Member Avatar for Daria Shmaria

I'm writing a spellchecking program using a hash table. The hash table (a char * array) serves as the dictionary, storing words inputted from the file wordlist.txt. When I call the lookup function, a floating point exception occurs. Any suggestions you have on how to find what's causing the exception would be greatly appreciated.


Descriptions of the attached files:

1. spellcheck.cpp contains the main function, calling the Dictionary class member function "add" on every word in the file wordlist.txt.
2. hashTable.cpp contains definitions for functions in the Dictionary class. The "add" and "getIndex" functions are the only ones that are functional right now.
3. hashTable.h contains declarations.
4. wordlist.txt contains all the words that need to go in the dictionary.
5. input.txt is the input to the program, but isn't actually being used for anything yet.

#include "hashTable.h"
#include "string.h"
#include <iostream>

using namespace std;

Dictionary::Dictionary()
{
  char* htl[139079];
  int status[139079];
  for(int i = 0; i < 139079; i++)
  {
    status[i] = 0;
  }
}

Dictionary::~Dictionary()
{
  // deallocate memory of ht and status arrays
  // delete[] ht;
  // delete[] status;
}

// assignemnt operator overloading 
/*
Dictionary& Dictionary::operator=(Dictionary& obj)
{

}
*/

unsigned int Dictionary::add(char* item)
{

  int r = 139067; 
  int m = 139079;

  int length;
  length = strlen(item); // determine length of item
  unsigned int myIndex; 
  myIndex = Dictionary::getIndex(item, length); 
  //cout << "myIndex = " << myIndex << endl;
  // check if the hash table at that index is available
  if((status[myIndex] == 0) || (status[myIndex] == 1)) // if the location is empty or deleted
    {  
      ht[myIndex] = item;
      cout << "word inserted. ht[" << myIndex << "] = " << ht[myIndex] << endl;
      status[myIndex] = 2;
    }
  else // if the location is already taken
    {
      int i = 1; // index of how many times you've attempted hashing the table
      // double hashing until the hash table location is empty
      //cout << "ht[myIndex] = " << ht[myIndex] << endl << "item = " << item << endl;
      while(status[myIndex] == 2)
	{
	  if(strcmp(ht[myIndex], item) == 0) // if the word has already been added, exit
	    {
	      //cout << "The item has already been inserted." << endl;
	      return 1;
	    }
	  else 
	    {
	      // rehash
	      //cout << "status = " << status[myIndex] << endl;
	      unsigned int originalIndex = myIndex;
	      myIndex = (originalIndex + (i * (r - (originalIndex % r))) ) % m;
	      i++;
	      //cout << "i incremented. i = " << i << endl;
	    }
	}
      ht[myIndex] = item;
      cout << "word inserted. ht[" << myIndex << "] = " << ht[myIndex] << endl;
      status[myIndex] = 2;      
    }
return 0;
}

int Dictionary::returnStatus(int index)
{
  cout << "Status[" << index << "] = " <<  status[index] << endl;
  return 0;
}

bool Dictionary::lookup(char* item) 
{
  // hash item
  int length;
  length = strlen(item); 
  unsigned int myIndex; 
  myIndex = Dictionary::getIndex(item, length); 
  cout << "Initial myIndex = " << myIndex << endl;
  unsigned int originalIndex = myIndex;     
  int i = 1; // index of how many times you've attempted hashing the table
  
  if(status[myIndex] == 0) // if the hash table is empty at that location
    {
      cout << "Item " << item << " does not exist." << endl;
      return false; // item does not exist
    }
  else if(status[myIndex] == 1) // if the entry at the location was deleted
    {
      // rehash again until you find it, or return to your original index
      myIndex = (originalIndex + (i * (r - (originalIndex % r)))) % m; // rehash
      while(((status[myIndex] == 2) || (status[myIndex] == 1))  && (myIndex != originalIndex))
      {
        if(strcmp(ht[myIndex], item) == 0) // if you've found the word
          {
            cout << "The word exists. Yay!" << endl;
            return true;
          }
        else
          {
            myIndex = (originalIndex + (i * (r - (originalIndex % r)))) % m; // hash it again
	    i++;
          }
      }
      cout << "The word does not exist in the dictionary." << endl;
      return false;
    }
  else if(status[myIndex] == 2) // if there's an entry there
    {
      // rehash again until you find it, or return to your original index
      myIndex = (originalIndex + (i * (r - (originalIndex % r))) ) % m; // rehash
      while(((status[myIndex] == 2) || (status[myIndex] == 1))  && (myIndex != originalIndex))
      {
        if(strcmp(ht[myIndex], item) == 0) // if you've found the word
          {
            cout << "The word exists. Yay!" << endl;
            return true;
          }
        else
          {
            myIndex = (originalIndex + (i * (r - (originalIndex % r))) ) % m; // hash it again
	    i++;
          }
      }
      cout << "The word does not exist in the dictionary." << endl;
      return false;
    }
}

