Section 3 with Remik (rz33@cornell.edu) 
Meeting #1Notes
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Administrivia:  Assignment #1 is out
		            
Review:		RPN Calculator
		      Stack
          Java syntax

RPN Calculator: Why do we use stacks?
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In 1951 a Polish mathematician wrote a book on logic.
His name was Jan Lukasiewicz and he showed that rules
of precedence and parentheses could be dropped
by rewriting expressions. For example,

  1+(2*3) can be written +1*23  (prefix notation)
          and also as    32*1+  (postfix or RPN)

RPN means reverse Polish notation.  So we can scan an
RPN expression from left to right just push values onto
a stack as they come, and pop them when an operator is
encountered.  This was actually adopted for machines to
use, namely the HP calculators.


RPN Java Framework: Using a Stack
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import java.util.Stack;

/** Do arithmetic RPN style.  Input must look like 3 4 + 1 +.
*/
class RPN {
  Stack stack = new Stack();

  /** @param string   an array of elements in an RPN expression
  */
  int eval(String[] string) { 
    for(int i = 0; i < string.length; ++i) {
      if (s[i].equals("+")) {
        // do addition and push the result
        int a = ((Integer)stack.pop()).intValue();
        int b = ((Integer)stack.pop()).intValue();

        stack.push( new Integer(a + b));
      
      } else {
        // must be a number
        stack.push( new Integer(s[i]));
      }
    }

    return ((Integer)stack.pop()).intValue();
  }
}