public static int evalDigit() throws InvalidExpression{
                                                                                    // expects e[i] to be a digit
                                                                                    // returns the int value of the digit
                                                                                    // advances i to the position after the digit
        if (e[i]>='0' && e[i]<='9'){
          int result = e[i]-(int)'0';                                                // code for characters '0' to '9' are 48 to 57
          i++;
          return result;
        } else {
          throw new InvalidExpression("index "+i);
        }
      }

                                                                                    // YOU WILL MODIFY THIS METHOD TO WORK WITH TERMS BASED ON PARENTHESES
public static int evalTerm() throws InvalidExpression{
                                                                                    //returns the value of the term starting at index i
                                                                                    //for simple expressions, a term is a single digit
            return evalDigit();
      }

public static int evalExpr() throws InvalidExpression {
                                                                                    //evaluate the simple expression in the char array e
        int result=evalTerm();
        while (e[i]=='+' || e[i]=='-'){
          switch (e[i]){
            case '+':
              i++;
              result += evalTerm();
              break;
            case '-':
              i++;
              result -= evalTerm();
              break;
          }
        }
        return result;
      }