I'm currently solving old exam questions for school and ran into trouble with handling references and pointers. The task is to determine if any number 'n' is divisible by 7 using a graph, nodes and different arcs.
Graph to determining divisibility of number 'n' by 7
In the task I was given a node structure:
struct Node {
int value;
Node * grey_line;
Node * red_line;
};
I was also given a template code for a class 'Mod7' which I have written code in:
class Mod7 {
public:
Mod7() {
// Create the graph here
for (int i {}; i<7; i++) {
nodes.push_back(Node{i});
}
}
int remainder(string number) {
// change this member function to make the test pass
//The n_vector contains 2, 3 and 4.//
vector<int> n_vector = string_to_vector_int(number);
Node * current_node {&nodes.at(0)};
int counter{};
for (int steps : n_vector) {
do { //step along the grey line//
cout << "no Segmentation fault up to this point" << endl;
current_node = current_node -> grey_line;
cout << "Segmentation fault occurs here on the 2nd iteration of the while loop" << endl;
steps--;
} while (steps < 1);
counter++;
//Don't follow the red line if this was the last digit in v//
if (counter == n_vector.size()) {
return current_node -> value;
}
//step along the red line//
current_node = current_node -> red_line;
}
return current_node -> value;
}
// Insert your help functions here
void set_arcs() {
//set grey lines//
for (int i{}; i < nodes.size(); i++) {
nodes.at(i).grey_line = &nodes.at( (i + 1) % 7);
}
//set red lines//
nodes.at(0).red_line = &nodes.at(0);
nodes.at(1).red_line = &nodes.at(3);
nodes.at(2).red_line = &nodes.at(6);
nodes.at(3).red_line = &nodes.at(2);
nodes.at(4).red_line = &nodes.at(5);
nodes.at(5).red_line = &nodes.at(1);
nodes.at(6).red_line = &nodes.at(4);
}
vector<Node> get_node_vector() {
return nodes;
}
private:
// Insert extra data members if needed
vector<Node> nodes{};
};
The Main function looks like this:
int main() {
string test1{"243"};
Mod7 modder{};
//A print-out I made just to make sure I created the nodes okay//
for (Node n : modder.get_node_vector()) {
cout << n.value << " <- Node" << endl;
}
// The examiner's required testing//
if (modder.remainder("243") == 5) {
cout << "Correct" << endl;
}
if (modder.remainder("11111111111111111111111111111") == 2) {
cout << "Correct" << endl;
}
}
The issues i have is I keep getting Segmentation fault which I think is due to the fact that I'm assigning new values to the current nodes' arcs. I don't know really how to continue from here, and I would greatly appreciate any help.
I don't know if the output is particularly giving, but I'll include it too:
> $ ./a.out
> 0 <- Node
> 1 <- Node
> 2 <- Node
> 3 <- Node
> 4 <- Node
> 5 <- Node
> 6 <- Node
> no Segmentation fault up to this point
> Segmentation fault occurs here on the 2nd iteration of the while loop
> Segmentation fault (core dumped)
UPDATE: I just now noticed that the member function 'set_arcs()' was never called. I added a call to it after the for-loop in the class' constructor.
Mod7() {
// Create the graph here
for (int i {}; i<7; i++) {
nodes.push_back(Node{i});
}
set_arcs();
}
Now the program get stuck in the while-loop. So I guess I'm still having issues with overwriting pointers.
Related
i am stuck in my uni assignment....
i have an linked list of 20 elements, i have to take the value from user and if user enter 5 then print the last 5 elements of linked list
void traverse(List list) {
Node *savedCurrentNode = list.currentNode;
list.currentNode = list.headNode;
for(int i = 1; list.next() == true; i++)
{
std::cout << "Element " << i << " " << list.get() << endl;
}
list.currentNode = savedCurrentNode;
}
im trying this but this method prints all the elements of my linked list
For what little code you have, a review:
// Why are you passing the list by value? That is wasteful.
void traverse(List list) {
// I don't see you taking a value anywhere; surely you know how to do that
// What is happening here? Can't you just assign the head to something
// directly?
Node *savedCurrentNode = list.currentNode;
list.currentNode = list.headNode;
// Like you said, this traverses the entire list, it's also poorly
// formed. You literally don't need i.
