[英]How to return structure array in C++
所以我一直在嘗試實現Kruskal的算法,首先我想明確這個問題與算法的實現無關。 我創建了一個graph.hpp
文件,一個kruskalsAlgo.hpp
和main.cpp
,分別如下:
#pragma once
struct Edge
{
int source;
int destination;
int weight;
};
struct Graph
{
int V;
int E;
Edge* edge;
};
Graph* create_graph(int V, int E)
{
Graph* graph = new Graph;
graph -> V = V;
graph -> E = E;
graph -> edge = new Edge[E];
return graph;
}
#include <stdlib.h>
#include <tuple>
#include "../Graph/Graph.hpp"
class Kruskals_Algo
{
private:
struct subset
{
int parent;
int rank;
};
void make_set(subset*, int);
int find_set(subset*, int);
void _union(subset*, int, int);
public:
Edge* kruskal(Graph*);
void print_kruskals_MST(Edge*, int);
};
void Kruskals_Algo::make_set(subset* subsets, int V)
{
subsets[V].parent = V;
subsets[V].rank = 0;
}
int Kruskals_Algo::find_set(subset* subsets, int V)
{
if(subsets[V].parent != V)
subsets[V].parent = find_set(subsets, subsets[V].parent);
return subsets[V].parent;
}
void Kruskals_Algo::_union(subset* subsets, int x, int y)
{
int xroot = find_set(subsets, x);
int yroot = find_set(subsets, y);
if(subsets[xroot].rank < subsets[yroot].rank)
subsets[xroot].parent = yroot;
else if(subsets[xroot].rank > subsets[yroot].rank)
subsets[yroot].parent = xroot;
else
{
subsets[yroot].parent = xroot;
subsets[xroot].rank++;
}
}
inline int myComp(const void* a, const void* b)
{
Edge* a1 = (Edge*)a;
Edge* b1 = (Edge*)b;
return a1 -> weight > b1 -> weight;
}
Edge* Kruskals_Algo::kruskal(Graph* graph)
{
int V = graph -> V;
Edge result[V];
Edge* result_ptr = result;
int e = 0;
int i = 0;
qsort(graph -> edge, graph -> E, sizeof(graph -> edge[0]), myComp);
subset* subsets = new subset[(V * sizeof(subset))];
for (int v = 0; v < V; ++v)
make_set(subsets, v);
while(e < V - 1 && i < graph -> E)
{
Edge next_edge = graph -> edge[i++];
int x = find_set(subsets, next_edge.source);
int y = find_set(subsets, next_edge.destination);
if (x != y)
{
result[e++] = next_edge;
_union(subsets, x, y);
}
}
//return std::make_tuple(res, e);
return result_ptr;
}
void Kruskals_Algo::print_kruskals_MST(Edge* r, int e)
{
int minimumCost = 0;
for(int i=0; i<e; ++i)
{
std::cout << r[i].source << " -- "
<< r[i].destination << " == "
<< r[i].weight << std::endl;
minimumCost = minimumCost + r[i].weight;
}
std::cout << "Minimum Cost Spanning Tree: " << minimumCost << std::endl;
}
#include <iostream>
#include "Graph/Graph.hpp"
#include "Kruskals_Algo/kruskalsAlgo.hpp"
//#include "Prims_Algo/primsAlgo.hpp"
using namespace std;
class GreedyAlgos
{
public:
void kruskals_mst();
//void prims_mst();
};
void GreedyAlgos::kruskals_mst()
{
Kruskals_Algo kr;
int V;
int E;
int source, destination, weight;
cout << "\nEnter the number of vertices: ";
cin >> V;
cout << "\nEnter the number of edges: ";
cin >> E;
Edge* res;
Graph* graph = create_graph(V, E);
for(int i=0; i<E; i++)
{
cout << "\nEnter source, destinstion and weight: ";
cin >> source >> destination >> weight;
graph -> edge[i].source = source;
graph -> edge[i].destination = destination;
graph -> edge[i].weight = weight;
}
//std::tie(result, E) = kr.kruskal(graph);
res = kr.kruskal(graph);
kr.print_kruskals_MST(res, E);
}
int main()
{
int choice;
GreedyAlgos greedy;
greedy.kruskals_mst();
return 0;
}
所以我的問題是,當我調試程序時,結構數組Edge result[V]
中的值在 position [0] [1] [2]
處被正確計算,如下圖所示:
但是當從主調用 function print_kruskals_MST(res, E)
時,打印的值是不同的:
我做錯了什么指針的事情嗎? 提前致謝。 PS忽略評論!
這個答案可能不會直接回答你的問題,但它應該對這個問題有所了解。
首先,是的,你有很多指針問題......
其次,將new
運算符的任何使用與delete
運算符配對。 就目前而言,您有一堆 memory 泄漏。
另外,為什么create_graph
? 而是為Graph
創建一個構造函數(以及一個析構函數,因為 class 有一個需要處理的Edge* edge
)。
struct Graph
{
int V;
int E;
Edge* edge;
// constructor
Graph(int V, int E)
{
this->V = V;
this->E = E;
this->edge = new Edge[E];
}
// destructor
~Graph()
{
// nullify the member variable before deleting its memory is just a safety measure pertaining to multithreading.
Edge* _edge = this->edge;
this->edge = nullptr;
delete _edge;
}
};
然后改變Graph* graph = create_graph(V, E);
進入Graph* graph = new Graph(V, E);
並在使用完畢后delete graph
。
確保刪除所有 memory 泄漏,我們可以 go 繼續討論引用正確的數據(例如,通過我更改我的答案)。
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