How to implement priority queue in C++? - c++

Question

I am implementing a train boarding system using priority queue. I have the working code but I need to make the following changes..
The priority levels will be : High, Medium and Low. So the passenger should input his/her name and the priority level. The train can have up to 30 passengers.
In the end I will sort the passengers accordingly... Here what I have so far my problem is taking strings as arguments instead of the integers I have now.
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <string>
using namespace std;
#define High 1
#define Medium 2
#define Low 3
/*
* Node Declaration
*/
struct node
{
int priority;
int info;
struct node *link;
};
/*
* Class Priority Queue
*/
class Priority_Queue
{
private:
node *front;
public:
Priority_Queue()
{
front = NULL;
}
/*
* Insert into Priority Queue
*/
void insert(int item, int priority)
{
node *tmp, *q;
tmp = new node;
tmp->info = item;
tmp->priority = priority;
if (front == NULL || priority < front->priority)
{
tmp->link = front;
front = tmp;
}
else
{
q = front;
while (q->link != NULL && q->link->priority <= priority)
q = q->link;
tmp->link = q->link;
q->link = tmp;
}
}
/*
* Delete from Priority Queue
*/
void del()
{
node *tmp;
if (front == NULL)
cout << "Queue Underflow\n";
else
{
tmp = front;
cout << "Deleted item is: " << tmp->info << endl;
front = front->link;
free(tmp);
}
}
/*
* Print Priority Queue
*/
void display()
{
node *ptr;
ptr = front;
if (front == NULL)
cout << "Queue is empty\n";
else
{
cout << "Queue is :\n";
cout << "Priority Item\n";
while (ptr != NULL)
{
cout << ptr->priority << " " << ptr->info << endl;
ptr = ptr->link;
}
}
}
};
/*
* Main
*/
int main()
{
int choice, item, priority;
Priority_Queue pq;
do
{
cout << "1.Insert\n";
cout << "2.Delete\n";
cout << "3.Display\n";
cout << "4.Quit\n";
cout << "Enter your choice : ";
cin >> choice;
switch (choice)
{
case 1:
cout << "Input the item value to be added in the queue : ";
cin >> item;
cout << "Enter its priority : ";
cin >> priority;
pq.insert(item, priority);
break;
case 2:
pq.del();
break;
case 3:
pq.display();
break;
case 4:
break;
default:
cout << "Wrong choice\n";
}
} while (choice != 4);
return 0;
}

Answer 1

I would avoid using string values for the queue and just translate plain-text priorities to the respective integer value, think
int ParsePriority(string plainTextPriority)
{
switch(plainTextPriority) {
case "High": return 1;
case "Medium": return 2;
case "Low": return 3;
}
throw "Unknown priority class";
}
You'll then be able to use the priority values like you did before.
case 1:
cout << "Input the item value to be added in the queue : ";
cin >> item;
cout << "Enter its priority : ";
do {
string plainTextPriority;
cin >> plainTextPriority;
try {
priority = ParsePriority(plainTextPriority)
}
catch {
priority = 0;
cout << "Could not parse the priority, please enter one of High, Medium, Low" << endl;
}
} while (priority == 0);
pq.insert(item, priority);
break;
I added the loop to take the case that a value that does not represent a valid priority has been entered into account.

Answer 2

Here you can use mapping.
int main()
{
int choice, item;
string priority;
map<string,int>m;
m["High"]=1;
m["Low"]=3;
m["Medium"]=2;
Priority_Queue pq;
do
{
cout << "1.Insert\n";
cout << "2.Delete\n";
cout << "3.Display\n";
cout << "4.Quit\n";
cout << "Enter your choice : ";
cin >> choice;
switch (choice)
{
case 1:
cout << "Input the item value to be added in the queue : ";
cin >> item;
cout << "Enter its priority : ";
cin >> priority;
pq.insert(item, m[priority]);//look change
//... rest of the code will same
and you need to include #include<map> also.
Thank you.