// for (; list.next(); /* However your list increments here */)
for(int i = 1; list.next() == true; i++)
{
std::cout << "Element " << i << " " << list.get() << endl;
}
// What is the purpose of this?
list.currentNode = savedCurrentNode;
}
For someone who is writing a linked list, this code seems to be fundamentally flawed. My expectation of someone tackling a linked list is that they are about to stop being a beginner, but I'm not seeing that here in the code and what structure of the list class is implied. The list class is weird to say the least.
And just to be clear, my expectation stems from where I place the linked list assignment in my curriculum. It's also more idiomatic than this list.
With that out of the way, this task is trivial if you took the time to think the project through. Most students skip the planning step and create unnecessary headaches for themselves.
Knowing that you would need the total size of the list, why not just make it member data? Any function that adds to the list will increment the value accordingly. And any function that subtracts from the list will decrement accordingly. That way you always know the size of the list at all times.
Knowing the size of the list is most of the battle. You then need to do the arithmetic necessary to advance in the list to satisfy your requirement. And now you can print.
#include <iostream>
class SList {
public:
SList() = default;
//
// Rule of 5 intentionally left out
//
void push_front(int val) {
m_head = new Node{val, m_head};
++m_size; // The magic happens here
}
std::size_t size() const { return m_size; }
void traverse_last(int numElements, std::ostream& sout = std::cout) const {
int placement = m_size;
Node* walker = m_head;
// Move our walker node the appropriate amount of steps
while (walker && placement > numElements) {
walker = walker->next;
--placement;
}
// Now that we're in position, we can print
while (walker) {
sout << walker->data << ' ';
walker = walker->next;
}
sout << '\n';
}
private:
struct Node {
int data;
Node* next = nullptr;
};
Node* m_head = nullptr;
std::size_t m_size = 0ULL;
};
int main() {
SList test;
for (int i = 5; i > 0; --i) {
test.push_front(i);
}
std::cout << "Size: " << test.size() << '\n';
for (int i = 1; i <= 5; ++i) {
test.traverse_last(i);
}
test.traverse_last(10);
}
Output:
❯ ./a.out
Size: 5
5
4 5
3 4 5
2 3 4 5
1 2 3 4 5
1 2 3 4 5
void traverse(List list, int printFrom)
{
Node *savedCurrentNode = list.currentNode;
list.currentNode = list.headNode;
for(int i=1; list.next(); i++)
{
if(i > printFrom)
{
cout << "Element " << (i - printFrom) << " " << list.get() << endl;
}
}
list.currentNode = savedCurrentNode;
}
solved my prblem by this there printFrom is a variable whose value is number of elemenets that skipped like if my linked list have size of 20 and user want to see last 5 then printFrom stores 15 and skipped 15 values and print last 5
This thing has been driving me crazy for a while now.
I need to create and traverse (post order) a general tree where each node (a structure) is added by the user via console.
I am NOT allowed to use STL.
The user specifies how many nodes will be added, and how many 'child' nodes it can hold (number) and the name of the node (string).
Example input:
5
1 A
2 B
1 C
1 D
3 E
The above means that 5 nodes will be added. The first one (A) can accept one 'child' node, (B) can accept 2 such nodes and (C) can accept 1 etc.
The newly added nodes have to always be added to the 'highest' possible node from the top (if it still can accept a new 'child' node, if not possible you go to the next one).
The idea is to create an array (I know how many nodes will be added in total) and put those nodes specified by the user there and 'link' them accordingly using array of pointers inside of a structure.
