parse.cpp

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// parse.cpp
// evaluates arithmetic expressions composed of 1-digit numbers
#include <iostream>
#include <cstring>                   //for strlen(), etc
using namespace std;
const int LEN = 80;    //length of expressions, in characters
const int MAX = 40;    //size of stack
////////////////////////////////////////////////////////////////
class Stack
   {
   private:
      char st[MAX];                  //stack: array of chars
      int top;                       //number of top of stack
   public:
      Stack()                        //constructor
         { top = 0; }
      void push(char var)            //put char on stack
         { st[++top] = var; }
      char pop()                     //take char off stack
         { return st[top--]; }
      int gettop()                   //get top of stack
         { return top; }
   };
////////////////////////////////////////////////////////////////
class express                        //expression class
   {
   private:
      Stack s;                       //stack for analysis
      char* pStr;                    //pointer to input string
      int len;                       //length of input string
   public:
      express(char* ptr)             //constructor
         {
         pStr = ptr;                 //set pointer to string
         len = strlen(pStr);         //set length
         }
      void parse();                  //parse the input string
      int solve();                   //evaluate the stack
   };
//--------------------------------------------------------------
void express::parse()                //add items to stack
   {
   char ch;                          //char from input string
   char lastval;                     //last value
   char lastop;                      //last operator

   for(int j=0; j<len; j++)          //for each input character
      {
      ch = pStr[j];                  //get next character

      if(ch>='0' && ch<='9')         //if it's a digit,
         s.push(ch-'0');             //save numerical value
                                     //if it's operator
      else if(ch=='+' || ch=='-' || ch=='*' || ch=='/')
         {
         if(s.gettop()==1)           //if it's first operator
            s.push(ch);              //put on stack
         else                        //not first operator
            {
            lastval = s.pop();       //get previous digit
            lastop = s.pop();        //get previous operator
            //if this is * or / AND last operator was + or -
            if( (ch=='*' || ch=='/') &&
                (lastop=='+' || lastop=='-') )
               {
               s.push(lastop);       //restore last two pops
               s.push(lastval);
               }
            else                     //in all other cases
               {
               switch(lastop)        //do last operation
                  {                  //push result on stack
                  case '+': s.push(s.pop() + lastval); break;
                  case '-': s.push(s.pop() - lastval); break;
                  case '*': s.push(s.pop() * lastval); break;
                  case '/': s.push(s.pop() / lastval); break;
                  default:  cout << "\nUnknown oper"; exit(1);
                  }  //end switch
               }  //end else, in all other cases
            s.push(ch);              //put current op on stack
            }  //end else, not first operator
         }  //end else if, it's an operator
      else                           //not a known character
         { cout << "\nUnknown input character"; exit(1); }
      }  //end for
   }  //end parse()
//--------------------------------------------------------------
int express::solve()                 //remove items from stack
   {
   char lastval;                     //previous value

   while(s.gettop() > 1)
      {
      lastval = s.pop();             //get previous value
      switch( s.pop() )              //get previous operator
         {                           //do operation, push answer
         case '+': s.push(s.pop() + lastval); break;
         case '-': s.push(s.pop() - lastval); break;
         case '*': s.push(s.pop() * lastval); break;
         case '/': s.push(s.pop() / lastval); break;
         default:  cout << "\nUnknown operator"; exit(1);
         }  //end switch
      }  //end while
   return int( s.pop() );            //last item on stack is ans
   }  //end solve()
////////////////////////////////////////////////////////////////
int main()
   {
   char ans;                         //'y' or 'n'
   char string[LEN];                 //input string from user
   
   cout << "\nEnter an arithmetic expression"
           "\nof the form 2+3*4/3-2."
           "\nNo number may have more than one digit."
           "\nDon't use any spaces or parentheses.";
   do {
      cout << "\nEnter expresssion: ";
      cin >> string;                        //input from user
      express* eptr = new express(string);  //make expression
      eptr->parse();                        //parse it
      cout << "\nThe numerical value is: " 
           << eptr->solve();                //solve it
      delete eptr;                          //delete expression
      cout << "\nDo another (Enter y or n)? ";
      cin >> ans;
      } while(ans == 'y');
   return 0;
   }