C++ Primer Plus第六版编程练习12.5解答

queue.h

// queue.h -- interface for a queue
#ifndef QUEUE_H_
#define QUEUE_H_
// This queue will contain Customer items
class Customer
{
private:
long arrive; // arrival time for customer
int processtime; // processing time for customer
public:
Customer() : arrive(0), processtime (0){}
void set(long when);
long when() const { return arrive; }
int ptime() const { return processtime; }
};

typedef Customer Item;

class Queue
{
private:
// class scope definitions
// Node is a nested structure definition local to this class
struct Node { Item item; struct Node * next;};
enum {Q_SIZE = 10};
// private class members
Node * front; // pointer to front of Queue
Node * rear; // pointer to rear of Queue
int items; // current number of items in Queue
const int qsize; // maximum number of items in Queue
// preemptive definitions to prevent public copying
Queue(const Queue & q) : qsize(0) { }
Queue & operator=(const Queue & q) { return *this;}
public:
Queue(int qs = Q_SIZE); // create queue with a qs limit
~Queue();
bool isempty() const;
bool isfull() const;
int queuecount() const;
bool enqueue(const Item &item); // add item to end
bool dequeue(Item &item); // remove item from front
};
#endif

queue.cpp
// queue.cpp -- Queue and Customer methods
#include "queue.h"
#include <cstdlib> // (or stdlib.h) for rand()

// Queue methods
Queue::Queue(int qs) : qsize(qs)
{
front = rear = NULL; // or nullptr
items = 0;
}

Queue::~Queue()
{
Node * temp;
while (front != NULL) // while queue is not yet empty
{
temp = front; // save address of front item
front = front->next;// reset pointer to next item
delete temp; // delete former front
}
}

bool Queue::isempty() const
{
return items == 0;
}

bool Queue::isfull() const
{
return items == qsize;
}

int Queue::queuecount() const
{
return items;
}

// Add item to queue
bool Queue::enqueue(const Item & item)
{
if (isfull())
return false;
Node * add = new Node; // create node
// on failure, new throws std::bad_alloc exception
add->item = item; // set node pointers
add->next = NULL; // or nullptr;
items++;
if (front == NULL) // if queue is empty,
front = add; // place item at front
else
rear->next = add; // else place at rear
rear = add; // have rear point to new node
return true;
}

// Place front item into item variable and remove from queue
bool Queue::dequeue(Item & item)
{
if (front == NULL)
return false;
item = front->item; // set item to first item in queue
items--;
Node * temp = front; // save location of first item
front = front->next; // reset front to next item
delete temp; // delete former first item
if (items == 0)
rear = NULL;
return true;
}

// customer method

// when is the time at which the customer arrives
// the arrival time is set to when and the processing
// time set to a random value in the range 1 - 3
void Customer::set(long when)
{
processtime = std::rand() % 3 + 1;
arrive = when;
}

bank.cpp
// bank.cpp -- using the Queue interface
// compile with queue.cpp
#include <iostream>
#include <cstdlib> // for rand() and srand()
#include <ctime> // for time()
#include "queue.h"
const int MIN_PER_HR = 60;

bool newcustomer(double x); // is there a new customer?

int main()
{
using std::cin;
using std::cout;
using std::endl;
using std::ios_base;
// setting things up
std::srand(std::time(0)); // random initializing of rand()

cout << "Case Study: Bank of Heather Automatic Teller\n";
cout << "Enter maximum size of queue: ";
int qs;
cin >> qs;
Queue line(qs); // line queue holds up to qs people

cout << "Enter the number of simulation hours: ";
int hours; // hours of simulation
cin >> hours;
// simulation will run 1 cycle per minute
long cyclelimit = MIN_PER_HR * hours; // # of cycles

Item temp; // new customer data
long turnaways; // turned away by full queue
long customers; // joined the queue
long served; // served during the simulation
long sum_line; // cumulative line length
long wait_time; // time until autoteller is free
long line_wait; // cumulative time in line
long cycle; // running time

cout << "Enter the average number of customers per hour: ";
double perhour; // average # of arrival per hour
double min_per_cust; // average time between arrivals
while (cin >> perhour)
{
turnaways = 0;
customers = 0;
served = 0;
sum_line = 0;
wait_time = 0;
line_wait = 0;
min_per_cust = MIN_PER_HR / perhour;
// running the simulation
for (cycle = 0; cycle < cyclelimit; cycle++)
{
if (newcustomer(min_per_cust)) // have newcomer
{
if (line.isfull())
turnaways++;
else
{
customers++;
temp.set(cycle); // cycle = time of arrival
line.enqueue(temp); // add newcomer to line
}
}
if (wait_time <= 0 && !line.isempty())
{
line.dequeue (temp); // attend next customer
wait_time = temp.ptime(); // for wait_time minutes
line_wait += cycle - temp.when();
served++;
}
if (wait_time > 0)
wait_time--;
sum_line += line.queuecount();
}

// reporting results
if (customers > 0)
{
cout << "customers accepted: " << customers << endl;
cout << " customers served: " << served << endl;
cout << " turnaways: " << turnaways << endl;
cout << "average queue size: ";
cout.precision(2);
cout.setf(ios_base::fixed, ios_base::floatfield);
cout << (double) sum_line / cyclelimit << endl;
cout << " average wait time: "
<< (double) line_wait / served << " minutes\n";
}
else
cout << "No customers!\n";
// cleat
while (!line.isempty())
line.dequeue(temp);

cout << "\nEnter the average number of customers per hour(q to quit): ";
}
cout << "Done!\n";
cin.get();
return 0;
}

// x = average time, in minutes, between customers
// return value is true if customer shows up this minute
bool newcustomer(double x)
{
return (std::rand() * x / RAND_MAX < 1);
}

这道题之前一直错误（输出的average wait time有时候会是负数），一直找不出原因。后来看了答案才发现原来是因为忘了清空队列！！

详细解释一下：

如果在队列不空的情况下开始下一次while循环，那么循环运行到71行 line_wait += cycle - temp.when() 的时候就会出错。由于队列不空，在第67行的判断 if (wait_time <= 0 && !line.isempty())会成功，而此时temp.when()返回的值（即上一次循环中最后一名顾客的到达时间）还未重置，这个值是在59990~60000之间的，但cycle是已经重置为0了的，于是此时line_wait
+= cycle - temp.when()就会是一个负值了。