The output of given example should be: E C D B A
I have written the whole thing as follows but I am unable to traverse the tree:
structure:
struct node {
string name = "";
int noChildrenAdded = 0;
int possibleNoChildren = 0;
int childrenFreeSlots = 0;
node* children = nullptr;
node* father = nullptr;
};
traverse function that's not working
void traverse(node* father)
{
cout << father->name << endl;
if (father == nullptr) {
return;
}
for (int i = 0; i < father->possibleNoChildren; i++) {
if (&father->children[i] == nullptr) {
continue;
}
traverse(&father->children[i]);
}
cout << father->name << "\n";
}
main
int main() {
int n = 0;
short g = 0;
string name;
cin >> n;
node* tree = new node[n];
node* tmp = nullptr;
//adding children to tree array
for (int i = 0; i < n; i++) {
cin >> g >> name;
tree[i].possibleNoChildren = tree[i].childrenFreeSlots = g;
tree[i].name = name;
tree[i].noChildrenAdded = 0;
tree[i].children = new node[1];
}
// making connections between nodes
for (int son = 1; son < n; son++) {
for (int father = 0; father < son; father++) {
if (tree[father].childrenFreeSlots > 0) {
//resizing array
if (tree[father].noChildrenAdded == 0) {
tree[father].children[0] = tree[son];
}
else {
int added = tree[father].noChildrenAdded;
tmp = new node[added + 1];
for (int i = 0; i < added; i++) {
tmp[i] = tree[father].children[i];
}
delete[] tree[father].children;
tree[father].children = nullptr;
tree[father].children = tmp;
tree[father].children[added] = tree[son];
tmp = nullptr;
}
tree[father].noChildrenAdded++;
tree[father].childrenFreeSlots -= 1;
break;
}
}
}
//this is how it should be
cout << "Father: " << tree[1].name << "\tchildren added: " << tree[1].noChildrenAdded << endl;
//tree[0].children[0] is holding pointer to drzewo[1] so the below should give me the same answer as above.
//this is giving me wrong answer
node* ptr = &tree[0].children[0];
cout << "Father: " << ptr->name << "\tchildren added: " << ptr->noChildrenAdded << endl;
//traverse(&tree[0]);
delete[] tree;
}
THE PROBLEMS
I am unable to access details of a structure (for example noChildrenAdded) - I am getting zero, despite the fact that noChildrenAdded is populated. When I access it via tree array I am getting the correct number but when I do it via pointer inside of a struct I am getting 0.
Example:
This is correct: cout << "Father: " << tree[1].name << "\tchildren added: " << tree[1].noChildrenAdded << endl;
But this is not (despite both should be giving the same number/answer):
//tree[0].children[0] is holding pointer to tree[1] so the below should give me the same answer as above.
//this is giving me wrong answer
node* ptr = &tree[0].children[0];
cout << "Father: " << ptr->name << "\tchildren added: " << ptr->noChildrenAdded << endl;
I expect I have messed up assigning children to the *children array inside of a struct. The name seems to be accessible fine but not the noChildren.
Both should be giving the same answer but they are not:
enter image description here
Any help would be greatly appreciated!
PS: when I use this code with static array of children everything is ok, traversal works fine but when I get a dynamic array it's broken. Static array alas won't do as it taking too much memory and takes way too long so my program fails the requirements.
Just as #igor-tandetnik suggested, using an array of node* pointers solved the problem. In my case solution was to use node** children not node *children.
I'd like to start with what I know about heaps and Huffman code.
For this project, we use a minimum heap. The top part of the upside-down tree (or root) holds the minimum element. Whenever something is added to the array, everything gets moved, so the root is always the minimum value element. Whenever an element is deleted, everything gets reconfigured with the top element holding the minimum once again. In class, we went over a (template) class called MaxHeap, which I converted into MinHeap without the template stuff.
My professor went over Huffman encoding, but I understood it best using this visual tool:
https://people.ok.ubc.ca/ylucet/DS/Huffman.html
The idea is to use a minimum heap as follows:
1. Delete two nodes
2. Create a new node with the deleted nodes as children. The frequency of this node is the summation of the two children frequencies.
3. Add this new node to the minimum heap
This process repeats until there is one node left in the heap (the root). Next, we find the encodings for each letter. To do this, travel down the tree with left movement being 0 and right movement being 1. Traveling right twice then left once would give 110 for the letter 'c' in my tree (image link can be found towards the bottom of my post).
Everything was going mostly fine until I needed to traverse from the root. I had no idea how to do this via code, so I tried googling the answers and found these two websites:
https://www.geeksforgeeks.org/huffman-coding-greedy-algo-3/
https://www.programiz.com/dsa/huffman-coding
I copied their function printCodes() into my code, but I didn't get see it work.
When I try manually going down the tree, I get two things. For example, I tried traveling left down the root and using cout to see the values. I expected to see 40, !, e, d; but when I tried I was getting gibberish number and characters (greek letters like theta, sigma, etc). It gets really weird because on line 207, yourRoot->left->freq gives me 40, but the same thing on the line 208 of code gives me a large number. When I traveled right, I got: Exception thrown: read access violation. yourRoot->right->right->letter was 0xCCCCCCCC.
