I'm attempting to create an array of pointers.
struct vertex
{
std::string id;
std::string name;
int networkID;
std::vector<adjVertex> friends;
bool visited;
};
struct hobbylist
{
std::string hobby;
std::vector<vertex*> list;
};
hobbylist * hobbies[HASHMAP_SIZE];
adding the user to the hobbies array:
int Graph::addUserToHobby(std::string hobby1, std::string id){
// initial key is based on the first 2 characters of the hobby name
int key = (hobby1[0] + hobby1[1]) % HASHMAP_SIZE;
cout << " initial hashmap key " << key << endl;
hobbylist *h = new hobbylist;
h->hobby = hobby1;
hobbies[key] = h;
}
my goal is to create an array of pointers with the hobbylist type, when attempting to print the contents of that array I end up with a very strange random symbol output:
GLIBC_2.2.5GLIBCXX_3.4.13GLIBCXX_3.4.14CXXABI_1.3GLIBCXX_3.4� P&y
I attempt to print it as so:
void Graph::displayHobbies(){
cout << "========================================\n";
cout << "DISPLAYING HOBBY INTERESTS =============" << endl;
for(auto const& value: hobbies)
{
cout << value->hobby << ":" << endl;
}
}
I was wondering if I am printing incorrectly or if I am adding the hobby to the hobbies array incorrectly.
Changed Code:
hobbylist *h = new hobbylist;
h->hobby = hobby1;
if(hobbies[key] ==NULL){
h->list.push_back(user);
hobbies[key] = h;
}
else if (hobbies[key]!=NULL){
h= hobbies[key];
h->list.push_back(user);
}
Changed code is above and I am getting a seg fault at the last line in the else statement when running the function the first time and I am confused why the function would go to the else statement when the array should be empty and therefore hobbies[key] should be null the first time the function is run?
You have at least two bugs.
hobbylist *h = new hobbylist;
h->hobby = hobby1;
hobbies[key] = h;
key is your hash key. If hobbies[key] already has a pointer, this is going to leak memory.
for(auto const& value: hobbies)
{
cout << value->hobby << ":" << endl;
}
This assumes that every slot in the hobbies hash array contains a pointer. This is unlikely. If a particular value in hobbies has never been initialized (none of the previously inserted hobbies mapped to that hash key), the pointer will be NULL, and value->hobby will attempt to dereference a NULL pointer, resulting in undefined behavior. That's your likely crash.
Related
I have just done a module on pointers and dynamic memory in C++ and am attempting to complete a personal assignment so that I can practice the concepts. The program manages an array of strings that are names. The goal that I set for myself is that the list is stored in the heap (to practice using "new"), and the list is dynamically sized as new names are entered.
Disclaimer: I realize that this is easily accomplished using vectors, and after struggling with this for hours I re-wrote my original code to use a vector for the list with no problems. However I want to learn where my understanding of how pointers work is broken.
The problem that I have with the program is this: I initialize the name array to have zero elements and have a function to add names that handles the dynamic sizing. When first called it seems to re-size the array correctly and add a new name to the the new array. Within the function to add a name, I can print the contents of the new array. I can also re-assign the old array pointer to the address of the new array on the heap. However when I call the print function from main after adding a name to the list, the list does not contain a name. By my understanding, since I'm using pointers I should be updating values directly, so after the add name function terminates, the values should persist. Also, if I attempt to add a second name the program crashes. What am I doing wrong with the handling of memory?
I've searched quite a bit and the closest that I can find for a resolution was this post:
How to make an array with a dynamic size? General usage of dynamic arrays (maybe pointers too)?
I modified my code based on what I understand from that but it still doesn't work properly.
