Priority Queue using array of linked list C++ [closed] - c++

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So I'm trying to create a priority queue using an array of linked lists in C++. I'm not finished but if I can fix the constructor I think I can do the rest on my own.
I have a data file, first line has the number of items in the file.
The next line thereafter will have a char and then the priority level starting from 0 to 9.
So I'm sorting the alphabet which has 26 letters (the items). Each letter is given a level of priority.
Ex. Q 5 (the letter Q has the priority 5)
When I run this, it says that the program stopped working and then it starts to look for a solution. Like an error for an infinity loop I think.
#include <iostream>
#include <fstream>
using namespace std;
class Queue
{
private:
struct linkedList
{
char data;
linkedList *next;
};
linkedList* PQ[10];
public:
//bool empty;
//bool empty(int priority);
void add(char info, int lvl);
//void remove();
Queue();
};
int main()
{
int size;
char Info;
int Lvl;
Queue Q;
ifstream dataIn;
dataIn.open("charQueueInput.txt");
if (dataIn.fail())
{
cout << "File does not exist." << endl;
exit(1);
}
dataIn >> size;
dataIn.get();
cout << size;
/*for (int i = 0; i < size; i++)
{
dataIn >> Info;
dataIn >> Lvl;
dataIn.get();
Q.add(Info, Lvl);
}*/
system("pause");
return 0;
}
Queue::Queue()
{
for (int i = 0; i < 10; i++)
{
PQ[i] = NULL;
}
for (int i = 0; i < 9; i++)
{
PQ[i]->next = PQ[i + 1];
}
PQ[9]->next = NULL;
}
void Queue::add(char info, int lvl)
{
if (lvl == 0)
{
PQ[0]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[1];
PQ[0]->next = temp;
}
else if (lvl == 1)
{
PQ[1]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[2];
PQ[1]->next = temp;
}
else if (lvl == 2)
{
PQ[2]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[3];
PQ[2]->next = temp;
}
else if (lvl == 3)
{
PQ[3]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[4];
PQ[3]->next = temp;
}
else if (lvl == 4)
{
PQ[4]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[5];
PQ[4]->next = temp;
}
else if (lvl == 5)
{
PQ[5]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[6];
PQ[5]->next = temp;
}
else if (lvl == 6)
{
PQ[6]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[7];
PQ[6]->next = temp;
}
else if (lvl == 7)
{
PQ[7]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[8];
PQ[7]->next = temp;
}
else if (lvl == 8)
{
PQ[8]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[9];
PQ[8]->next = temp;
}
else if (lvl == 9)
{
PQ[9]->data = info;
linkedList *temp = new linkedList;
temp->next = NULL;
PQ[1]->next = temp;
}
}
Here would be an example of the data file:
7
Q 5
W 3
T 0
Y 4
A 9
B 5
U 0
And you would read it as:
0: T -> U
1.
2.
3. W
4. Y
5. Q -> B
6.
7.
8.
9. A
T, U, W, Y, Q, B, A

Answer 1

The problem is that you access PQ before allocating memory.
class Queue
{
private:
struct linkedList
{
char data;
linkedList *next;
};
linkedList* PQ[10]; // Allocates pointer only
The class only allocates pointers to linkenList but not any instances.
Later you have:
// In constructor
Queue::Queue()
{
for (int i = 0; i < 10; i++)
{
PQ[i] = NULL;
}
for (int i = 0; i < 9; i++)
{
PQ[i]->next = PQ[i + 1]; // PQ[i] is NULL so run time error
}
and also later
void Queue::add(char info, int lvl)
{
if (lvl == 0)
{
PQ[0]->data = info; // Access to non-allocated element!
linkedList *temp = new linkedList;
temp->next = PQ[1];
where you access PQ[0]->data. But the element has not been allocated so you get a run time problem.
Instead of
if (lvl == 0)
{
PQ[0]->data = info;
linkedList *temp = new linkedList;
temp->next = PQ[1];
PQ[0]->next = temp;
}
you need something like:
if (lvl == 0)
{
if (PQ[0] == nullptr)
{
PQ[0] = new linkedList; // If head of queue is null, allocate
// new element for head
PQ[0]->next = nullptr;
}
linkedList *temp2 = PQ[0];
while(temp2->next != nullptr) // Search the linked list
{ // to find last element
temp2 = temp2->next;
}
// Now temp2 points to the last element in the list
temp2->next = new linkedList; // Allocate new element and add it
// to the list after temp2
// to get a linked list
temp2 = temp2->next; // Make temp2 point to the new element
temp2->next = nullptr; // Remember to initialize next of new element
temp2->data = info; // Save info
}
And in the constructor:
Queue::Queue()
{
for (int i = 0; i < 10; i++)
{
PQ[i] = nullptr; // Use nullptr instead of NULL
}
// Remove this block
// for (int i = 0; i < 9; i++)
// {
// PQ[i]->next = PQ[i + 1];
// }
// PQ[9]->next = NULL;
}
It is not the pointers in the array which shall be linked.
Each pointer is a pointer to the HEAD of a linked list, i.e. 10 independent linked lists. So don't link them.
And finally - use the array in the add() function! Don't do the big if-statement.
void Queue::add(char info, int lvl) // Tip: Consider making lvl an unsigned int
{
if ((lvl >= 10) || (lvl < 0)) return; // Ignore if lvl is out of range
if (PQ[lvl] == nullptr) // <---- USE lvl to index array
{
PQ[lvl] = new linkedList; // If head of queue is null, allocate
// new element for head
PQ[lvl]->next = nullptr;
}
linkedList *temp2 = PQ[lvl];
while(temp2->next != nullptr) // Search the linked list
{ // to find last element
temp2 = temp2->next;
}
// Now temp2 points to the last element in the list
temp2->next = new linkedList; // Allocate new element and add it
// to the list after temp2
// to get a linked list
temp2 = temp2->next; // Make temp2 point to the new element
temp2->next = nullptr; // Remember to initialize next of new element
temp2->data = info; // Save info
}
BTW - remember to create a destructor which deletes all elements in the 10 linked lists.