evaluate

Category: Token-Based File Processing

Author: Stuart Reges

Book Chapter: 6.2

Problem: evaluate

  Write a static method called evaluate
   that takes as a parameter a Scanner containing a series of tokens
   that represent a numeric expression involving addition and subtraction and
   that returns the value of the expression.  For example, if a Scanner called
   data contains the following tokens:

	4.2 + 3.4 - 4.1

   and we make the following call:

        evaluate(data)

   the method should evaluate the result as (4.2+3.4-4.1) = (7.6-4.1) = 3.5 and
   should return this value as its result.  Every expression will begin with a
   real number and then will have a series of operator/number pairs that
   follow.  The operators will be either "+" (addition) or "-" (subtraction).
   As in the example above, there will be spaces separating numbers and
   operators.  You may assume the expression is legal.

   Your program should evaluate operators sequentially from left to right.  For
   example, for this expression:

	7.3 - 4.1 - 2.0

   your method should evaluate the operators as follows:

	7.3 - 4.1 - 2.0 = (7.3 - 4.1) - 2.0 = 3.2 - 2.0 = 1.2

   The Scanner might contain just a number, in which case your method should
   return that number as its result.