#include <stdio.h>
#include <vector>
#include <iostream>
using namespace std;
void add_name_to_list(string * my_list, size_t * list_size);
string get_name();
void print_names(const string *const my_list, const size_t *const list_size);
int main()
{
string *name_list_ptr {nullptr};
name_list_ptr = new string [0];
size_t name_list_size{0};
size_t *name_list_size_ptr {&name_list_size};
print_names(name_list_ptr, name_list_size_ptr);
add_name_to_list(name_list_ptr, name_list_size_ptr);
print_names(name_list_ptr, name_list_size_ptr);
return 0;
}
void add_name_to_list (string * my_list, size_t *list_size)
{
string new_name{get_name()};
string *new_string_ptr{nullptr};
new_string_ptr = new string [*list_size+1];
// copy existing list into new list
cout << "List size is " << *list_size << " so *list size == 0 is " << (*list_size == 0) << endl;
if(*list_size == 0)
{
new_string_ptr[0] = new_name;
*list_size = *list_size +1;
cout << new_string_ptr[0] << " has been added to position " << *list_size << endl;
}
else
{
print_names(my_list, list_size);
for(size_t i{0}; i < *list_size; i++)
{
cout << "At position " << i << " original list is " << my_list[i] << endl;
new_string_ptr[i] = my_list[i];
cout << "The name " << new_string_ptr[i] << " has been added to position " << i << " of the new list" << endl;
}
new_string_ptr[*list_size - 1] = new_name;
*list_size = *list_size + 1;
}
print_names(new_string_ptr, list_size);
string *temp_ptr{nullptr};
temp_ptr = new string [*list_size-1];
cout << "temp ptr is " << temp_ptr << " and my list is " << my_list << endl;
temp_ptr = my_list;
cout << "temp ptr is " << temp_ptr << " and my list is " << my_list << endl;
my_list = new_string_ptr;
delete [] temp_ptr;
new_string_ptr = nullptr;
print_names(my_list, list_size);
}
string get_name()
{
cin.sync();
cin.clear();
string new_name{};
cout << "\nEnter the full name: ";
getline(cin, new_name);
cin.sync();
cin.clear();
if(new_name.size() <= 1)
return "0";
else
return new_name;
}
void print_names(const string *const my_list, const size_t *const list_size)
{
if(*list_size == 0)
cout << "The list is empty" << endl;
else
for(size_t j{0}; j < *list_size; j++)
cout << j << ". " << my_list[j] << endl;
}
One variation that I've tried based on what I learned from searching is:
cout << "temp ptr is " << temp_ptr << " and my list is " << my_list << endl;
//my_list = new_string_ptr;
//delete [] temp_ptr;
//new_string_ptr = nullptr;
delete [] my_list;
my_list = new string[*list_size];
my_list = new_string_ptr;
print_names(my_list, list_size);
Unfortunately the results are the same.
Without checking the logic of the implementation, your list doesn't update because you are assigning my_list = new_string_ptr; but your function received void add_name_to_list (string * my_list, size_t *list_size).
As you are newcomer to C++ world, let me explain clearly:
list_size is a pointer to a size_t, so if you modify the pointed memory, the change will persist, but if you modify the pointer itself, it will not.
list_size = new size_t; // This change doesn't persist
list_size++; // This change doesn't persist
*list_size++; // This change persists and the value of pointed memory was increased.
With my_list is happening exactly the same, you are trying to modify the pointer itself, not the pointed memory.
So, you should use:
void add_name_to_list (string * &my_list, size_t *list_size)
Or maybe you are more confortable with
void add_name_to_list (string ** my_list, size_t *list_size)
[...]
*my_list = new_string_ptr;
Hope this helps
I been programming a set of lists holing pointer to lists, and wanna create an convenient interface to brows them and save selected data from file to lists. The idea is that there is one "API list" with unique id, and whenever I find an unique id, I shall create a new API list. For now I'm stack on saving data to one "API list's" related lists.
The structure looks simply like this:
enum day { mon, tue, wed, thu, fri, sat, sun };
static const string enumValues[] = { "mon", "tue", "wed", "thu", "fri", "sat", "sun" };
struct _ListSub {
string h;
day d;
string gr;
string sub;
_ListSub *next = nullptr;
};
struct ListAPI {
string id;
ListAPI *next = nullptr;
_ListSub *head = nullptr;
};
And my function to read and save values from the file to the lists look like this:
ListAPI *createLists(string arg) {
ifstream f_in;
ListAPI *listGrip;
f_in.open(arg);
if (!f_in.is_open()) {
cout << "\"" << arg << "\": file does not exist!" << endl;
exit(EXIT_FAILURE);
}
listGrip = new ListAPI;
listGrip->head = new _ListSub;
while (true) {
// dummy data
string h;
string week = "";
string gr;
string id;
string sub;
if (!(f_in >> h >> week >> gr >> id >> sub)) {
break;
}
cout << "ID check: " << checkListID(id, listGrip) << endl;
listGrip->id = id;
listGrip->head->h = h;
listGrip->head->d = (day)enumerateDay(week);
listGrip->head->gr = gr;
listGrip->head->sub = sub;
listGrip->head ++;
listGrip->head = new _ListSub;
}
f_in.close();
return listGrip;
}
Any ways, the data is correct, it worked fine, so long didn't add this part (moving head pointer of _listSub and creating new instance of this object):
listGrip->head ++;
listGrip->head = new _ListSub;
All data that I'm getting out of this is id witch I save to listGrip (my list API), however all the data from the nested list list is gone.
