Problem with structure and dynamic array

[curiousEngine]

how to make provision of a dynamic array say of size 5 with each slot
holding a structure of type passenger?

/*
* Labsheet Queues
Question 1
Implement a program that shows a queue of people lining at a bus-stop.
The first one in the queue is the first one to get on the bus.
Your program should display the following data :
Id
First Name
Last Name
DestinationAddress
The id is to be incremented automatically. By default, it can be set
to 0.
*
* IMPLEMENTATION USING AN ARRAY
*/
#include <iostream>
using namespace std;

struct passenger{
int id;
char first_name[15];
char last_name[30];
char dest_add[20];
};

typedef passenger QueueElementType;

class Queue{
private:
QueueElementType * elements;
int MaxQueue;
int Front;
int Rear;
int fullqueue();
int emptyqueue();
public:
Queue(int MS);
void Enq(QueueElementType apassenger);
QueueElementType Deq();
void printQueue();
~Queue();
};

Queue::Queue(int MS){
elements = new QueueElementType;
MaxQueue = MS;
Front = MaxQueue -1;
Rear = MaxQueue -1;
}

Queue::~Queue(){
delete [] elements;
MaxQueue = 0;
Front = MaxQueue -1;
Rear = MaxQueue -1;
}

int Queue::fullqueue(){
return(Front = (Rear + 1) % MaxQueue);
}

int Queue::emptyqueue(){
return (Front == Rear);}

void Queue::Enq(QueueElementType apassenger){
if (fullqueue())
cout<<"Queue is Full";
else{
Rear = (Rear +1) % MaxQueue;
cout<<"Enter passenger first name:";
cin>apassenger.first_name;
cout<<endl<<"Enter passenger last name:";
cin>>apassenger.last_name;
cout<<endl<<"Enter destination address:";
cin>>apassenger.dest_add;
apassenger.id = 1;
elements[Rear] = apassenger;
}
}

QueueElementType Queue::Deq(){
QueueElementType mypass;
if (emptyqueue())
cout<<"Queue is empty";
else{
Front = (Front + 1) % MaxQueue;
mypass = elements[Front];
}
return(mypass);
}

void Queue::printQueue(){
for(int i = Front ; i<= Rear; i++){
cout<<elements[i].id;
cout<<elements[i].first_name;
cout<<elements[i].last_name;
cout<<elements[i].dest_add;
}
}
int main(int argc, char** argv)
{
Queue myqueue(5);
QueueElementType mypass;
myqueue.Enq(mypass);
myqueue.printQueue();

myqueue.Enq(mypass);
myqueue.printQueue();
return 0;
}

[Lionel B]

On Wed, 07 May 2008 02:57:31 -0700, curiousEngine wrote:

how to make provision of a dynamic array say of size 5 with each slot
holding a structure of type passenger?

/*
* Labsheet Queues
Question 1
Implement a program that shows a queue of people lining at a bus-stop.
The first one in the queue is the first one to get on the bus. Your
program should display the following data : Id
First Name
Last Name
DestinationAddress
The id is to be incremented automatically. By default, it can be set to
0.

Does your homework assignment specifically state that you aren't allowed
to use std::queue? Otherwise, you're re-inventing a (*very* wonky) wheel.

[...]

Queue::Queue(int MS){
elements = new QueueElementType;

Here you allocate *one* element to 'elements'. I think you probably meant:

elements = new QueueElementType[MS];

Queue::~Queue(){
delete [] elements;

Here you delete an *array* of elements. This is not going to work.

void Queue::Enq(QueueElementType apassenger){
if (fullqueue())
cout<<"Queue is Full";
else{
Rear = (Rear +1) % MaxQueue;
cout<<"Enter passenger first name:";
cin>apassenger.first_name;
cout<<endl<<"Enter passenger last name:";

cin>>apassenger.last_name;

cout<<endl<<"Enter destination address:";

cin>>apassenger.dest_add;

apassenger.id = 1;
elements[Rear] = apassenger; <------

Since you don't allocate an array of elements this is going to access an
unallocated element. Ditto many other places in your code.

This is a really bad attempt at implementing a queue. Either use
std::queue (*way* easier) or, if you have to implement it yourself, read
a good book on data structures (and C++) first - it's tricky.

--
Lionel B