I'm writing code to implement a priority queue using a heap. When I enter items into the queue with these priorities in this specific order 8 10 4 3 7 6 9 5 I get an error once I start popping them out with the get_front() function.
The problem is that an assertion fails for the swap_with_parent() function in the while-loop of the get_front() function . The argument is somehow growing larger than the number of items in the array, many_items. I'll post the entire code, if anybody can spot a problem I'd appreciate it if you could let me know. I apologize in advance for the lack of comments, I hope it's clear enough what I had going on.
// INVARIANT for the PriorityQueue Class:
// 1. The member variable many_items is the number of items in the
// PriorityQueue.
// 2. The items themselves are stored in the member variable heap,
// which is a partially filled array organized to follow the usual
// heap storage rules from Chapter 11 of the class notes.
// NOTE: Private helper functions are implemented at the bottom of this
// file along with their precondition/postcondition contracts.
#include <assert.h> // Provides assert function
#include <iomanip> // Provides setw
#include <iostream> // Provides cin, cout
#include <math.h> // Provides log2
#include "pqueue2.h"
using namespace std;
PriorityQueue::PriorityQueue( )
{
heap[CAPACITY];
many_items=0;
}
void PriorityQueue::insert(const Item& entry, unsigned int priority)
{
if(many_items==0)
{
heap[many_items].data= entry;
heap[many_items].priority= priority;
many_items++;
}
else
{
heap[many_items].data= entry;
heap[many_items].priority= priority;
unsigned int i= many_items;
many_items++;
while(parent_priority(i)<priority)
{
swap_with_parent(i);
i=parent_index(i);
}
}
}
PriorityQueue::Item PriorityQueue::get_front( )
{
assert(many_items>0);
if(many_items==1)
{
Item front_value=heap[0].data;
many_items--;
return front_value;
}
else
{
Item front_value=heap[0].data;
heap[0]=heap[many_items-1];
unsigned int priority= heap[many_items-1].priority;
unsigned int k=0;
while( (k<many_items) && !is_leaf(k) && big_child_priority(k)>priority)
{
unsigned int j=big_child_index(k);
swap_with_parent(big_child_index(k));
k= j;
}
many_items--;
return front_value;
}
}
bool PriorityQueue::is_leaf(size_t i) const
// Precondition: (i < many_items)
// Postcondition: If heap[i] has no children in the heap, then the function
// returns true. Otherwise the function returns false.
{
if(((2*i)+1)>many_items)
return 1;
else
return 0;
}
size_t PriorityQueue::parent_index(size_t i) const
// Precondition: (i > 0) && (i < many_items)
// Postcondition: The return value is the index of the parent of heap[i].
{
//assert( /*(i>0) && */(i<many_items));
return (i-1)/2;
}
unsigned int PriorityQueue::parent_priority(size_t i) const
// Precondition: (i > 0) && (i < many_items)
// Postcondition: The return value is the priority of the parent of heap[i].
{
return heap[ (i-1)/2].priority;
}
size_t PriorityQueue::big_child_index(size_t i) const
// Precondition: !is_leaf(i)
// Postcondition: The return value is the index of one of heap[i]'s children.
// This is the child with the larger priority.
{
assert(!is_leaf(i));
if((2*i)+2<many_items)
{
if(heap[(2*i)+1].priority>heap[(2*i)+2].priority)
{
return (2*i)+1;
}
else
{
return (2*i)+2;
}
}
else
{
return(2*i)+1;
}
}
unsigned int PriorityQueue::big_child_priority(size_t i) const
// Precondition: !is_leaf(i)
// Postcondition: The return value heap[big_child_index(i)].priority
{
return heap[big_child_index(i)].priority;
}
void PriorityQueue::swap_with_parent(size_t i)
// Precondition: (i > 0) && (i < many_items)
// Postcondition: heap[i] has been swapped with heap[parent_index(i)]
{
assert( i>0 && i<many_items);
OneItemInfo temp_parent=heap[parent_index(i)];
OneItemInfo temp_child=heap[i];
heap[i]=temp_parent;
heap[parent_index(i)]=temp_child;
}