How to free memory from minHeap C++ - c++

Question

I have an implementation of Dijkstra's algorithm from https://www.geeksforgeeks.org/dijkstras-algorithm-for-adjacency-list-representation-greedy-algo-8/ but there is significant memory leaks when I try to use it. The problem is with the minHeap according to when I run it with valgrind. I have tried freeing each pointer like:
for (int i=0; i<size; i+=1){
free(minHeap->array[i]);
}
but valgrind gives invalid free() and the leak is not solved. Here is the Dijkstra's code.
// C++ program for Dijkstra's shortest path algorithm for adjacency
// list representation of graph
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <float.h>
// A structure to represent a node in adjacency list
struct AdjListNode
{
int dest;
double weight;
struct AdjListNode* next;
};
// A structure to represent an adjacency list
struct AdjList
{
struct AdjListNode *head; // pointer to head node of list
};
// A structure to represent a graph. A graph is an array of adjacency lists.
// Size of array will be V (number of vertices in graph)
struct Graph
{
int V;
struct AdjList* array;
};
// A utility function to create a new adjacency list node
struct AdjListNode* newAdjListNode(int dest, double weight)
{
struct AdjListNode* newNode = (struct AdjListNode*) malloc(sizeof(struct AdjListNode));
newNode->dest = dest;
newNode->weight = weight;
newNode->next = NULL;
return newNode;
}
// A utility function that creates a graph of V vertices
struct Graph* createGraph(int V)
{
struct Graph* graph = (struct Graph*) malloc(sizeof(struct Graph));
graph->V = V;
// Create an array of adjacency lists. Size of array will be V
graph->array = (struct AdjList*) malloc(V * sizeof(struct AdjList));
// Initialize each adjacency list as empty by making head as NULL
for (int i = 0; i < V; ++i)
graph->array[i].head = NULL;
return graph;
}
// Adds an edge to an undirected graph
void addEdge(struct Graph* graph, int src, int dest, double weight)
{
// Add an edge from src to dest. A new node is added to the adjacency
// list of src. The node is added at the beginning
struct AdjListNode* newNode = newAdjListNode(dest, weight);
newNode->next = graph->array[src].head;
graph->array[src].head = newNode;
// Since graph is undirected, add an edge from dest to src also
newNode = newAdjListNode(src, weight);
newNode->next = graph->array[dest].head;
graph->array[dest].head = newNode;
}
// Structure to represent a min heap node
struct MinHeapNode
{
int v;
double dist;
};
// Structure to represent a min heap
struct MinHeap
{
int size; // Number of heap nodes present currently
int capacity; // Capacity of min heap
int *pos; // This is needed for decreaseKey()
struct MinHeapNode **array;
};
// A utility function to create a new Min Heap Node
struct MinHeapNode* newMinHeapNode(int v, double dist)
{
struct MinHeapNode* minHeapNode = (struct MinHeapNode*) malloc(sizeof(struct MinHeapNode));
minHeapNode->v = v;
minHeapNode->dist = dist;
return minHeapNode;
}
// A utility function to create a Min Heap
struct MinHeap* createMinHeap(int capacity)
{
struct MinHeap* minHeap = (struct MinHeap*) malloc(sizeof(struct MinHeap));
minHeap->pos = (int *)malloc(capacity * sizeof(int));
minHeap->size = 0;
minHeap->capacity = capacity;
minHeap->array =
(struct MinHeapNode**) malloc(capacity * sizeof(struct MinHeapNode*));
return minHeap;
}
// A utility function to swap two nodes of min heap. Needed for min heapify
void swapMinHeapNode(struct MinHeapNode** a, struct MinHeapNode** b)
{
struct MinHeapNode* t = *a;
*a = *b;
*b = t;
}
// A standard function to heapify at given idx
// This function also updates position of nodes when they are swapped.
// Position is needed for decreaseKey()
void minHeapify(struct MinHeap* minHeap, int idx)
{
int smallest, left, right;
smallest = idx;
left = 2 * idx + 1;
right = 2 * idx + 2;
if (left < minHeap->size &&
minHeap->array[left]->dist < minHeap->array[smallest]->dist )
smallest = left;
if (right < minHeap->size &&
minHeap->array[right]->dist < minHeap->array[smallest]->dist )
smallest = right;
if (smallest != idx)
{
// The nodes to be swapped in min heap
MinHeapNode *smallestNode = minHeap->array[smallest];
MinHeapNode *idxNode = minHeap->array[idx];
// Swap positions
