Min-Heap help

This is a discussion on Min-Heap help within the C++ Programming forums, part of the General Programming Boards category; Hey, I have to code a min-heap, along with other things to help compute minimum spanning tree. I coded up ...

  1. #1
    Registered User
    Join Date
    Apr 2010
    Posts
    6

    Min-Heap help

    Hey, I have to code a min-heap, along with other things to help compute minimum spanning tree.

    I coded up the min-heap pretty easily and I have no compile errors, however my delete fix up does not seem to be working properly.
    For example, I insert these ints into an empty min heap:
    10, 9, 7, 2, 5, 3, 8, 4.

    I get the min-heap:
    2, 4, 3, 5, 7, 9, 8, 10

    After the first extract minimum call I get the min-heap:
    3, 4, 8, 5, 7, 9, 10

    Which is correct, however, if I call min-heap again I get this heap:
    8, 4, 10, 5, 7, 9

    Which is incorrect, I've check my code and I still can't find the problem.

    My min-heap consist of "edges" and not ints. An edge is a class that consist of 3 ints that represent two vertices and a cost. My min-heap is sorted by cost.

    Here is the header file:

    Code:
    /* 
     * File:   heap.h
     * Author: Owner
     *
     * Created on April 24, 2010, 10:51 PM
     */
    
    #ifndef _HEAP_H
    #define	_HEAP_H
    #include <vector>
    #include "edge.h"
    
    using namespace std;
    
    class min_heap
    {
        public:
            min_heap(int size);
            ~min_heap();
            int get_min();
            void extract_min();
            bool isEmpty();
            void insert(edge newEdge);
            void bottom_up(int i);
            void top_down(int i);
            void display();
            void heapify(int i);
            vector<edge> theEdges;
    
    
       // private:
            int heapSize;
            int arraySize;
            int* data;
            int parent(int i);
            int lchild(int i);
            int rchild(int i);
            
    };
    
    #endif	/* _HEAP_H */
    Here is my heap.cpp file:

    Code:
    #include <vector>
    #include "heap.h"
    #include "edge.h"
    #include <string>
    #include <iostream>
    using namespace std;
    
    min_heap::min_heap(int size)
    {
        arraySize = 0;
        heapSize = 0;
        
        theEdges.resize(size);
    }
    
    min_heap::~min_heap()
    {
        delete[] data;
    }
    
    int min_heap::parent(int i)
    {
        //return i/2;
        return (i-1)/2;
    }
    
    int min_heap::lchild(int i)
    {
        //return 2*i;
        return 2*(i+1);
    }
    
    int min_heap::rchild(int i)
    {
        //return 2*i+1;
        return 2*i+2;
    }
    
    bool min_heap::isEmpty()
    {
        return (heapSize == 0);
    }
    
    int min_heap::get_min()
    {
        if(isEmpty())
        ;
    //        throw string("empty the heap is");
        else
            return theEdges[0].cost;
           
    }
    
    void min_heap::bottom_up(int i)
    {
        int p;
        int temp;
    
        if(i != 0)
        {
            p = parent(i);
    
            if(theEdges[p].cost > theEdges[i].cost)
            {
                temp = theEdges[p].cost;
                theEdges[p].cost = theEdges[i].cost;
                theEdges[i].cost = temp;
                bottom_up(p);
            }
        }
    }
    
    void min_heap::top_down(int i)
    {
        int min_element;
        int temp;
        int left = lchild(i);
        int right = rchild(i);
    
        if(right >= heapSize)
        {
            if(left >= heapSize)
                return;
            else
                min_element = left;
        }
    
        else
        {
            if(theEdges[left].cost <= theEdges[right].cost)
                min_element = left;
            else
                min_element = right;
        }
    
        if(theEdges[i].cost> theEdges[min_element].cost)
        {
            temp = data[min_element];
            theEdges[min_element].cost = theEdges[i].cost;
            theEdges[i].cost = temp;
            top_down(min_element);
        }
    }
    
    void min_heap:: insert(edge newEdge)
    {
    
        theEdges[heapSize] = newEdge;
        
       
       //top_down(heapSize-1);
        bottom_up(heapSize);
        
        heapSize++;
    
    }
    
    void min_heap::extract_min()
    {
        theEdges[0] = theEdges[heapSize-1];
        heapSize--;
    
        if(heapSize> 0)
        {
    
            heapify(0);
    
            //top_down(0);
        }
    }
    
    void min_heap::display()
    {
        for(int i = 0; i < heapSize; i++)
            cout<< "i is: " << i << ", data is: " << theEdges[i].cost << ", " << endl;;
    
            
    }
    
    void min_heap:: heapify(int i)
    {
        int left = lchild(i);
        int right = rchild(i);
        int smallest;
        int temp;
    
        if(left <= heapSize && theEdges[left].cost < theEdges[i].cost)
            smallest = left;
        else
            smallest = i;
    
        if(right <= heapSize && theEdges[right].cost < theEdges[smallest].cost)
            smallest = right;
    
        if(smallest != i)
        {
            temp = theEdges[smallest].cost;
            theEdges[smallest].cost = theEdges[i].cost;
            theEdges[i].cost = temp;
            heapify(smallest);
        }
    
    
        
    }
    The deletion fix up was the "top_down" function, but that didn't seem to work. So I created the "heapify" function, and that gives me the problem that I stated above. Any help at all would be appreciated.

    Here is the edge.h file if needed:

    Code:
    /* 
     * File:   edge.h
     * Author: Owner
     *
     * Created on April 28, 2010, 3:30 PM
     */
    
    #ifndef _EDGE_H
    #define	_EDGE_H
    
    class edge
    {
        public:
    
        int u;
        int v;
        int cost;
    
        edge()
        {
            u = -1;
            v = -1;
            cost = -1;
        }
    
        edge (int vertex1,int vertext2,int theCost)
        {
            u = vertex1;
            v = vertext2;
            cost = theCost;
        }
    };
    
    #endif	/* _EDGE_H */

  2. #2
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    Location
    New Zealand
    Posts
    6,288
    There's too much code there. Please remove bits that aren't relevant to the problem, compile the code to make sure it still produces the problem, and then repost it.

    First thing that jumps out at me is that there appears to be an uninitialised member called "data", which will surely cause a crash anywhere it is referenced. E.g. in the destructor.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

    Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"

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