package Arithmetic;
import Stack.*;
/** PostfixTranslator translates an infix expression into postfix */
public class PostfixTranslator
{
  private StringReader reader;  // holds the input infix expression
  private Stack st;             // a stack to help with the translation

  /** Constructor PostfixTranslator initializes the translator
    * @param r - the object that holds the input infix expression */
  public PostfixTranslator(StringReader r)
  { reader = r;
    st = new Stack();
  }

  /** translate does the translation from infix to postfix
    * @return a string that is the postfix translation of the infix
    *   expression that was supplied to the constructor method
    * @throw RuntimeException if there is a syntax error in the infix
    *   expression */
  public String translate()
  { String answer = "";
    char c = reader.nextChar();
    while ( c != StringReader.END_CHAR )
          { if ( c >= '0' && c <= '9' )  // a numeral?
               { answer = answer + c; }
            else if ( c == '+'  ||       // an operator?
                      c == '-'  ||
                      c == '*'  ||
                      c == '/' )
                    { st.push(new Character(c)); }
           else if ( c == ')' )         // right paren means pop an operator
                   { Character C = (Character)(st.pop());
                     answer = answer + C.charValue(); }
           else if ( c == '(' )         // left paren has no significance
                   { }
           else { throw new RuntimeException("Syntax error in expression"); }
         
           c = reader.nextChar();
         }
    if ( ! st.isEmpty() )
       { throw new RuntimeException("Syntax error in expression"); }
    return answer;
  }
}