Can someone tell me what I'm doing wrong here with pointers?
MAIN:
int main( int argc, char **argv ) {
/* Directly parse options in order to avoid accepting abbrevations. */
string ARGUMENT;
validate_arguments;
cout << "File path: " << ARGUMENT << endl;
ListAPI *listGrip;
listGrip = createLists( ARGUMENT );
//listGrip->head;
cout << "List has been created." << endl;
cout << "ID: " << listGrip->id << endl;
cout << "Subject: " << listGrip->head->sub << endl;
cout << "Time: " << listGrip->head->h << endl;
cout << "Day: " << getTextFromEnum((short)listGrip->head->d) << endl;
delete listGrip;
return EXIT_SUCCESS;
}
listGrip->head ++;
Makes no sense. head points to a single _ListSub. When you increment it, it will point to some random memory after the _ListSub you've allocated. This random memory is not yours and could lead to memory corruption or if you are lucky to a SegFault.
What you probably want is to insert at head:
auto new_head = new _ListSub;
new_head->next = listGrip->head;
listGrip->head = new_head;
Or even better use the Standard library. There's a std::list in there already, it's well tested and it works.
Linked-List is an abstract structure connecting loosely placed nodes. As nodes are not placed next to each other we can not preform pointer arithmetics to get to the next node. In stead we can create an insert function witch is going to redirect pointers for us.
Redirecting nodes:
Classical Linked-List contain the next pointer in it's structure, pointing to the next node of the same type. For convince we can add previous pointer and a size variable to keep track of list size.
void insertSub(string h, day d, string gr, string sub) {
auto newNode = new _ListSub;
//initialize
newNode->h = h;
newNode->d = d;
newNode->gr = gr;
newNode->sub = sub;
// point new node to the currently previous and next node
newNode->next = node->next;
newNode->prev = node;
// point currently next and previous node to the new node
node->next->prev = newNode;
node->next = newNode;
// increment size
++ node->size;
}
This works for my structure, but probably you wanna use constructor for object initialization.
This may be a dumb question but I'm getting pretty desperate at this point.
I am attempting to create an array of pointers:
struct vertex
{
std::string id;
std::string name;
int networkID;
std::vector<adjVertex> friends;
bool visited;
};
struct hobbylist
{
std::string hobby;
std::vector<vertex*> list;
};
hobbylist * hobbies[HASHMAP_SIZE];
int Graph::addUserToHobby(std::string hobby1, std::string id){
//cout << "Adding to hobby: " << hobby1 << " user: " << id << endl;
vertex *user = findVertex(id);
int collisions = 0;
// initial key is based on the first 2 characters of the hobby name
int key = (hobby1[0] + hobby1[1]) % HASHMAP_SIZE;
//cout << " initial hashmap key " << key << endl;
hobbylist *h = new hobbylist;
if(hobbies[key] == NULL){
h->hobby = hobby1;
h->list.push_back(user);
hobbies[key] = h;}
else if (hobbies[key]!=NULL){
hobbies[key]->list.push_back(user);
collisions++;}
return collisions;
}
I am getting a seg fault at the last line in the else statement in the addUserToHobby function when running the function the first time and I am confused why the function would go to the else statement when the array should be empty and therefore hobbies[key] should be null the first time the function is run? Upon further inspection the function will always enter the else statement, so the array values are never null?
Each location is the array is not set to null by default, it's just whatever trash was in there before you allocated it.
I am having a problem with pointers and scope. I am trying to maintain an array of linked lists of pointers to objects. When I try to push_front() in one function, it works. However, if I try to iterate through the list in another part of my program, the list no longer contains any data, and my pointers are bad.
This is part of my parseCommands function. the problem is when printList is called:
Administrator *adminPtr = new Administrator(); // create new Administrator pointer
//local variables...
