C++ program ends for no reason ? (Hash table) - c++

Question

I'm trying to implement hash table with Separate chaining collision resolution and I have a problem.
This is my code (little modified to simplify, but error still same):
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <fstream>
#include <cstring>
#include <string>
using namespace std;
int ascii(char character)
{
return character;
}
int hashFunction(string word, int num)
{
char* str = new char[word.length() + 1];
strcpy(str, word.c_str());
return ((3 * ascii(str[0]) + 5 * ascii(str[1]))) % num;
}
typedef struct tab
{
string data;
struct tab* next;
}Node;
typedef struct link
{
Node* head;
Node* tail;
}List;
List* createList()
{
List* list = new List();
if (list)
{
list->head = NULL;
list->tail = NULL;
}
return list;
}
void insert(List* list, string data)
{
//if list is empty
if (list->head == NULL) //!!!!!!!!!!!!!!!!ERROR OCCURE HERE !!!!!!!!!!!!!!!
{
list->head = new Node();
list->head->data = data;
list->head->next = NULL;
list->tail = list->head;
}
//if list already contains some data
else
{
list->tail->next = new Node();
list->tail->next->data = data;
list->tail = list->tail->next;
list->tail->next = NULL;
}
}
int main(int argc, char* argv[])
{
int size = 8; //Size of hash table (number of indexes)
List* table[12];
string A[8] = { "hello","world","car","notebook","science","starwars","lollypop","anything" };
//Insert elements from array A into a hash table
int index;
for (int i = 0; i < size; i++)
{
index = hashFunction(A[i], size);
if (table[index] == NULL)
table[index] = createList();
insert(table[index], A[i]);
}
return 0;
}
When I run the .exe file (or start from cmd), program ends up with message that app.exe has stopped working. I tried debug the program and got this:
http://imgur.com/a/yOhRV
Can anyone help me how to fix this? I've figured out problem must be in insert() function, probably in the condition, but I don't know what is wrong.

Answer 1

you derefernce a pointer without checking it: if (list->head == NULL)...
What you are doing here, is taking list and cheking if the value pointed by it is NULL, but since you haven't checked if (list) then it's possible that list == NULL and that causes segfault when dereferencing it

Answer 2

You are declaring List* table[12] but it is never initialized. So it does contain garbage.
You have to do the following in order to initialize it: List* table[12] = {NULL};

Answer 3

As a general rule you should never have uninitialized variables in your code (unless for optimization purposes when you know exactly what you are doing).
Add default contructors to your structs and make use of initialization list. Also keep you variables as local as possible (move index inside the loop).
You don't need ascii() cause char is integer type. char+char and char*int get promoted to int.