To reiterate cout << yourRoot->left->freq << endl; will give me 40 the first time I call it, but the second time I get a random number. I expected the same output twice in a row. Am I supposed to keep a pointer or pointer-to-pointer to the address of yourRoot or something?
Another problem is in createHuffmanTree(), if I put return root; outside the while loop I get this error and the code doesn't run at all:
potentially uninitialized local pointer variable 'root' used
Both of these things were odd problems and I assume it has to do with the way I'm using & and * symbols. I tried using something like this:
MinHeap yourHeap = MinHeap(6);
node *item = newNode(30, 'f');
yourHeap.Insert(*item);
item = newNode(20, 'e');
yourHeap.Insert(*item);
item = newNode(20, 'd');
yourHeap.Insert(*item);
item = newNode(15, 'c');
yourHeap.Insert(*item);
item = newNode(10, 'b');
yourHeap.Insert(*item);
item = newNode(5, 'a');
yourHeap.Insert(*item);
delete item;
This works the same as the yourList[] code I have in main(), but I figured "keep it simple stupid" and avoid using pointers since I clearly have some issues with them.
I uploaded an output without any error causing code and a drawing of what I expect my tree to look like with the values I want to use (https://imgur.com/a/Vpx7Eif). If the link doesn't work, please let me know so I can fix it.
The code I have thus far is:
#include <iostream>
using namespace std;
#define MAX_TREE_HEIGHT 20
//exception is thrown if wrong input
class NoMem
{
public:
NoMem() { cout << "Heap is full\n"; }
};
class OutOfBounds
{
public:
OutOfBounds() { cout << "Heap is empty\n"; }
};
struct node
{
int freq;
char letter;
struct node *left, *right;
};
// initialize node with frequency and letter
node* newNode(int freq, char letter)
{
node *temp = new node;
temp->freq = freq;
temp->letter = letter;
temp->left = nullptr;
temp->right = nullptr;
return temp;
}
// initialize node using two nodes as children
node* newNode(node& a, node& b)
{
node *temp = new node;
temp->freq = a.freq + b.freq;
temp->letter = '!';
temp->left = &a;
temp->right = &b;
return temp;
}
class MinHeap {
public:
MinHeap(int MSize)
{
MaxSize = MSize;
heap = new node[MaxSize + 1];
Size = 0;
}
~MinHeap() { delete[] heap; }
MinHeap& Insert(node& x);
MinHeap& Delete(node& x);
void Display();
int Size;
private:
int MaxSize;
node *heap;
};
MinHeap& MinHeap::Insert(node& x)
{
if (Size == MaxSize) throw NoMem();
else
{
printf("Inserting '%c' with frequency of %d. ", x.letter, x.freq);
int i = ++Size;
while (i != 1 && x.freq < heap[i / 2].freq)
{
heap[i] = heap[i / 2];
i /= 2;
}
heap[i] = x;
Display();
return *this;
}
}
MinHeap& MinHeap::Delete(node& x)
{
if (Size == 0) throw OutOfBounds();
x.freq = heap[1].freq; // root has the smallest key
x.letter = heap[1].letter;
printf("Deleting '%c' with frequency of %d. ", x.letter, x.freq);
node y = heap[Size--]; // last element
int vacant = 1;
int child = 2; //make child = left child
while (child <= Size)
{
if (child < Size && heap[child].freq > heap[child + 1].freq) ++child;
// right child < left child
if (y.freq <= heap[child].freq) break;
heap[vacant] = heap[child]; // move smaller child
vacant = child; // new vacant
child = child * 2; // new child of vacant
}
heap[vacant] = y;
Display();
return *this;
}
void MinHeap::Display()
{
printf("Your heap contains: ");
for (int i = 1; i <= Size; i++)
printf("'%c' = %d, ", heap[i].letter, heap[i].freq);
printf("\n");
}
node* createHuffmanTree(MinHeap& yourHeap)
{
cout << "--- Creating Huffman Tree ---\n";
node left, right, *root;
while (yourHeap.Size > 1)
{
yourHeap.Delete(left);
yourHeap.Delete(right);
root = newNode(left, right);
