pass struct node **fro

Quote:

---

Originally Posted by Griever

3rd level enqueue (called in showqueue): ??? (pointer of pointer of pointer of structure in order to be passed from showqueue to enqueue)

---

Again pass struct node *fro to enqueue instead of struct node ***fro.

Quote:

---

Originally Posted by Griever

tried *&fro,&fro,&*fro in showqueue to pass the double pointer to enqueue but nothing worked.

---

How about simply struct node *fro.

Quote:

---

Originally Posted by Griever

did you understand where is the problem?

---

Trying...others may have better insights / ideas.

ah yes you are absolutely right... i wanted to convert my code in using pointers functioning like when i was using global variables that i did not make a simple check of what is actually needed to be pointed...i corrected almost everything.also i found that *qsz++ or *qsz-- did not work and i corrected it putting the ++ change in a variable (qq++; *qsz =qq;) and updating the pointer at the end of the process.thanks again for your quick responses and your time.

Quote:

---

Originally Posted by Griever

ah yes you are absolutely right... i wanted to convert my code in using pointers functioning like when i was using global variables that i did not make a simple check of what is actually needed to be pointed...i corrected almost everything.also i found that *qsz++ or *qsz-- did not work and i corrected it putting the ++ change in a variable (qq++; *qsz =qq;) and updating the pointer at the end of the process.thanks again for your quick responses and your time.

---

IMHO that is the correct approach; keep it simple and call each function with a single level of indirection ie struct node *fro.
*qs++ or *qs-- won't work as it increments or decrements the pointer qs not the object it points to; use (*qs)++ or (*qs)-- instead.

i upload the final version of my code to anyone who is interested(i changed every message and comment to english).could not have done it without your help.

code contains DOS commands ( system("PAUSE"); ,system("CLS"); )so it will not compile on non-windows environments.delete system("CLS"); and replace system("PAUSE"); with getchar(); to fix this.

thanks a lot.