Reputation: 25
linked list bug
I was working on a program in C++ where I was supposed to play against a computer at hangman, and I am supposed to convert a particular .cpp file from using vectors to using Linked Lists. While I seem to almost be done, I keep getting this strange bug that causes my program to crash every time the computer guesses a letter incorrectly. I ran the program through the debugger and a message saying something along the lines of "program received SIGSEGV, segmentation fault" pops up along with this information in the call stack:
libstdc++6!_ZNSsC1ERKSs()
Guesser::guessACharacter(this=0x28feac) Line 56
main(argc=1, argv=0x7725a8) Line 30
The crash seems to lead back to a problem with line 9 of my Guesser::guessACharacter function, where I try to put information from the current node into a temporary variable. It looks like this:
string word = current->data;
Here is the full code:
#include "guesser.h"
#include "game.h"
#include <iostream>
#include <fstream>
using namespace std;
const std::string Guesser::alphabet = "abcdefghijklmnopqrstuvwxyz";
// Initialize the guesser for a game wit hthe indicated wordlength,
// using words from an indicated file.
Guesser::Guesser (int wordLength, const char* wordListFilename)
{
for (int i = 0; i < 26; ++i)
charactersTried[i] = false;
string word;
ifstream in (wordListFilename);
while (in >> word)
{
if (word.size() == wordLength)
{
// word is of desired length
if (word.find_first_not_of(alphabet) == string::npos) {
// word contains only lowercse alphabetics
//cerr<<"data before possible soln.:/n"<<possibleSolutions->data<<endl;
possibleSolutions->data = word;
//cerr<<"data after possible soln.:/n"<<possibleSolutions->data<<endl;
possibleSolutions = possibleSolutions -> next;
}
}
}
in.close();
}
/**
* Scan the words that are possible solutions so far, counting, for
* each letter not already tried, the number of words with that letter.
* Guess the letter that occurs in the most words.
*/
char Guesser::guessACharacter()
{
int counts[26];
for (int i = 0; i < 26; ++i)
counts[i] = 0;
// Count the number of words in which each letter can be found
for(ListNode* current = possibleSolutions; current != NULL; current = current-> next)
{
//cerr<<"data before possible soln.:/n"<<possibleSolutions->data<<endl;
string word = current->data;
//cerr<<"data in word:/n"<<word<<endl;
for (char c = 'a'; c <= 'z'; ++c)
{
if (!charactersTried[c- 'a'])
{
// Character c has not been tried yet
if (word.find(c) != string::npos)
// c is in this word
++counts[c - 'a'];
}
}
}
// Find the character that occurs in the most words
char guess = ' ';
int maxSoFar = -1;
for (char c = 'a'; c <= 'z'; ++c)
{
if (counts[c - 'a'] > maxSoFar)
{
guess = c;
maxSoFar = counts[c - 'a'];
}
}
if (maxSoFar <= 0)
{
guess = 'a';
while (charactersTried[guess-'a'])
++guess;
}
charactersTried[guess-'a'] = true;
return guess;
}
/**
* Following a successful guess of a letter in the word, make a pass through
* the possibleSolutions, removing all words that do not contain the
* guess character in the positions indicated in wordSoFar.
*/
void Guesser::characterIsInWord (char guess, const string& wordSoFar)
{
ListNode* remainingSolutions = 0;
for(ListNode* current = possibleSolutions; current != NULL; current = current-> next)
{
string wd = current-> data;
bool OK = true;
for (int k = 0; OK && k < wordSoFar.size(); ++k)
{
if (wordSoFar[k] == guess)
{
if (wd[k] != guess)
{
OK = false;
}
}
}
if (OK)
{
//cerr << "Keeping " << wd << endl;
remainingSolutions->data = wd;
//possibleSolutions->next = remainingSolutions;
}
}
possibleSolutions = remainingSolutions;
}
/**
* Following a mistaken guess of a letter in the word, make a pass through
* the possibleSolutions, removing all words that contain the
* guess character.
*/
void Guesser::characterIsNotInWord (char guess)
{
ListNode* remainingSolutions;
for(ListNode* current = possibleSolutions; current != NULL; current = current-> next)
{
string wd = current->data;
if (wd.find(guess) == string::npos)
{
remainingSolutions = new ListNode(wd, remainingSolutions);
}
}
possibleSolutions = remainingSolutions;
}
/**
* Guesser has lost the game. Look at the provider's actual word
* and gripe a bit about losing.
*/
void Guesser::admitToLoss (std::string actualWord, const string& wordSoFar)
{
bool match = actualWord.size() == wordSoFar.size();
for (int i = 0; match && i < actualWord.size(); ++i)
{
match = wordSoFar[i] == Game::FILL_CHARACTER ||
wordSoFar[i] == actualWord[i];
}
if (!match)
{
cout << "Ummm...your word '" << actualWord
<< "' does not match the patterh '"
<< wordSoFar <<"'.\nDid you make a mistake somewhere?"
<< endl;
}
else
{
for (int i = 0; match && i < actualWord.size(); ++i)
{
if (wordSoFar[i] == Game::FILL_CHARACTER
&& charactersTried[actualWord[i]-'a'])
{
cout << "Did you forget to mention the '"
<< actualWord[i]
<< "' in position " << i+1 << "?"
<< endl;
return;
}
}
for (ListNode* current = possibleSolutions;(!match) && current !=
NULL; current = current-> next)
{
match = (actualWord == current->data);
current = current->next;
match = match && (current != 0);
}
//match = match && (current != 0);
if (match)
{
cout << "OK, I might have guessed that eventually." << endl;
}
else
{
cout << "Interesting, I don't know that word. Are you sure you\n"
<< "spelled it correctly?." << endl;
}
}
}
I feel like the answer to this is staring me right in my face, but I just can't figure it out. I was wondering if someone else can easily spot what I'm doing wrong here.
Upvotes: 2
Views: 287
Answers (1)
Reputation: 6451
In the following code snippet
ListNode* remainingSolutions = 0;
for(ListNode* current = possibleSolutions; current != NULL; current = current-> next)
{
....
if (wordSoFar[k] == guess)
{
if (wd[k] != guess)
{
OK = false;
}
}
}
if (OK)
{
remainingSolutions->data = wd;
}
}
possibleSolutions = remainingSolutions;
remainingSolutions will be 0 or NULL if accessed and that can be one of the reasons.
Also at the end it will assign NULL to possibleSolutions(which I assume is the head of linkedlist) so the next time you access it, maybe by calling guessACharacter(), it will cause segmentation fault.
Upvotes: 2