Computer Science 3310
Program 4
Your assignment is to write a program that evaluates postfix expressions. The form of a postfix expression and the
algorithm for evaluating them are discussed in chapter 7 of your text on pages 349-351. In particular, the algorithm
uses a stack for storing the operands until used in carrying out a calculation. For this program, the operands will be
upper case letters and the operators will be from the set of characters {+,-,*,/}. The semantics for these operators
conforms to their semantics in Java. Integer arithmetic will be used. In order to assign values to the upper case
letters, the first portion of the input consists of assignments of integer values to letters. Any letter not assigned a
value will have the default value of 0. The second portion of the input consists of postfix expressions to evaluate.
Input Format: The first portion of the input (one or more lines) consists of assignments of integer values to upper
case letters. The letters are in no particular order, but no letter will be assigned two values. An example assignment
line is
D = -45
The integer values assigned to letters will be in the range of a 32 bit integer. The assignment lines always begin with
the letter in column 1 of the input line. You may assume that there will be one or more spaces between each token
(i.e. the <letter>, <asst_op>, and <number>). A sentinel line of the form $PART2 indicates the end of the
assignment lines. The second portion of the input (one or more lines) consists of postfix expressions. An example
expression line is
CBCAE-++-X-
Each of the input expressions will consist of only upper case letters and the four operators. The expression lines
begin in column 1 and contain no embedded spaces. Any letter appearing in an expression but not assigned a value
is assumed to have the default value of 0. You may also assume that the values of any calculations involved in
evaluating an expression will all be in the range of a 32 bit integer.
The input is terminated by the sentinel line $END.
Input Processing:
• Read from standard input. Use the scanner class described on pages 44-45 of your text. Individually process
each token of the assignment lines.
• Process the expressions as strings. The String functions length() and charAt() should be very helpful.
Error Handling: The following error checking should be performed.
• After the last character of an expression is processed, the stack should consist of one number, which is the result.
• When an operator is encountered, there should be at least two operands on the stack.
If any of these errors occur, a suitable error message should be printed, and the rest of the input expression ignored.
You may use exception handling or traditional conditional logic for doing your error handling. Processing should
resume with the next line. Error handling that causes termination of execution of the program will be severely
penalized.
Output Format: See sample test case. The expression line should be echoed before the result is printed.
Data Structures: To keep up with the assigned values of the operands, use an array of 26 integers. A slight
complication is that the array subscripts will run from 0 to 25, while the ASCII codes associated with the upper case
letters range between 65 and 90. However, the following function will map an upper case letter into a number
between 0 and 25 where 0 will correspond to ‘A’, and 25 correspond to ‘Z’.