minHeap->pos[smallestNode->v] = idx;
minHeap->pos[idxNode->v] = smallest;
// Swap nodes
swapMinHeapNode(&minHeap->array[smallest], &minHeap->array[idx]);
minHeapify(minHeap, smallest);
}
}
// A utility function to check if the given minHeap is ampty or not
int isEmpty(struct MinHeap* minHeap)
{
return minHeap->size == 0;
}
// Standard function to extract minimum node from heap
struct MinHeapNode* extractMin(struct MinHeap* minHeap)
{
if (isEmpty(minHeap))
return NULL;
// Store the root node
struct MinHeapNode* root = minHeap->array[0];
// Replace root node with last node
struct MinHeapNode* lastNode = minHeap->array[minHeap->size - 1];
minHeap->array[0] = lastNode;
// Update position of last node
minHeap->pos[root->v] = minHeap->size-1;
minHeap->pos[lastNode->v] = 0;
// Reduce heap size and heapify root
--minHeap->size;
minHeapify(minHeap, 0);
return root;
}
// Function to decreasy dist value of a given vertex v. This function
// uses pos[] of min heap to get the current index of node in min heap
void decreaseKey(struct MinHeap* minHeap, int v, double dist)
{
// Get the index of v in heap array
int i = minHeap->pos[v];
// Get the node and update its dist value
minHeap->array[i]->dist = dist;
// Travel up while the complete tree is not hepified.
// This is a O(Logn) loop
while (i && minHeap->array[i]->dist < minHeap->array[(i - 1) / 2]->dist)
{
// Swap this node with its parent
minHeap->pos[minHeap->array[i]->v] = (i-1)/2;
minHeap->pos[minHeap->array[(i-1)/2]->v] = i;
swapMinHeapNode(&minHeap->array[i], &minHeap->array[(i - 1) / 2]);
// move to parent index
i = (i - 1) / 2;
}
}
// A utility function to check if a given vertex
// 'v' is in min heap or not
bool isInMinHeap(struct MinHeap *minHeap, int v)
{
if (minHeap->pos[v] < minHeap->size)
return true;
return false;
}
// The main function that calulates distances of shortest paths from src to all
// vertices. It is a O(ELogV) function
void dijkstra(struct Graph* const graph, vector<double> &dist, vector<double> const &max, vector<double> const &P_s, const int src){
int size = graph->V;
// minHeap represents set E
struct MinHeap* minHeap = createMinHeap(size);
// Initialize min heap with all vertices. dist value of all vertices
for (int v = 0; v < size; ++v)
{
// Testing different initialization
dist[v] = max[v];
// Make dist value of src vertex as 0 so that it is extracted first
if (v==src){
dist[v] = 0;
}
minHeap->array[v] = newMinHeapNode(v, dist[v]);
minHeap->pos[v] = v;
}
decreaseKey(minHeap, src, dist[src]);
// Initially size of min heap is equal to size
minHeap->size = size;
// In the followin loop, min heap contains all nodes
// whose shortest distance is not yet finalized.
while (!isEmpty(minHeap))
{
// Extract the vertex with minimum distance value
struct MinHeapNode* minHeapNode = extractMin(minHeap);
int u = minHeapNode->v; // Store the extracted vertex number
// Traverse through all adjacent vertices of u (the extracted
// vertex) and update their distance values
struct AdjListNode* pCrawl = graph->array[u].head;
while (pCrawl != NULL)
{
int v = pCrawl->dest;
// If shortest distance to v is not finalized yet, and distance to v
// through u is less than its previously calculated distance
if (isInMinHeap(minHeap, v) && dist[u] != DBL_MAX &&
(pCrawl->weight)/P_s[v] + dist[u] < dist[v])
{
dist[v] = dist[u] + pCrawl->weight/P_s[v];
// update distance value in min heap also
decreaseKey(minHeap, v, dist[v]);
}
pCrawl = pCrawl->next;
}
}
free(minHeap->pos);
free(minHeap->array);
free(minHeap);
}

Answer 1

I agree with the previous comments, this code is pure C, it should not be tagged as C++. Writing it in C++ would make it quite easier to read and to debug. Using std::vector would prevent your program from loosing any memory, because memory allocs are encapsulated in this dedicated class.
You may want to follow this tutorial about std::vector and object oriented programming : https://www.tutorialspoint.com/cplusplus/cpp_stl_tutorial.htm
Generally, new and delete (C++) are less error prone than malloc and free (C). They help to avoid some common mistakes (wrong size buffer allocation...)
Classes (C++) have initialisation (constructor) and destruction mechasism that helps to guarantee a valid state, they are a better alternative to C structures...