int Dictionary::remove(char* item)
{
  cout << "Remove function called." << endl;
  return 0;
}

unsigned int Dictionary::getIndex(char * p, int len) // hash the item using a cyclic shift hash code
{
  unsigned int h = 0;
  for(int i = 0; i < len; i++)
    {
      h = (h<<5)|(h>>27);
      h += (unsigned int)p[i]; 
    }
  
  //cout << "getIndex function called." << endl;
  //cout << "h = " << h << endl;
  h = (h % 69539); // make sure the index is possible given the size of the hash table
  //cout << "h after modding = " << h << endl;
  return h; 
}
#ifndef _HASHTABLE_H
#define _HASHTABLE_H

class Dictionary
{
 public:
  Dictionary(); // constructor
  ~Dictionary(); // destructor
  // Dictionary& operator=(Dictionary& obj); // assignment operator overloading
  unsigned int add(char* item); // add an item
  int remove(char * item); // remove an item
  bool lookup(char * item); // find an item
  unsigned int getIndex(char * p, int len); // hash an item
  int returnStatus(int index);

 private:
  char* ht[139079];
  int status[139079];
  int m; // size of hash table for add formula
  int r; // prime number less than m in double hash function
};

#endif
In computer science, a data stucture is a wayof storing datta in a computur so that it can be used efficeintly.  Often a careffully chosen data structure willallow the moste efficient algorthm to be used.
#include "iostream"
#include "fstream"
#include "string.h"

#include "hashTable.h"

using namespace std;

int main(int argc, char** argv)
{
  if(argc != 2)
    {
      cerr << "Incorrect number of inputs." << endl;
      return -1;
    }

  // Read the input file

  ifstream myInput(argv[1]);
  
  if(myInput)
    {
      cout << "The input file was opened. Yay!" << endl;
      char firstInputLetter;
      myInput >> firstInputLetter;
      cout << "The first letter of the input file: " << firstInputLetter << "." << endl;
    }
  else if(!myInput)
    {
      cerr << "The input file could not be opened. Try again!" << endl;
      exit(1);
    }

  // Read the wordlist file

  ifstream wordlist("./wordlist.txt");


  Dictionary* htl;
  htl =new Dictionary();
  if(wordlist)
    {
      cout << "The wordlist file was opened." << endl;
      string line;
      cout << "made a string called line" << endl;
      wordlist.ignore(7, '\n');  // ignore the first entry (the number of words in the wordlist)
      cout << "ignored the first 7 chars" << endl;
      while(getline(wordlist, line, '\n'))
	{
	  //cout << "in the while loop" << endl;
	  char * convline = new char[line.size() + 1];
	  //cout << "made char * convline" << endl;
	  strcpy(convline, line.c_str());
	  //cout << "strcpy'd" << endl;
	  htl->add(convline);
	  //cout << "added " << convline << endl;

	  // delete convline; // caused segmentation fault
	}
    }
  else if(!wordlist)
    {
      cerr << "The wordlist could not be read." << endl;
      exit(2);
    }

  cout << "Checking if the item language exists" << endl;
  htl->lookup("language");

  cout << "Checking if the item beedril exists" << endl;
  htl->lookup("beedril");
  // call destructor 

  delete htl;
  return 0;
}

// at the end, go through entire hash table and delete memory pointed to by hash table
// OR allocate the memory in the hash table
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Member Avatar for Stinomus

First thing to look for are cases where you are maybe dividing by zero or attempting to take the square root of a negative number. Output your floating point values before using them as a sanity check.

Member Avatar for Salem

> Any suggestions you have on how to find what's causing the exception would be greatly appreciated.
Use a debugger.
It will catch the exception, and then show you the line of code that TRIGGERED the exception.

From there, you look back through the code to mentally construct how it got to that point until you find where the root CAUSE of the problem happened (this is the bug).

Fix the bug, and re-test.

Member Avatar for Daria Shmaria

Thank you both very much! You helped a lot.

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