string adminName; //administrator's name
int adminMNum; //administrator's M Number
string adminEmail; //administrator's email address
string adminTitle; // administrator's title
// read in & store data for new administrator
inData >> adminName; //read in data
adminPtr->setName(adminName); //set admin name
inData >> adminMNum;
adminPtr->setMNum(adminMNum); // set admin M Number
inData >> adminEmail;
adminPtr->setEmail(adminEmail); // set admin email address
inData >> adminTitle;
adminPtr->setTitle(adminTitle); //set admin title
// finished storing new administrator info
// add Administrator to list
cout << "Adding Administrator: " << endl;
cout << "in records office adminPtr/newPerson: " << adminPtr << endl;
universityList.addPerson(adminPtr); // call addPerson--hashTable
//universityList.printPerson(adminPtr); // print admin info using polymorphic method
//cout << "The load factor (alpha) is: " << universityList.getLength()/universityList.getMaxTableSize() << endl; // print alpha
universityList.printList(adminPtr->getMNum()); // print all items at table[loc]--breaks here
cout << endl;
The addPerson function where printList works fine:
template <typename T>
void HashTable<T>::addPerson(T newPerson) { //newPerson is a pointer to a person object
int loc; // array location provided by hashFunction
cout << "in hashtable newPerson: " << newPerson << endl;
loc = hashFunction(newPerson->getMNum()); // get loc
table[loc].push_front(&newPerson); // add to list at table[loc] passing address of pointer to person
printList(newPerson->getMNum()); // print all items at table[loc]--works here
size++; // increment size
} //end function
The printList function that works when called in addPerson, but not in parseCommands:
template <typename T>
void HashTable<T>::printList(searchKeyType key) { //print list of items held in array location
int loc = hashFunction(key); // get array location
if (table[loc].empty()) { // if list is empty
cout << "Can not print person M" << key << " NOT found" << endl << endl;
} //end if empty
else{
list<T*>::iterator iter; // stl iterator
iter = table[loc].begin();
cout << "in printList table[loc]begin " << *iter << endl; //print address of table[loc]begin.()--where iter points
cout << "in printList item in table[loc]begin " << **iter << endl; // print address of the pointer that iter points to
while(iter != table[loc].end()) { // for each item in the list
(**iter)->print(); // print person info using polymorphic method
++iter;
} //end for
} // end else
} // end printList
The print function:
void Administrator::print()const {
// print Administrator info
cout << " " << "Full Name: " << getName() << endl;
cout << " " << "M Number : "<< getMNum() << endl;
cout << " " << "Email Addr: " << getEmail() << endl;
cout << " " << "Title: " << getTitle() << endl;
}; // end print function
The hashTable class:
template<typename T>
class HashTable{
public:
HashTable(); // constructor
bool isEmpty()const; //determines if the hash table is empty
int getLength() const; // returns (size) number of Persons in table (accessor)
int getMaxTableSize() const; // returns tableSize (size of array)
void addPerson(T person); // adds new Person
void removePerson(searchKeyType key); // deletes Person from the HashTable
void printPerson(T person); // prints Person info
T getNodeItem(int mNumber); //returns person object (accessor)
void printList(searchKeyType key); //print list of items held in array location
private:
int size; // number of Persons in table
static const int tableSize = 1; // number of buckets/array size -- planning on using 70001--assuming 35,000 entries at once; largest prime > 2*35000
list <T*> table[tableSize]; // array of STL lists for chains
int hashFunction(searchKeyType searchKey); // hash function to return location (array index) of item
}; //end HashTable class
I pass adminPtr to addPerson, and it seems to add it to the list. Why am I losing the data when I return to the parseCommands function? Is it a stack vs. heap issue? Do I need "new" somewhere? There are a few extra lines in there where I was printing out the address of the pointers trying to figure out what's going on.
This was a programming problem for a class that I was unable to resolve. We had to simulate a hash table using an array of STL linked lists. We were not allowed to use vectors, maps, etc. The program involves an abstract base class (Person) with derived classes (Administrator, etc.) and a templated hash table class. There is one more class (RecordsOffice) that holds the hash table.
class RecordsOffice {
public:
RecordsOffice(); // default constructor
void parseCommands(string fileName); // function to parse commands from a file to maintain the StudentList
private:
HashTable <Person*> universityList; // creates empty hashtable
};
The problem is in these two places.
universityList.addPerson(adminPtr);
//...
You are passing a copy of adminPtr.
template <typename T>
void HashTable<T>::addPerson(T newPerson) { //newPerson is a pointer to a person object
// ...
table[loc].push_front(&newPerson); // add to list at table[loc] passing address of pointer to person
// ....
}
newPerson is a local variable to addPerson. When it returns it is no more valid. But you are adding it´s address in to the table.
the issue is that
list <T*> table[tableSize];
is storing pointers to pointers of Person.
I don't think passing by reference would solve the problem too. Because then you will be dependent on the automatically created pointer here.
Administrator *adminPtr = new Administrator();
What adminPtr pointer points to will stay but not adminPtr itself. So you can not depend on its address (unless you are sstill in the same function that created it). One possible way to solve it would be to allocate adminPtr itself dynamically.
Administrator **adminPtr = new Administrator*;
adminPtr = new Administrator();
But maybe you should revise the requirements.