cout << "-> New Node: freq = " << root->freq << ", letter = " << root->letter << ", left: " << root->left->letter << ", right: " << root->right->letter << endl;
yourHeap.Insert(*root);
if (yourHeap.Size < 2)
{
return root;
}
}
//return root; // potentially uninitialized local pointer variable 'root' used
}
void outputHuffmanCode(node* root, int arr[], int top)
{
// left movement is 0
if (root->left)
{
arr[top] = 0;
outputHuffmanCode(root->left, arr, top + 1);
}
// right movement is 1
if (root->right)
{
arr[top] = 1;
outputHuffmanCode(root->right, arr, top + 1);
}
// if reached leaf node, must print character as well
if (!(root->left) && !(root->right))
{
cout << "'" << root->letter << "' = ";
for (int i = 0; i < top; ++i)
cout << arr[i];
cout << endl;
}
}
int main()
{
node yourList[6];
yourList[0].freq = 5;
yourList[0].letter = 'a';
yourList[1].freq = 10;
yourList[1].letter = 'b';
yourList[2].freq = 15;
yourList[2].letter = 'c';
yourList[3].freq = 20;
yourList[3].letter = 'd';
yourList[4].freq = 20;
yourList[4].letter = 'e';
yourList[5].freq = 30;
yourList[5].letter = 'f';
cout << "Here is your list: ";
for (int i = 0; i < 6; i++)
{
cout << "'" << yourList[i].letter << "' = " << yourList[i].freq;
if (i < 5) cout << ", ";
} cout << endl;
MinHeap yourHeap(6);
yourHeap.Insert(yourList[5]);
yourHeap.Insert(yourList[4]);
yourHeap.Insert(yourList[3]);
yourHeap.Insert(yourList[2]);
yourHeap.Insert(yourList[1]);
yourHeap.Insert(yourList[0]);
/*
MinHeap yourHeap = MinHeap(6);
node *item = newNode(30, 'f');
yourHeap.Insert(*item);
item = newNode(20, 'e');
yourHeap.Insert(*item);
item = newNode(20, 'd');
yourHeap.Insert(*item);
item = newNode(15, 'c');
yourHeap.Insert(*item);
item = newNode(10, 'b');
yourHeap.Insert(*item);
item = newNode(5, 'a');
yourHeap.Insert(*item);
delete item;
*/
node *yourRoot = newNode(0, NULL);
yourRoot = createHuffmanTree(yourHeap);
// same cout twice in a row, different results
//cout << yourRoot->left->freq << endl;
//cout << yourRoot->left->freq << endl;
cout << "L0 Root: freq = " << yourRoot->freq << ", letter = " << yourRoot->letter << ", left freq: " << yourRoot->left->freq << ", right freq: " << yourRoot->right->freq << endl;
cout << "L11 Left: freq = " << yourRoot->left->freq << ", letter = " << yourRoot->left->letter << ", left: " << yourRoot->left->left->letter << ", right: " << yourRoot->left->right->letter << endl;
//cout << "R11 Left: freq = " << yourRoot->right->freq << ", letter = " << yourRoot->right->letter << ", left: \n";
//<< yourRoot->right->left->letter << ", right: " << yourRoot->right->right->letter << endl;
//int arr[MAX_TREE_HEIGHT], top = 0;
//outputHuffmanCode(yourRoot, arr, top);
system("pause");
return 0;
}
I'd like to thank whoever reads and replies to this post in advance. I think I've given as much information as I could. If I did anything that's against community rules, please let me know so I can fix my mistake(s).
In your createHuffmanTree Function, you create the node's left and right...
with root = newNode(left, right); you let the left/right member of your struct point to the address of the (temporary) node you've created in createHuffmanTree (that means in
node* newNode(node& a, node& b)
the address of a and b is always the same..
and the node goes out of scope after leaving the createHuffmanTree function - i think this causes your problem. You know what I mean?
I think I am close to finishing this implementation of A* but my mind is becoming fried and am looking for pointers on what I should be doing to complete it.
My current problem is that my function that runs through A* remains stuck on the same node, as in the current node never moves into any other of the open nodes.
