How to Write a Spelling Corrector
Greetings, i wrote a C++ version of the Norvig’s spelling corrector. All theory and other language implementations can be found in his website. Here’s my reduced version with less number of lines, that scored 66 lines of code (Implementation in the same source that the declaration): SpellCorrector in C++. Or just take a look at the complete version:
If you want to use the Spelling Corrector in your project feel free, but just send me an email to let me know about it.
SpellCorrector.h
/*
* SpellCorrector.h
*
* Copyright (C) 2007 Felipe Farinon <felipe.farinon@gmail.com>
*
* Version: 1.4
* Author: Felipe Farinon <felipe.farinon@gmail.com>
* Maintainer: Felipe Farinon <felipe.farinon@gmail.com>
* URL: http://scarvenger.wordpress.com/
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*
* Commentary:
*
* See http://scarvenger.wordpress.com/.
*
* Code:
*/
#ifndef _SPELLCORRECTOR_H_
#define _SPELLCORRECTOR_H_
#include <vector>
#include <tr1/unordered_map>
class SpellCorrector
{
private:
typedef std::vector<std::string> Vector;
typedef std::tr1::unordered_map<std::string, int> Dictionary;
Dictionary dictionary;
void edits(const std::string& word, Vector& result);
void known(Vector& results, Dictionary& candidates);
public:
void load(const std::string& filename);
std::string correct(const std::string& word);
};
#endif
SpellCorrector.cpp
/*
* SpellCorrector.cpp
*
* Copyright (C) 2007 Felipe Farinon <felipe.farinon@gmail.com>
*
* Version: 1.4
* Author: Felipe Farinon <felipe.farinon@gmail.com>
* Maintainer: Felipe Farinon <felipe.farinon@gmail.com>
* URL: http://scarvenger.wordpress.com/
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*
* Commentary:
*
* See http://scarvenger.wordpress.com/.
*
* Code:
*/
#include <fstream>
#include <string>
#include <algorithm>
#include <iostream>
#include "SpellCorrector.h"
using namespace std;
bool sortBySecond(const pair<std::string, int>& left, const pair<std::string, int>& right)
{
return left.second < right.second;
}
char filterNonAlphabetic(char& letter)
{
if (isalpha(letter))
return tolower(letter);
return '-';
}
void SpellCorrector::load(const std::string& filename)
{
ifstream file(filename.c_str(), ios_base::binary | ios_base::in);
file.seekg(0, ios_base::end);
int length = file.tellg();
file.seekg(0, ios_base::beg);
string line(length, '0');
file.read(&line[0], length);
transform(line.begin(), line.end(), line.begin(), filterNonAlphabetic);
string::size_type begin = 0;
string::size_type end = line.size();
for (string::size_type i = 0; 
{
while (line[++i] == '-' && i < end); //find first '-' character
if (i >= end) { break; }
begin = i;
while (line[++i] != '-' && i < end); //find first not of '-' character
dictionary[line.substr(begin, i - begin)]++;
}
}
string SpellCorrector::correct(const std::string& word)
{
Vector result;
Dictionary candidates;
if (dictionary.find(word) != dictionary.end()) { return word; }
edits(word, result);
known(result, candidates);
if (candidates.size() > 0) { return max_element(candidates.begin(), candidates.end(), sortBySecond)->first; }
for (unsigned int i = 0;i < result.size();i++)
{
Vector subResult;
edits(result[i], subResult);
known(subResult, candidates);
}
if (candidates.size() > 0) { return max_element(candidates.begin(), candidates.end(), sortBySecond)->first; }
return "";
}
void SpellCorrector::known(Vector& results, Dictionary& candidates)
{
Dictionary::iterator end = dictionary.end();
for (unsigned int i = 0;i < results.size();i++)
{
Dictionary::iterator value = dictionary.find(results[i]);
if (value != end) candidates[value->first] = value->second;
}
}
void SpellCorrector::edits(const std::string& word, Vector& result)
{
for (string::size_type i = 0;i < word.size(); i++) result.push_back(word.substr(0, i) + word.substr(i + 1)); //deletions
for (string::size_type i = 0;i < word.size() - 1;i++) result.push_back(word.substr(0, i) + word[i+1] + word.substr(i + 2)); //transposition
for (char j = 'a';j <= 'z';++j)
{
for (string::size_type i = 0;i < word.size(); i++) result.push_back(word.substr(0, i) + j + word.substr(i + 1)); //alterations
for (string::size_type i = 0;i < word.size() + 1;i++) result.push_back(word.substr(0, i) + j + word.substr(i) ); //insertion
}
}
I am working in some optimizations yet but it should be ready to use. Using the corrector is pretty easy, just download the words database, and see the example:
#include "SpellCorrector.h"
using namespace std;
int main()
{
SpellCorrector corrector;
corrector.load("big.txt");
string request;
while (request != "quit"
{
cout << "Type one word\n";
cin >> request;
string correct(corrector.correct(request));
if (correct != ""
cout << "Did you mean: " << correct << "?\n\n\n";
else
cout << "No corrections avaiable\n\n\n";
}
return 0;
}
