C++ primer plus(sixth edition) 编程练习答案（answers for programing exercises）第七章(chapter 7) 1-5

7.1//calculate the harmonic average//program end until you enter  zero#include <iostream>using namespace std;double h_average(double,double);
int main(){ double a,b,average; cout<<"Please enter two numbers:\n"; while(cin>>a>>b&&a!=0&&b!=0)//a*b!=0  {    average=h_average(a,b);    cout<<"Harmonic average:"<<average<<endl;    cout<<"Please enter two numbers:\n";    continue;    }    cout<<"Bye!!";    return 0;//dd}
double h_average(double a,double b){ double average; average=2*a*b/(a+b); return average;}
7.2//enter at most ten groups of your golf scores//display,calculate the average,show the average.#include <iostream>using namespace std;int read(double*,int);double calculate(double*,int);void show(double*,int,double);const int Size=10;
int main(){ double scores[Size]; int count; cout<<"Enter at most ten groups of your golf scores.(q to quit)\n"; count=read(scores,Size); double average; average=calculate(scores,count); show(scores,count,average); return 0; //dd
}
int read(double scores[],int n){ int i=0; cout<<"# "<<i+1<<" : "; for(i;i<n;i++) { if(cin>>scores[i])        {                  cout<<"# "<<i+2<<" : ";        } else    break; }
return i;}
double  calculate(double scores[],int count){ double average=0.0,sum=0.0; for(int i=0;i<count;i++)        sum+=scores[i];    average=sum/count;        return average;    }
void show(double scores[],int count,double average){    cout<<"Here are the scores: ";    for(int i=0;i<count;i++)    cout<<scores[i]<<' '; cout<<endl; cout<<"Average: "<<average;    }7.3//using a program to test//two functions ,one that passes structure by value//the other passes structure by address#include <iostream>using namespace std;struct box{ char maker[40]; float height; float width; float length; float volume;};
void show1(box);void show2(box*);void setbox(box*);
int main(){ box car = { "Mercedes benz", 200, 100, 2, };    setbox(&car);    show1(car);    cout<<endl;    show2(&car);        return 0;}
void show1(box car){ cout<<"Maker: "<<car.maker<<endl; cout<<"height: "<<car.height<<endl; cout<<"width: "<<car.width<<endl; cout<<"length: "<<car.length<<endl; cout<<"volume: "<<car.volume<<endl;}
void show2(box* car){ cout<<"Maker: "<<car->maker<<endl; cout<<"height: "<<car->height<<endl; cout<<"width: "<<car->width<<endl; cout<<"length: "<<car->length<<endl; cout<<"volume: "<<car->volume<<endl;}
void setbox(box*pb){ pb->volume=pb->height*pb->width*pb->length;}
7.4// lotto.cpp -- probability of winning#include <iostream>// Note: some implementations require double instead of long doublelong double probability(unsigned numbers, unsigned picks);int main(){    using namespace std;    double total, choices,total2,choices2,p2;    cout << "Enter the total number of choices on the game card and\n"            "the number of picks allowed:\n";    while ((cin >> total >> choices) && choices <= total)    {    cout<<"Enter the total number of the second field number\n"         "and you can choose at most one or not.\n";    while((cin>>total2>>choices2)&&choices2<=1)    {    if(choices2==1){              p2=probability(total2,choices2);           break; }    else      {           p2=1;           break;       }    }        cout << "You have one chance in ";        cout << probability(total, choices)*p2;            cout << " of winning.\n";        cout << "Next two numbers (q to quit): ";    }    cout << "bye\n";    // cin.get();    // cin.get();    return 0;//dd}
// the following function calculates the probability of picking picks// numbers correctly from numbers choiceslong double probability(unsigned numbers, unsigned picks){    long double result = 1.0;  // here come some local variables    long double n;    unsigned p;
    for (n = numbers, p = picks; p > 0; n--, p--)        result = result * n / p ;     return result;}7.5//using recursive function to caculate factorial#include <iostream>using namespace std;long long factorial(int);
int main(){ int num; cout<<"Please enter a integer number(q to quit): "; while(cin>>num&&num>=0) { cout<<num<<"! = "<<factorial(num)<<endl;        cout<<"Please enter a integer number: ";        continue;         } cout<<"Bye!!"; return 0; //dd }
long long factorial(int num){ if(num==0)        return 1;        else if(num==1)        return 1;        return num*factorial(num-1);}