Here is my main function, note that the heuristic(Node &n1, Node &n2) function is currently set to always to return 0, so it should currently be working more like a Dijkstra algorithm rather than A*. Also, movement is restricted to the NESW plane, no diagonal movement, so distance_between(Node &n1, Node &n2) always returns 1.
void astar(Node start_, Node end_) {
Node start = start_;
Node end = end_;
// compute f,g,h for the start node
start.g = 0;
start.h = heuristic(start, end);
start.f = start.g + start.h;
// insert start node into the open set
openNodes.insert(&start);
// while the set of open nodes is not empty
while (openNodes.size() > 0) {
// pick the most promising node to look at next
Node currentNode;
cout << "currentNode before: ";
currentNode.displaylocation();
// go through all the open nodes and find the one with the smallest 'f' value
Node* minf = (*openNodes.begin()); // set initial value for minimum f to be the first node in the set of open nodes
for (auto n : openNodes) {
if (n->f <= minf->f) {
minf = n;
}
}
currentNode = *minf; // set the current node to the node that holds the smallest 'f' value
cout << "currentNode after: ";
currentNode.displaylocation();
// if the current node is the end node, then we have found a path
if (currentNode.type == -3) {
break;
}
// remove the current node from the set of open nodes, and add it to the set of closed nodes
openNodes.erase(¤tNode);
closedNodes.insert(¤tNode);
// go through the currents node's neighbours
for (auto n : neighbours(currentNode)) {
cout << "neighbour local: " << n.location.x << "," << n.location.y << "\n";
if (closedNodes.count(&n) == 0 && n.type != -2) { // if this node is neither closed or a blocker
int new_g = currentNode.g + distance_between(currentNode, n);
if (openNodes.count(&n) != 0) { // if we have not seen this node before, add to the open set
openNodes.insert(&n);
}
else if (new_g >= n.g) { // else if we have seen this node before, and already found a shorter path to it from the starting node
}
n.g = new_g;
n.f = n.g + heuristic(n, end);
n.parent_ = ¤tNode;
}
}
cout << "\n A* run success! \n";
//break;
}
}
Here is the deceleration of things like the Node struct and the global variables:
// The size of the grid
#define WIDTH 6
#define HEIGHT 6
// Holds values for x and y locations on the grid
struct Coord {
int x, y;
};
// holds data for each node required for A*
struct Node {
int type; // used for defining if this node is a blocker, empty, start or end
Coord location;
int g = 0;
int h = 0;
int f = g + h;
Node *parent_; // pointer to this node's parent
std::string debugmessage;
void displaylocation() {
std::cout << "I am the node at: " << location.x << "," << location.y << "\n";
}
};
// The 2D grid array for A*, requiring a Node struct to store the data of each cell
Node astarArray[WIDTH][HEIGHT];
// Sets for A*
std::set<Node *> openNodes; // contains the nodes that are yet to be considered (if this is empty, then there are no more paths to consider or there is no path)
std::set<Node *> closedNodes; // contains the nodes that have already been considered (if the end node is placed in here, a path has been found)
// stores the start and end values for A*
Node start_A, end_A;
void astar(Node start_, Node end_);
int distance_between(Node& n1, Node& n2);
int heuristic(Node& n1, Node& n2);
std::list<Node> neighbours(Node& n_);
// returns the distance between two nodes for A*
int distance_between(Node& n1, Node& n2) {
return 1; // always return 1 as we are working in a grid restricted to NSEW movement
}
int heuristic(Node& n1, Node& n2) {
return 0; // return 0 to work as a Dijkstra algorithm rather than A*
}
// finds a node's neighbours for A*
std::list<Node> neighbours(Node& n_) {
std::list<Node> neighbours_;
int x = n_.location.x;
int y = n_.location.y;
// start at the location belonging to 'n_'
//for (int y = n_.location.y; y < HEIGHT; y++) {
//for (int x = n_.location.x; x < WIDTH; x++) {
// east
if (x < WIDTH - 1) {
neighbours_.push_back(astarArray[x + 1][y]);
}
// west
if (x > 0) {
neighbours_.push_back(astarArray[x - 1][y]);
}
// south
if (y < HEIGHT - 1) {
neighbours_.push_back(astarArray[x][y + 1]);
}
// north
if (y > 0) {
neighbours_.push_back(astarArray[x][y -1]);
}
//}
//}
return neighbours_;
}
Thank you very much for reading and for any help you can give. I will provide more code if required.
