queues FIFO implementation

So I have been working on this program from my instructors notes, google and lots of brain power. Basically I just have to implement inserting, deleting and printing a queue using the array way. I am now at the point where I need you guys to tell me what isn't correct in my code. Right now I am only inserting and printing a queue and later (hopefully) I'll do the delete part. I'll appreciate your help.
Q.H (header file)

Code:

struct Queue_struct;
typedef struct Queue_struct Q_type;


/* Allocate a new Queue and initialize it to be empty. */
Q_type *Q_new();


/* Deallocate a Queue and free the memory */
void Q_free(Q_type *);


/* Print the Queue in an attractive fashion */
void Q_print (Q_type *);


/* error codes for errorflag returned by Q_add and Q_delete */
#define ERROR_FULL_QUEUE 0
#define ERROR_EMPTY_QUEUE -1
#define EVERYTHING_OK 1


/* add an item at the end of the Queue */
void Q_add(Q_type *, int, int *errorflag);


/* remove an item from the front of the Queue */
int Q_delete(Q_type *, int *errorflag);

main.c

Code:

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include "Q.h"    /* Queue header file */




void showcommands()
{
  printf("Queue Processing Commands:\n");
  printf("==========================\n");
  printf("n : \n");
  printf("p : \n");
  printf("a : \n");
  printf("d : \n");
  printf("v : \n");
  printf("e : \n");
  printf("f : \n");
}
main(void)
{
  Q_type *myQ;
  myQ = Qnew();  /* create an empty Q data object */


  char command;   // store current command
  int errorflag;      // possible error code
  int value;          // value to be inserted or returned
  float avg;          // average of queue values


  show_commands();
  printf("Queue command? ");
  scanf("%c", &command);


  while (command != 'q')
    {
      switch (command)
        {
   case 'n': free(myQ);
          myQ = Q_new();
          break;
        case 'p': Q_print(myQ);
          break;
        case 'a': printf("Enter value to insert: ");
          scanf("%d", &value);
          Q_add(myQ, value, &errorflag);
          if (errorflag == ERROR_FULL_QUEUE)
            printf("Queue full -- insert failed.\n");
          break;
      default: printf("Command not recognized\n");
          show_commands();
        }
      printf("Queue command? ");
      scanf("%c",&command);
    }


  Q_free (myQ);
  return 0;
}

Q.c (THE REAL STUFF)

Code:

#include <stdio.h>
#include <stdlib.h>
#include "Q.h"


/*********************************************************
Implementation of the Queue ADT using a fixed size array.
*********************************************************/


/* Queue size must be an array of size 10 */
#define Q_MAX_SIZE 10




typedef struct Q_struct {
  int contents[Q_MAX_SIZE];
  int front;
  int count;
} Q_struct;


Q_type *Qnew()


{
  Q_type *Q;


  Q = (Q_type *)malloc(sizeof(Q_type));


  Q->front = 0;
  Q->count = 0;


  return Q;
}


void Q_add(Q_type *Q, int value, int *errorflag)
{
  int newElementIndex;


  if (Q->count >= Q_MAX_SIZE)
    { *errorflag = ERROR_FULL_QUEUE;
  return;


  }
*errorflag = EVERYTHING_OK;
  newElementIndex = (Q->front + Q->count) % Q_MAX_SIZE;
  Q->contents[newElementIndex] = value;


  Q->count++;
}
void Q_print (Q_type *Q)
{
  int i;
  if (front < 0)
    return;
  if ( count >= front)
    {
      for(i = front; i <= count; i++)
        {
          printf("\n i = %d", i);
          printf(" %d ", contents[i]);
        }
    }
  else
    {
      for(i = front; i < Q_MAX_SIZE_; i++)
        {
          printf("\n i = %d", i);
          printf(" %d", contents[i]);
        }
      for(i = 0; i <= count; i++)
        {
          printf("\n i = %d", i);
          printf(" %d ", contents[i]);
        }
    }
}

Well you forgot to write why you are not happy with your code.

