C++ Primer Pluse(第6版)中文版 第12章编程练习第5题_heather银行进行的研究表明,atm客户不希望排队时间不超过 1分钟。使用程序清单12.

题目:

     Heather银行进行的研究表明，ATM客户不希望排队时间不超过1分钟。请使用书中程序清单12.10中的模拟，找出要使平均等候时间为1分钟，每小时到达的客户数应为多少(试验时间不短于100小时)？

思路:

     通过网络搜索,分析网友的回答，解决这个问题有两种思路：

     一、直接使用书中的例题源代码12.10、12.11和12.12,通过输入不同的参数，人工找出运行结果里平均等候时间为1分种的客户数。

     （参考：1、http://zhidao.baidu.com/link?url=2GVmQkvQYIStDYIUDuxvCh_lib0CLHBjEwseIOPKkuIp-V6oLsrwtTyOmY5XKlsE8OaYAmLMshyDeANPXzw8ba  2、http://blog.csdn.net/qq844352155/article/details/24327151）

     二、问题的关键在于对“平均等候时间为1分钟”条件的设定。简单修改一下12.12中的部分代码，通过设定条件，自动获取平均等候时间为1分钟的客户数（参考：http://tieba.baidu.com/p/1826136344）。我理解，不太可能完全获得平均值时间正好为1分钟的值，只能是非常接近。应用常用的知识点，使一个浮点数无限接近于某一个值（本题中是1.0）的办法，程序中使用了fabs((double)line_wait / served-1)<0.1 这个条件表达式。在测试过程中发现，设置的精度不能太高，否则得到的结果是每小时到达的客户接近或者60，那么客户到达的平均时间间隔少于1分钟，根据书中所述，此时是无效的。现将12.12的修改代码附上，结合书中的12.10、12.11即可运行。

// bank.cpp -- using the Queue interface
// compile with queue.cpp
#include <iostream>
#include <cstdlib> // for rand() and srand()
#include <ctime>   // for time()
#include <cmath>
#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
 
	double perhour=1;  
 	double min_per_cust;  
   
    long turnaways = 0;     //  turned away by full queue
	long customers = 0;     //  joined the queue
    long served = 0;        //  served during the simulation
	long sum_line = 0;      //  cumulative line length
	int wait_time = 0;      //  time until autoteller is free
	long line_wait = 0; 
	
	while(1)
	{
		min_per_cust = MIN_PER_HR / perhour;
		Item temp;              //  new customer data
	    turnaways = 0;     //  turned away by full queue
		customers = 0;     //  joined the queue
    	served = 0;        //  served during the simulation
		sum_line = 0;      //  cumulative line length
		wait_time = 0;      //  time until autoteller is free
		line_wait = 0;  	
		
		for (int 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();
   		 }
		
		if(fabs((double)line_wait / served-1.0)<0.1)
		{
		   	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";
             cout<<"fabs((double)line_wait / served-1.0):"<<fabs((double)line_wait / served-1.0)<<endl;
			cout<<"average number of customer per hour: "<<perhour<<endl;
           
		 
		   break;
		}
		
		perhour++;
		
	}
 
	cin.get();
	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); 
}