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how would i replace all c style strings with the string class i would it look.

I will show u my code


double g_dblLastResult;

double EvaluateExpression ( Expression* pExpr )
{
switch(pExpr->op)
{
case OP_ADD : g_dblLastResult = pExpr->leftValue + pExpr->rightValue; break;
case OP_SUB : g_dblLastResult = pExpr->leftValue - pExpr->rightValue; break;
case OP_MULT: g_dblLastResult = pExpr->leftValue * pExpr->rightValue; break;
case OP_DIV : g_dblLastResult = pExpr->leftValue / pExpr->rightValue; break;
};

return g_dblLastResult;
};

int GetOperator ( char op )
{
switch(op)
{
case '+' : return OP_ADD;
case '-' : return OP_SUB;
case '*' : return OP_MULT;
case '/' : return OP_DIV;
};

return 0;
};

Expression* ParseExpression ( LPCTSTR lpstrExpression )
{
//Temporary cache for values
char szBuffer[100] = {0};
LPTSTR pBuffer = szBuffer;

Expression* pExpr = new Expression;
char ch;
enum STATE { FIRST_VALUE, OP, SECOND_VALUE, DONE } state = FIRST_VALUE;

//State machine - either we are looking for values or we are looking for operations
//Skip spaces and tabs
size_t nIdx = 0;
size_t nLen = strlen(lpstrExpression);
while (nIdx < nLen)
{
ch = lpstrExpression[nIdx];
switch(state)
{
case FIRST_VALUE :
case SECOND_VALUE :
{
//Store the value as needed
if (isdigit(ch))
{
*pBuffer = ch;
++pBuffer;

++nIdx; //Next character
} else if (((ch == 'A') || (ch == 'a')) && (nIdx == 0))
{
//Special of the special value of ANS
char chN = lpstrExpression[nIdx+1];
char chS = lpstrExpression[nIdx+2];
if (((chN == 'N') || (chN == 'n')) &&
((chS == 'S') || (chS == 's')))
{
//Use the previously stored value and move to the next state
if (state == FIRST_VALUE)
{
pExpr->leftValue = g_dblLastResult;
state = OP;
} else
{
pExpr->rightValue = g_dblLastResult;
state = DONE;
};

nIdx += 3; //Skip characters
} else
return NULL;
} else
{
//Convert the value to a double and enter op state
if (state == FIRST_VALUE)
{
pExpr->leftValue = atof(szBuffer);
state = OP;
} else
{
pExpr->rightValue = atof(szBuffer);
state = DONE;
};
};

break;
};

case OP :
{
pExpr->op = GetOperator(ch);
if (pExpr->op < 0)
return NULL;

//Return to value state
state = SECOND_VALUE;

//Reset temporary buffer
memset(szBuffer, 0, sizeof(szBuffer));
pBuffer = szBuffer;

++nIdx; //Next character
break;
};
};
};

//Whatever is left is the second value
pExpr->rightValue = atof(szBuffer);

return pExpr;
};

int main ( void )
{
char buffer[100];

while(true)
{
printf("Enter an expression (i.e. 5 + 4). Previous answer is available as ANS: ");
std::cin >> buffer;
if (strcmpi(buffer, "Q") == 0)
break;

Expression* pExpr = ParseExpression(buffer);
if (pExpr != NULL)
{
printf("%g\n", EvaluateExpression(pExpr));
delete pExpr;
} else
printf("Error parsing expression.\n");
};

return 0;
}