Tip : The second scanf in main should be with a space before %, thus

Code:

scanf(" %d",...);

Why?

Oh because my code isn't compiling and I don't see why not

well in the Q_print function my compiler says that count, front, contents aren't defined?
WHAT?
clearly they were defined when I was using them in the add function?

front count contents of what?????

In your add function you request a member of a struct, but in the print you don't

could you elaborate I don't think I understand. From my view I think I am requesting a struct for both. Could you show me where I am requesting a struct in my add

In add function, you nicely request for count member of Q (where Q is a pointer to your struct)

Code:

Q->count

Now in the print you say

Code:

count

I won't get the joy of you to 'kill' the bug !

In order to kill it, use this weapon

Thanks, now that they are not undefined anymore, a new error exists in Q_new, Q_add and Q_print. Its the same error it says:
"dereferencing pointer to incomplete type"
I'm pretty sure it has to do with all of the Q-> to something lines
I have no clue what the error means though

You are welcome Would you like to post your updated code?

Sure.
Q.c

Code:

#include <stdio.h>
#include <stdlib.h>
#include "Q.h"


/*********************************************************
Implementation of the Queue ADT using a fixed size array.
*********************************************************/


/* Queue size must be an array of size 10 */
#define Q_MAX_SIZE 10




typedef struct Q_struct {
  int contents[Q_MAX_SIZE];
  int front;
  int count;
} Q_struct;


Q_type *Qnew()


{
  Q_type *Q;


  Q = (Q_type *)malloc(sizeof(Q_type));


  Q->front = 0;
  Q->count = 0;


  return Q;
}


void Q_add(Q_type *Q, int value, int *errorflag)
{
  int newElementIndex;


  if (Q->count >= Q_MAX_SIZE)
    { *errorflag = ERROR_FULL_QUEUE;
  return;


  }
   */
  *errorflag = EVERYTHING_OK;
  newElementIndex = (Q->front + Q->count) % Q_MAX_SIZE;
  Q->contents[newElementIndex] = value;


  Q->count++;
}


void Q_print (Q_type *Q)
{
  int i;
  if (Q->front <= 0)
    return;
  if ( Q->count >= Q->front)
    {
      for(i = Q->front; i <= Q->count; i++)
        {
          printf("\n i = %d", i);
 printf(" %d ", Q->contents[i]);
        }
    }
  else
    {
      for(i = Q->front; i < Q_MAX_SIZE_; i++)
        {
          printf("\n i = %d", i);
          printf(" %d", Q->contents[i]);
        }
      for(i = 0; i <= Q->count; i++)
        {
          printf("\n i = %d", i);
          printf(" %d ", Q->contents[i]);
        }
    }
}

In the code above you never define what Q_type is unless you meant to use Q_struct which you do typedef.

Does "Q.h" contain the complete definition of Q_type? Because the compiler is telling you that you are trying to dereference a pointer to a type that it hasn't
yet seen the full definition of (Q_type in this case).

Originally Posted by nonpuz

In the code above you never define what Q_type is unless you meant to use Q_struct which you do typedef.

Does "Q.h" contain the complete definition of Q_type? Because the compiler is telling you that you are trying to dereference a pointer to a type that it hasn't
yet seen the full definition of (Q_type in this case).

Spend some seconds to scroll up to the first post...

I have a question and this may sound stupid but when I wrote this:
typedef struct Queue_struct Q_type;

what does the Q_type mean?
Because I thought that the typedef struct was going to Q_type also.

You created a typename. However you never defined what a struct Queue_Struct contains, so Q_type is an "incomplete type".

Originally Posted by kiwi101

I have a question and this may sound stupid but when I wrote this:
typedef struct Queue_struct Q_type;

what does the Q_type mean?
Because I thought that the typedef struct was going to Q_type also.

It means that Q_type is exactly the same with struct Queue_struct They are like sinonims.