Creating a function that converts to postfix

[superrun]

I am currently trying to create a calculator that takes an infix equation and converts it to postfix, then calculates. I've got the calculator to convert when fully parenthesized, but I cannot get the equation to convert it without full parenthesis. I have tried to impliment the conversion as shown below. The only problem is that it crashes, saying that deque iterator isn't dereferencable, when I do not have a deque or iterator in the problem. Any Input is greatly appreciated.

string convert_infix(string infix)
//Accepts an infix expression and converts it to rpn format
{
infix+="!";
const char DECIMAL = '.';
const char LEFT_PARENTHESIS = '(';
stringstream ins (infix);
stringstream outs;
stack<char> operators;
string n_equation;
char operate;
int number;
int prec_in=0, prec_top=1;

while(ins && ins.peek() != '!')
{
if(ins.peek() == LEFT_PARENTHESIS)
{
ins>>operate;
operators.push(operate);
}
else
{
if(isdigit(ins.peek()) || ins.peek()== DECIMAL)
{
ins>>number;
outs<<number;
outs<< " ";
}
else
{


if(strchr("+-*/%", ins.peek()) != NULL)
{
switch(ins.peek())
{
case '+': case '-' : prec_in=1;
break;
case '/': case '*': case '%': prec_in=2;
break;
}

switch(operators.top())
{
case '+': case '-' : prec_top=1;
break;
case '/' : case '*': case '%' : prec_top=2;
break;
}


while(!operators.empty() && operators.top() != '(' && prec_top>=prec_in)
{
outs<<operators.top();
operators.pop();
}
ins>>operate;
operators.push(operate);
}
else
{
if(ins.peek() == ')')
{
ins.ignore();
while(operators.top() != '(' && !operators.empty())
{
outs<<operators.top();
outs<<" ";
operators.pop();
}
operators.pop();
}
}
}
}
}
while(!operators.empty())
{
if(strchr("/*-+%",operators.top()) != NULL)
{
outs<<operators.top();
operators.pop();
}
}
n_equation=outs.str();
return n_equation;
}




double read_and_calc(string s)
//takes a rpn expression and calculates it
{

const char DECIMAL = '.';
stringstream ins (s);
deque<double> temp (100);
stack<double> numbers (temp);
double number;
double operand1;
double operand2;
char operate;

//Loop goes until there is a new line in the istream
while(ins && ins.peek() != '\n')
{
if((isdigit(ins.peek()) || (ins.peek() == DECIMAL)) && ins.peek() != ' ')
{
ins>>number;
numbers.push(number);
}
else
{
if(strchr("+-*/%", ins.peek()) != NULL)
{
operand2=numbers.top();
numbers.pop();
operand1=numbers.top();
numbers.pop();

ins>>operate;
switch(operate)
{
case '+': numbers.push(operand1+operand2);
break;
case '-': numbers.push(operand1-operand2);
break;
case '*': numbers.push(operand1*operand2);
break;
case '/': numbers.push(operand1/operand2);
break;
case '%': numbers.push((int)operand1%(int)operand2);
}
}
else
{
ins.ignore();
}
}
}
return numbers.top();
}


int main ()
{
displayIntro("Derek Krzysiak",__FILE__,"Calculates values and shows them in postfix format");

double result;
string entry;
int i=0;

cout<<"Please enter an expression. Enter ""!"" to end the program."<<endl;
getline(cin,entry);
while(entry!="!")
{
entry=convert_infix(entry);
result=read_and_calc(entry);
cout<<entry<<" = "<<result<<endl;
cout<<"Please enter an expression. Enter ""!"" to end the program."<<endl;
getline(cin,entry);
}

pause();
return 0;


Any input would be helpful.