Hey I'm having problems with this class I wrote. It's for the Boyer Moore Algorithm, I basically copied my working Horspool's Algorithm class and added the suffix table part, that's the only change. I haven't gotten the BoyerMooreAlgorithm to run yet. So if you notice I have a constructor which gets passed a string called pattern. It's saved in this->pattern which I can access throughout everything in that constructor and any functions the constructor calls for setup. However, when I call the search algorithm, the (this->pattern).length() errors and it appears from testing that this is do to this->pattern being null. For the life of me I can't figure this out because all that I do is the following in main.cpp: BoyerMooreAlgorithm* boyer = new BoyerMooreAlgorithm( "hello" ); boyer->search("Text to find hello in"); This really isn't a boyer moore question since I will be able to implement it fine once I can do a test run of this code. I just need to figure out what happens to the this->pattern string. Any help would be so gratefully appreciated and I'm just giving a thanks ahead of time for all of you for even looking at it let alone if you can help me. Thanks. ---- BoyerMooreAlgorithm.h

#pragma once
#include <string>
#include <iostream>
using namespace std;

class BoyerMooreAlgorithm
{
private:
	string pattern;
	int shiftTable[256];
	int suffixTable[256];
    
	void findShiftTable(void);
	void findSuffixTable(void);
	
public:
	BoyerMooreAlgorithm(string);
	~BoyerMooreAlgorithm(void);
	int search(string);
};

---- BoyerMooreAlgorithm.cpp

#include "boyermoorealgorithm.h"

BoyerMooreAlgorithm::BoyerMooreAlgorithm( string pattern )
{
	this->pattern = pattern;
	this->findShiftTable();
	this->findSuffixTable();
}

BoyerMooreAlgorithm::~BoyerMooreAlgorithm(void)
{
}

int BoyerMooreAlgorithm::search( string text )
{
	int k;
	bool match = true;
	int i = ((this->pattern).length())-1;
	int patternLength = this->pattern.length();
	
	while( i < text.length() )
	{
		match = true;
		k = 0;

		for( ; k < patternLength && match ; k++ )
		{
			if( !( this->pattern[ patternLength - k - 1 ] == text[ i - k ] ) )
			{
                match = false;
			}
		}

		if( k == (patternLength - 1) && match )
		{ return i - patternLength + 1; }
		else
		{
			i = i + max( max( this->shiftTable[(int)text[i]] , 1 ) , this->suffixTable[k] );;
		}

	}

	return -1;
}

void BoyerMooreAlgorithm::findShiftTable( void )
{
	int letter;
	int patternLength = (this->pattern).length();


	/*** Initialize Shift Table ***/
	for( int i = 0 ; i < 257 ; i++ )
	{
		this->shiftTable[i] = patternLength;
	}

	/*** Compute Shift Table ***/
	for( int i = 0 ; i < patternLength - 1 ; i++ )
	{
		this->shiftTable[(int)this->pattern[i]] = patternLength - 1 - i;
	}

}

void BoyerMooreAlgorithm::findSuffixTable( void )
{
	int letter;
	int patternLength = (this->pattern).length();
	string pattern = this->pattern;

	for( int i = 1 ; i < patternLength && i < 255 ; i++ ) {
		int temp = pattern.rfind( pattern.substr( patternLength - i , i ) , patternLength - i - 1 );

		if( temp == string::npos ) {
			suffixTable[i] = patternLength - 1;
		} else {
			suffixTable[i] = patternLength - temp - i;
		}
	}	
}

edit: Oh and I'm compiling with Visual Studio .NET hence the pragma once [edited by - Hexxagonal on April 20, 2004 3:17:28 PM] [edited by - Hexxagonal on April 20, 2004 3:18:16 PM]

thanks i figured it out, for some reason, the array i store the return int in is causing the null.

edit: return

[edited by - Hexxagonal on April 20, 2004 3:38:03 PM]

actually we get downgraded here @ my university if we don''t use this... they consider it improper coding style. plus I have to do that in PHP, which is what i''m used to so it just sits well with me. and pattern is a private member.

quote:Original post by Hexxagonal
actually we get downgraded here @ my university if we don''t use this...

That''s a really strange convention, and I have never seen it used before in a real project, except in some some examples that shows how to do it.

I have a few suggestions for you. You should almost always pass strings by reference, not by value. The constructor and search method should look like this:

BoyerMooreAlgorithm::BoyerMooreAlgorithm(string & pattern) // Note the ''&''
BoyerMooreAlgorithm::search(string & text)

And also, the following line can be simplified from

int i = ((this->pattern).length())-1;

to

int i = this->pattern.length()-1;

because the dot and arrow operators have equal precedence and bind left-to-right. They both have equal precedence with parentheses and brackets, too, so you can have a nasty expression such as

myClass.getObj(5).getPointer->getArray[6].activate();

and it will evaluate just as you would expect.