How do I write a parser in C or Objective-C withou

Question

I am trying to make a calculator in C or Objective-C that accepts a string along the lines of

8/2+4(3*9)^2

and returns the answer 2920. I would prefer not to use a generator like Lex or Yacc, so I want to code it from the ground up. How should I go about doing this? Other than the Dragon book, are there any recommended texts that cover this subject matter?

Answer 1

If I remember correctly, you can solve this problem with two stacks, one for the operators, the other for the operands.

// OPTR stack: store operators
// OPND stack: store operands
// OP: predefined set of operators
OperandType EvaluateExpression(){  

   InitStack(OPET);Push(OPTR,'#');  
   initStack(OPND);c=getchar();  
   while(c!='#'||GetTop(OPTR)!='#'){  
     if(!In(c,OP)){Push((OPND,c);c=getchar();} //Push to stack if not operator
     else 
       switch(Precede(GetTop(OPTR),c){
         //Top element in stack has a lower priority  
         case '<':
               Push(OPTR,c); c=getch();  
               break;
         case '=':
               Pop(OPTR,x); c=getch();  
               break;
         //Pop top element and push back the calculated result 
         case '>':
               Pop(OPTR,theta);  
               Pop(OPND,b); Pop(OPND,a);  
               Push(OPND,Operate(a,theta,b));  
               break;  
         } 
     } 
     return GetTop(OPND);
   }