The main problem you have is that you are using the pointers (mem address) to find out if a node is in your set or not.
currentNode = *minf; // set the current node to the node that holds the smallest 'f' value
Then you copy to currentNode the contents of minf.
currentNode will have a different address from the pointer to minf
openNodes.erase(¤tNode); will not remove minf because currentNode does not have the same address.
I would suggest you investigate more on how to implement A* as you are missing some steps. Look for priority queues.
Instead of the mem address of the node, use the position index for that node in the grid (pos.x * numCols) + pos.y
I'm trying to implement the Skip List using this article Skip List.
Code:
#include<iostream>
#include<cstdlib>
#include<ctime>
#include<limits>
using namespace std;
template<class T>
class SkipList{
private:
class SkipNode{
public:
T* key; //Pointer to the key
SkipNode** forward; //Forward nodes array
int level; //Node level
//SkipNode constructor
SkipNode(T* key, int maxlvl, int lvl){
forward = new SkipNode*[maxlvl];
this->key=key;
level=lvl;
}
//Method that print key and level node
print(){
cout << "(" << *key << "," << level << ") ";
}
};
SkipNode *header,*NIL; //Root and End pointers
float probability; //Level rate
int level; //Current list level
int MaxLevel; //Maximum list levels number
//Function that returns a random level between 0 and MaxLevel-1
int randomLevel(){
int lvl = 0;
while( (float(rand())/RAND_MAX < probability) && (lvl < MaxLevel-1) )
lvl++;
return lvl;
}
public:
//SkipList constructor
SkipList(float probability, int maxlvl){
this->probability = probability;
MaxLevel = maxlvl;
srand(time(0));
header=new SkipNode(NULL,MaxLevel,0); //Header initialization
T* maxValue = new T;
*maxValue = numeric_limits<T>::max(); //Assign max value that T can reach
NIL = new SkipNode(maxValue,0,0); //NIL initialization
level=0; //First level
for(int i=0; i<MaxLevel; i++){ //Every header forward node points to NIL
header->forward[i]=NIL;
}
}
//SkipList destructor
~SkipList(){
delete header;
delete NIL;
}
//Method that search for a key in the list
SkipNode* search(T* key){
SkipNode* cursor = header;
//Scan the list
for(int i=level; i>=0; i--)
while(*(cursor->forward[i]->key) < (*key))
cursor=cursor->forward[i];
cursor=cursor->forward[0];
if(*(cursor->key) == *key)
return cursor;
return NULL;
}
//Method that insert a key in the list
SkipList* insert(T* key){
SkipNode* cursor = header;
SkipNode* update[MaxLevel]; //Support array used for fixing pointers
//Scan the list
for(int i=level; i>=0; i--){
while(*(cursor->forward[i]->key) < *(key))
cursor=cursor->forward[i];
update[i]=cursor;
}
cursor=cursor->forward[0];
if(*(cursor->key) == *(key)){ //Node already inserted
return this;
}
int lvl = randomLevel(); //New node random level
if(lvl > level){ //Adding missing levels
for(int i=level+1; i<=lvl; i++)
update[i]=header;
level=lvl;
}
SkipNode* x = new SkipNode(key,MaxLevel,lvl); //New node creation
for(int i=0; i<=lvl; i++){ //Fixing pointers
x->forward[i] = update[i]->forward[i];
update[i]->forward[i] = x;
}
return this;
}
//Method that delete a key in the list
SkipList* erase(T* key){
SkipNode* cursor = header;
SkipNode* update[MaxLevel]; //Support array used for fixing pointers
//Scan the list
for(int i=level; i>=0; i--){
while(*(cursor->forward[i]->key) < *(key))
cursor=cursor->forward[i];
update[i]=cursor;
}
cursor=cursor->forward[0];
if(*(cursor->key) == *(key)){ //Deletetion of the founded key
for(int i=0; i<=level && update[i]->forward[i] == cursor; i++){
update[i]->forward[i] = cursor->forward[i];
}
delete cursor;
while(level>0 && header->forward[level]==NIL){
level=level-1;
}
}
return this;
}
//Method that print every key with his level
SkipList* print(){
SkipNode* cursor = header->forward[0];
int i=1;
while (cursor != NIL) {
cursor->print();
cursor = cursor->forward[0];
if(i%15==0) cout << endl; i++;
}
cout << endl;
return this;
}
};
main(){
SkipList<int>* list = new SkipList<int>(0.80, 8);
int v[100];
for(int i=0; i<100; i++){
v[i]=rand()%100;
list->insert(&v[i]);
}
list->print();
cout << endl << "Deleting ";
for(int i=0; i<10; i++){
int h = rand()%100;
cout << v[h] << " ";
list->erase(&v[h]);
}
cout << endl;
list->print();
cout << endl;
for(int i=0; i<10; i++){
int h = rand()%100;
cout << v[h] << " ";
if(list->search(&v[h]))
cout << " is in the list" << endl;
else
cout << " isn't in the list" << endl;
}
delete list;
}
It gives me Segmentation Fault on line 59 (the for-cycle on the insert), but I can't understand why. May you help me please? I will accept any other improvement that you suggest. My deadline is on two days, that's why I'm asking for help.