Your table is declared like this:
list <T*> table[tableSize];
That means any pointers it contains need to be dynamically allocated, or need to remain in scope for the entire lifetime of the container. This is not the case. In your function addPerson you add the address of a local variable:
table[loc].push_front(&newPerson);
You should do one of the following:
Change the table to an array of list<T> objects.
Copy the data dynamically. eg table[loc].push_front(new T(newPerson))
Because this is a list, I would go for option 1 because the list will copy locally anyway, and you won't have to clean up the pointers later. A third option would be to use list<unique_ptr<T> > or similar.
I'm writing a code to index the skills available to a user in a game, constructed as a linked list. I've throughly tested the function that populates the list and it seems to be working correctly (so the head pointer for the list shouldn't be null). When I attempt to traverse the list to set values in the skill, before any of the code which writes to memory within the list gets to execute the program is crashing when I initialise the temp pointer within the search function of the list to the head pointer.
What makes this additionally weird to me is that it worked fine (and I had tested this fairly thuroughly) until I added in a list to store a list of available items, and may just be missing an odd interaction between the two when I populate them.
The specific error is that the pointer is supposedly accessing memory index 0x000000c to write to, but I don't see how the code at that point is dealing with a null pointer at all (since after 10 runs of the program the OS shouldn't be allocating that block of memory to the temp pointer every time and nothing else should be null.
I'm probably just ramblind at this point so here's the code:
The function that causes the error according to the debugger:
void Mechanics::setSkillValue(int index, int value)
{
Skill *temp = FirstSkill; // << The error is happening on this line //
while((temp != NULL)&&(temp->index != index))
{
temp = temp->next;
}
if (temp == NULL)
{
cout << "%";
}
else temp->setValue(value);
// cout << temp->returnValue(); //This was a test check, not needed for anything
}
The Function that's supposed to populate the skill and item lists.
void Mechanics::Populate()
{
ifstream skillstream("Skills.txt");
if(skillstream.is_open())
{
while(skillstream.good())
{
Skill *newskill;
int indexval;
string skillindex;
string skillname;
string skilldescription;
cout << "TP4" << endl; //TEST POINT
getline(skillstream, skillindex);
cout << skillindex;
getline(skillstream, skillname);
cout << skillname;
getline(skillstream, skilldescription);
cout << skilldescription; cout << endl;
indexval = atoi(skillindex.c_str());
newskill = new Skill(skillname, skilldescription,indexval);
//cout << "TP5" << endl; //TEST POINT
if(newskill == NULL) cout << "NULL!!!";
addSkill(newskill);
}
}
ifstream itemstream("Items.txt");
if(itemstream.is_open())
{
while(itemstream.good())
{
Item *newitem;
int indexval;
string skillindex;
string skillname;
string skilldescription;
string abilitydescription;
string valueSTR;
string typeSTR;
int value;
int type;
int numeric[5];
// cout << "TP4" << endl; //TEST POINT
getline(itemstream, skillindex);
// cout << skillindex;
getline(itemstream, skillname);
// cout << skillname;
getline(itemstream, skilldescription);
// cout << skilldescription;
getline(itemstream, abilitydescription);
getline(itemstream, valueSTR);
value = atoi(valueSTR.c_str());
getline(itemstream,typeSTR);
type = atoi(typeSTR.c_str());
for (int i=0; i < 5; i++)
{
string numericSTR;
getline(itemstream,numericSTR);
numeric[i]=atoi(numericSTR.c_str());
}
indexval = atoi(skillindex.c_str());
newitem = new Item(indexval, skilldescription, skillname, abilitydescription, value, type, numeric);
//cout << "TP5" << endl; //TEST POINT
// if(newskill == NULL) cout << "NULL!!!";
addItem(newitem);
}
}
The function that's supposed to actually add a skill to the skill list:
void Mechanics::addSkill(Skill *nskill)
{
Skill *temp = FirstSkill;
if(FirstSkill == NULL)
{
FirstSkill = nskill;
//cout << "TP1" << endl; //TEST POINT
//FirstSkill->printname();
}
else
{
while((temp->next != NULL))
{
temp = temp-> next;
//cout << "TP2" << endl; //TEST POINT
//temp->printname(); cout << endl;
}
if (FirstSkill != NULL)
{
temp->next = nskill;
nskill->next = NULL;
}
}
}
The code that I have is somewhat extensive so I'm only going to include the blocks which are potentially interacting with the function that's throwing up the error.
Thanks in advance for reading through this, and any assistance you're able to offfer, I've been banging my head against this for about 6 hours now and I've lost the perspective to actually track this one down.