Priority Queue using Heap - c++

Question

I have written this code after studying from Introduction to Algorithm. I am unable to find out what is the problem with the code. I have written the code for heapsort and it runs well. heap_extract_max() will return the maximum value. heap_increase_key() increase the priority of an element. Here I have written program for priority queue using singly linked list which runs well.
#include <iostream>
#include <vector>
#include <algorithm>
template<typename T>
void max_heapify(std::vector<T>& array, size_t index)
{
size_t largest;
size_t left = (2*index) + 1;
size_t right = left + 1;
if(left < array.size() && array[left] > array[index])
largest = left;
else
largest = index;
if(right < array.size() && array[right] > array[largest])
largest = right;
if(largest != index)
{
int tmp = array[index];
array[index] = array[largest];
array[largest] = tmp;
max_heapify(array, largest);
}
}
template<typename T>
void build_max_heap(std::vector<T>& array)
{
for(size_t i = (array.size()/2) - 1; i >= 0; i--)
max_heapify(array, i);
}
template<typename T>
T heap_maximum(std::vector<T>& array)
{
return array[0];
}
template<typename T>
T heap_extract_max(std::vector<T>& array)
{
if(array.size() < 0)
{
std::cerr << "Heap Underflow \n";
}
else
{
T max = array[0];
array[0] = array[array.size() - 1];
//array.size() = array.size() - 1;
max_heapify(array, 1);
return max;
}
}
template<typename T>
void heap_increase_key(std::vector<T>& array, size_t index, T value)
{
if(value < array[index])
{
std::cerr <<"New value is smaller than the current value\n";
return;
}
else
{
array[index] = value;
while(index > 0 && array[(index/2) - 1] < array[index])
{
std::swap(array[index], array[(index/2) - 1]);
index = (index/2) - 1;
}
}
}
template<typename T>
void max_heap_insert(std::vector<T>& array, T value)
{
array[array.size()] = -1;
heap_increase_key(array, array.size(), value);
}
int main()
{
std::vector<int> v({1, 2, 6, 3, 7});
build_max_heap(v);
std::cout << heap_extract_max(v)<<"\n";
for(size_t i = 0; i < v.size(); i++)
{
std::cout << v[i] << " ";
}
std::cout << "\n";
}
It is not showing any output. I am writing commands
$ g++ -std=c++11 priorityqueue.cpp -o priorityqueue
$ ./priorityqueue

Answer 1

Problem is with the below function what will be return for if case
because of that
Control may reach end of non-void function
template<typename T>
T heap_extract_max(std::vector<T>& array)
{
if(array.size() < 0)
{
std::cerr << "Heap Underflow \n";
}
else
{
T max = array[0];
array[0] = array[array.size() - 1];
//array.size() = array.size() - 1;
max_heapify(array, 1);
return max;
}
}
Add return -1 or anything else for if case
if(array.size() < 0)
{
std::cerr << "Heap Underflow \n";
return -1;
}
second problem
for(size_t i = (array.size()/2) - 1; i >= 0; i--)
Comparison of unsigned expression >= 0 is always true
Try changing to:(as suggested by Serge Rogatch)
for(int64_t i = (int64_t(array.size())/2) - 1; i >= 0; i--)
As well as there is problem with extracting it
Before change result was
7
2 3 6 1 2
Program ended with exit code: 0
Because of wrong handling of remove .Handled removed properly in below code
template<typename T>
T heap_extract_max(std::vector<T>& array)
{
if(array.size() < 0)
{
std::cerr << "Heap Underflow \n";
return -1;
}
else
{
T max = array[0];
array[0] = array[array.size() - 1];
array.erase(std::remove(array.begin(), array.end(), array[0]),
array.end());
array.shrink_to_fit();
max_heapify(array, 0);
return max;
}
}
After change
7
6 3 1 2
Program ended with exit code: 0

Answer 2

It seems your program enters an infinite loop or crashes. I see a problem here:
for(size_t i = (array.size()/2) - 1; i >= 0; i--)
Because i is unsigned, it is always i >= 0. Also, for array.size() <= 1, you initialize it with some large positive integer, because i tries to go negative, but size_t is never negative, so the number wraps.
Try changing to:
for(int64_t i = (int64_t(array.size())/2) - 1; i >= 0; i--)
Also you seem to confuse 0- and 1-based array indexing, and you should do array.pop_back() in place of //array.size() = array.size() - 1;. Furthermore, it seems you intended array[1] instead of array[0] here:
T max = array[0];
array[0] = array[array.size() - 1];
If you stick with 1-based indexing, you should place and keep a dummy element at index 0 in your array:
std::vector<int> v({/*dummy*/-1, 1, 2, 6, 3, 7});
Array size is never negative, so you don't need if(array.size() < 0) and what's in then clause.
Though 0-based indexing is more natural in C++ and heap can be implemented with it too. For this you would need to revise all the index arithmetic like:
size_t left = (2*index) + 1;
size_t right = left + 1;
and
array[(index/2) - 1]