EDIT:
I've corrected the code with bebidek suggestions (Thanks). Now first level is 0. It seems to be working, but sometimes some nodes is not inserted correctly and the search give a bad result.
LAST EDIT:
It works, thanks to all
ONE MORE EDIT:
Added comments to code, if you have any suggestion you're welcome
The biggest problem in your code is probably NIL=new SkipNode(numeric_limits<T*>::max());
First of all i suspect you want the key pointer to point to a memory address that contains the biggest possible int value.
But that's not what's actually happening here. Instead the key pointer points to the biggest possible memory-address which is most likely not available for your process.
Also the forward property probably contains an array of junk pointers.
Then when the first loop in the insert method is executed this leads to 2 problems:
while(*(cursor->forward[i]->key) < *(key)) will compare the key value to an invalid pointer
cursor=cursor->forward[i]; will re-assign cursor to an invalid pointer
I would first suggest you'd change the design to let SkipNode keep a value to T instead of a pointer:
class SkipNode{
public:
T key;
SkipNode* forward[100];
This will make a lot of pointer related code unnecessary and make the code simpler so less likely to run into access violation.
Also it might be cleaner to use an actual NULL (or event better nullptr) value instead of a dummy NIL value to indicate the end of the list.
So, first problem is when you create NIL node:
NIL=new SkipNode(numeric_limits<T*>::max());
As argument you should use pointer to existing variable, for example:
T* some_name = new T;
*some_name = numeric_limits<T>::max();
NIL = new SkipNode(some_name);
Notice, I used T instead of T* in numeric_limits. Of course you have to remember about deleting this variable in destructor.
Second problem is that level variable in your code sometimes is inclusive (I mean level number level exists) as in line 61, and sometimes exclusive (level number level doesn't exist) as in line 71. You have to be consistent.
Third problem is in line 52. You probably mean cursor=cursor->forward[1];, but after loop i = 0, and forward[0] doesn't have any sense in your code.
EDIT:
Fourth and fifth problem is in erase function.
cursor->~SkipNode();
It won't delete your node, but only run empty destructor. Use delete cursor; instead.
And in loop you probably wanted to write update[i]->forward[i] == cursor instead of !=.
ONE MORE EDIT:
You haven't implemented any destructor of SkipList and also you forgot about delete list; at the end of main(). These two will give you a memory leak.
ANOTHER EDIT:
srand(time(0));
This line should be executed once at the beginning of main and that's all. If you execute it before each random generation, you will get the same result every time (as time(0) counts only seconds and your program can run function randomLevel() more than once a second).
You also forgot about rewriting precision variable in constructor of SkipList.
NEXT EDIT:
In your insert function you don't have level randomization. I mean, you do not have ability of inserting node of level less than level of whole skip list. It's not error which will crash your program or give wrong results, but time complexity of queries in your structure is O(n) instead of O(log n).
You should use lvl instead of level in this loop in insert function:
for(int i=1; i<level; i++){
x->forward[i] = update[i]->forward[i];
update[i]->forward[i] = x;
}
And also minimum result of your random function randomLevel should be 1 instead of 0, as you don't want node witch level=0.