segmentation fault in c++

rushman8282 · #1 25-Jan-2005, 00:50

hi,
i dunno why i get this segmentation fault whenever i run the codes.
this program calculates a fully bracketed mathematical expression recursively. i.e. (1+(2*(2*4)))
can anybody tell me how to fix the segmentation fault part plz?

CPP / C++ / C Code:

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

int eval(char expression[]);
int eval(char expression[])
{
	int expres_size;
	int new_expres_size;
	int bracket;
	int oper_position;
	char result;
	char interval1[100];	
	char interval2[100];

	expres_size = strlen(expression);
	
	if (expres_size == 1)
	{
		return (int)expression[0]-(int)'0';
	}

	else
	{	
		if (expression[0] == '(' )
        	{
			// taking out the brackets () by assigning to another array
                	char new_expression[expres_size-2];

	                for (int r=0; r < (expres_size-2); r++)
	                {
        	                new_expression[r] = expression[r+1];
	                }
                
			new_expres_size = strlen(new_expression);
			expres_size = new_expres_size;
			expression = new_expression;
		}
	
		bracket=0;	
	
		for (int j= 0; j < expres_size; j++)
		{
			if (expression[j] == '(' )
				bracket += 1;
			if (expression[j] == ')' )
				bracket -= 1;
			if (bracket == 0)
			{
				switch (expression[j])
				{
					case '*':
						oper_position = j;											break;	
					case '/':
						oper_position = j;
						break;
					case '+':
						oper_position = j;
						break;
					case '-':
						oper_position = j;
						break;
				}
			}
		}

		// splitting the expression character array into 2 arrays
		for (int k= 0; k < oper_position-1; k++)
		{
			interval1[k] = expression[k];
		}

		for (int p= (oper_position+1); p < expres_size; p++)	
		{
			interval2[p] = expression[p];
		}	
			
		switch (expression[oper_position])
	        {
       			case '*':
       				return (eval(interval1))*(eval(interval2)); 
	                        break;
               		case '/':
	                        return (eval(interval1))/(eval(interval2));
               		        break;
	                case '+':
               		        return (eval(interval1))+(eval(interval2));
	                        break;
               		case '-':
	                        return (eval(interval1))-(eval(interval2));
               		        break;
		}	
	}	
}

int main()
{	
	// declaring variables
	char inExpress[100];
	int str_size;
	
	// display out messages
	cout << "This program calculates a given mathematical"; 
	cout << " expression.\n";
	cout << "Please enter a mathematical expression.\n";
	cin >> inExpress;
	
	str_size = strlen(inExpress);
	cout << str_size;
	cout << "\n";

	int countBracket = 0;
	char fixExpress[str_size];

	for (int i= 0; i < str_size; i++)
	{
	//	fixExpress[i] = inExpress[i];
	
		if (inExpress[i] == '(' ) 
			countBracket += 1;

		if (inExpress[i] == ')' )
			countBracket -= 1;
	}
	
	if (countBracket != 0)
	{	
		cout << "Unmatched brackets!!\n";
		cout << "Please re-enter the expression.\n";
	}
	
	cout << eval(inExpress);	
	
	return 0;
}

dsmith · #2 25-Jan-2005, 11:42

Hello rushman8282. Welcome to GIDForums™.

Your seg fault is coming because of the way that you split your strings and recall eval. You never null terminate your strings and then you are passing them back to eval at which point strlen looks for a null termination and doesn't find it until tromping all through unallocated memory. In addition your counting method was not quite right. Here is a replacement that is a bit closer:

CPP / C++ / C Code:

    int k;
    for (k= 0; k < oper_position; k++)
    {
      interval1[k] = expression[k];
    }
	interval1[k] = 0;
	cout << interval1 << "\n";
    for (int p= (oper_position+1),k=0; p < expres_size; p++,k++)
    {
      interval2[k] = expression[p];
    }
	interval2[k]=0;
	cout << interval2 << "\n";

You will notice that I added a couple of couts just to tell me what was going on. With this fix I got the rudimentary equation of 3+2 to return the proper value, but I think there are other more deep rooted problems with this routine.

My advice is to dump a bunch of print commands into this thing to try to figure out where things are going wrong.

In addition if you are using linux, I find that valgrind is an invaluable tool to track down memory leaks at exactly where they are happening.

Good luck.

rushman8282 · #3 26-Jan-2005, 03:38

thank you dsmith,
i took a look and changed main algorithm.
i tested out and everything works fine except the last switch part.
all i need to do now is to call eval function recursively but not successful..
T.T what should i do??

CPP / C++ / C Code:

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

int eval(char expression[]);

int eval(char expression[])
{
	int expres_size;
	int new_expres_size;
	int bracket;
	int oper_position;
	int size1;
	int size2;
	char interval1[100];	
	char interval2[100];

	expres_size = strlen(expression);
	
	if (expres_size == 1)
	{
		return (int)expression[0]-(int)'0';
	}

	else
	{	
		if (expression[0] == '(')
        	{
	                for (int r=0; r < (expres_size-2); r++)
	                {
        	                expression[r] = expression[r+1];
	                }
                	
			expres_size = expres_size-2;
		}
	
		cout << expres_size;
		cout << "\n";
				
		for (int i=0; i < expres_size; i++)
			cout << expression[i];
			
		
		// initializing bracket counter variable
		bracket = 0;
			
		for (int j= 0; j < expres_size; j++)
		{
			if (expression[j] == '(')
				bracket += 1;
			if (expression[j] == ')')
				bracket -= 1;
			if ((bracket == 0) && (expression[j] == '*'))
				oper_position = j;			
			if ((bracket == 0) && (expression[j] == '/'))
				oper_position = j;
			if ((bracket == 0) && (expression[j] == '+'))
				oper_position = j;
			if ((bracket == 0) && (expression[j] == '-'))
				oper_position = j;									
		}			
		
		
		// splitting the expression character array into 2 arrays
		for (int k = 0; k < oper_position; k++)
		{
			interval1[k] = expression[k];		
			cout << interval1[k];
		}
	
		for (int p = (oper_position+1), k = 0; p < expres_size; p++,k++)	
		{
			interval2[k] = expression[p];
			cout << interval2[k];
		}	


		switch (expression[oper_position])
	        {
     			case '*':
       				return eval(interval1)*eval(interval2); 
				break;		                        
             		case '/':
	                        return eval(interval1)/eval(interval2);
             		        break;
	                case '+':
               		        return eval(interval1)+eval(interval2);
	                        break;
               		case '-':
	                        return eval(interval1)-eval(interval2);
             		        break;
		}
	
	}
}

int main()
{	
	// declaring variables
	char inExpress[100];
	int str_size;
	
	// display out messages
	cout << "This program calculates a given mathematical"; 
	cout << " expression.\n";
	cout << "Please enter a mathematical expression.\n";
	cin >> inExpress;
	
	str_size = strlen(inExpress);
	
	int countBracket = 0;

	for (int i= 0; i < str_size; i++)
	{
		if (inExpress[i] == '(') 
			countBracket += 1;

		if (inExpress[i] == ')')
			countBracket -= 1;
	}
	
	if (countBracket != 0)
	{	
		cout << "Unmatched brackets!!\n";
		cout << "Please re-enter the expression.\n";
	}
	
	cout << eval(inExpress);
	cout << "\n";
	
	return 0;
}

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segmentation fault in c++

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