Java Reference
In-Depth Information
c)
Method
precedence
, which determines whether the precedence of
operator1
(from the
infix expression) is less than, equal to or greater than that of
operator2
(from the stack).
The method returns
true
if
operator1
has lower precedence than
operator2
. Other-
wise,
false
is returned.
d)
Method
peek
(this should be added to the stack class), which returns the top value of
the stack without popping the stack.
21.13
(Postfix Evaluator)
Write class
PostfixEvaluator
that evaluates a postfix expression such as
6 2 + 5 * 8 4 / -
The program should read a postfix expression consisting of digits and operators into a
StringBuf-
fer
. Using modified versions of the stack methods implemented earlier in this chapter, the pro-
gram should scan the expression and evaluate it (assume it's valid). The algorithm is as follows:
a) Append a right parenthesis
')'
to the end of the postfix expression. When the right-
parenthesis character is encountered, no further processing is necessary.
b) Until the right parenthesis is encountered, read the expression from left to right.
If the current character is a digit, do the following:
Push its integer value onto the stack (the integer value of a digit character is its
value in the Unicode character set minus the value of
'0'
in Unicode).
Otherwise, if the current character is an
operator
:
Pop the two top elements of the stack into variables
x
and
y
.
Calculate
y
operator
x
.
Push the result of the calculation onto the stack.
c) When the right parenthesis is encountered in the expression, pop the top value of the
stack. This is the result of the postfix expression.
[
Note:
In b) above (based on the sample expression at the beginning of this exercise), if the operator
is
'/'
, the top of the stack is
4
and the next element in the stack is
40
, then pop
4
into
x
, pop
40
into
y
, evaluate
40
/
4
and push the result,
10
, back on the stack. This note also applies to operator
'-'
.] The arithmetic operations allowed in an expression are:
+
(addition),
-
(subtraction),
*
(mul-
tiplication),
/
(division),
^
(exponentiation) and
%
(remainder).
The stack should be maintained with one of the stack classes introduced in this chapter. You
may want to provide the following methods:
a) Method
evaluatePostfixExpression
, which evaluates the postfix expression.
b) Method
calculate
, which evaluates the expression
op1
operator
op2
.
21.14
(Postfix Evaluator Modification)
Modify the postfix evaluator program of Exercise 21.13
so that it can process integer operands larger than 9.
21.15
(Supermarket Simulation)
Write a program that simulates a checkout line at a supermarket.
The line is a queue object. Customers (i.e., customer objects) arrive in random integer intervals of
from 1 to 4 minutes. Also, each customer is serviced in random integer intervals of from 1 to 4 min-
utes. Obviously, the rates need to be balanced. If the average arrival rate is larger than the average
service rate, the queue will grow infinitely. Even with “balanced” rates, randomness can still cause
long lines. Run the supermarket simulation for a 12-hour day (720 minutes), using the following
algorithm:
a)
Choose a random integer between 1 and 4 to determine the minute at which the first
customer arrives.
b)
At the first customer's arrival time, do the following:
Determine customer's service time (random integer from 1 to 4).
Begin servicing the customer.
Schedule arrival time of next customer (random integer 1 to 4 added to the current time).
