AC++第三章答案

3-1 如本章内容所述，计算数值集合中值有三点：

1）中值区分数值集合中比其大的一半和比其小的一半；

2）数值集合必须由大到小排序以抉择出哪个数值可以成为中值；

3）中值必须由整个数值集合中所有数决定。

由上述三点决定了整个数值集合一旦有了变化，那么很可能会影响到中值的判断。

比如有一个数值集合已经读入了1，3， 5三个元素，中值为3。下一个读入的为15，则中值为3和5的均值，即4。此时再次读入一个数20。如果舍弃1或3，则中值则为5和15的均值；如果舍弃了5，中值为3和15的均值，如果舍弃15,中值为3和5的均值。并且舍弃两个数，也同样会导致中值结果发生变化，因此不可以随便舍弃。

3-2 源代码：

#include <algorithm>#include <iostream>#include <ios>#include <iomanip>#include <vector>using namespace std;int main(int argc, char *argv[]){cout << "Enter the numbers until the end of the file: ";int num;vector<int> vec_nums;while (cin >> num) {vec_nums.push_back(num);//cout<<num<<endl;}typedef vector<int>::size_type vec_sz;vec_sz size = vec_nums.size();if (size / 4 == 0) {cout << endl << "You must enter at least 4 numbers." << endl;//cout<<size;return 1;}sort(vec_nums.begin(), vec_nums.end());vec_sz mid_1;vec_sz mid_2;vec_sz mid_3;mid_2 = size / 2;mid_1 = size / 4;mid_3 = size * 3 / 4;vec_sz count = 0;cout << "Now print the first part of the numbers: " << endl;for (count = size; count > mid_3; --count) {cout << vec_nums[count - 1] << " ";}cout << endl;cout << "Now print the second part of the numbers: " << endl;for (; count > mid_2; --count) {cout << vec_nums[count - 1] << " ";}cout << endl;cout << "Now print the third part of the numbers: " << endl;for (; count > mid_1; --count) {cout << vec_nums[count - 1] << " ";}cout << endl;cout << "Now print the last part of the numbers: " << endl;for (; count > 0; --count) {cout << vec_nums[count - 1] << " ";}cout << endl;    system("PAUSE");    //return EXIT_SUCCESS;    return 1;}

我这个写的比较麻烦，下面贴上github上某前辈简洁之作：（但同样在eclipse中无法直接ctrl+Z结束，输入完要先按enter，然后点击红色结束按钮才能看到结果，而直接打开运行的.exe文件可以识别数字之间的空格）

#include <algorithm>#include <iostream>#include <vector>using namespace std;int main() {  vector<int> integers;  cout << "Integers: ";  int x;  while (cin >> x)    integers.push_back(x);  if (integers.size() == 0) {    cout << endl << "No integers!" << endl;    return 1;  }  sort(integers.rbegin(), integers.rend());  typedef vector<int>::size_type vec_sz;  cout << "1st quartile" << endl;  for (vec_sz i = 0; i < integers.size() / 4; ++i)    cout << integers[i] << endl;  cout << "2nd quartile" << endl;  for (vec_sz i = integers.size() / 4; i < integers.size() / 2; ++i)    cout << integers[i] << endl;  cout << "3rd quartile" << endl;  for (vec_sz i = integers.size() / 2; i < integers.size() * 3 / 4; ++i)    cout << integers[i] << endl;  cout << "4th quartile" << endl;  for (vec_sz i = integers.size() * 3 / 4; i < integers.size(); ++i)    cout << integers[i] << endl;  system("PAUSE");  return 0;}

两程序执行结果相同，说明只要将sort的参数分别定义为*.rbegin() 和 *.rend()则代表将排序变为递减。

3-3 源代码：

#include <iostream>#include <string>#include <vector>#include <algorithm>using namespace std;int main() {  typedef vector<string>::size_type vec_sz;  vector<string> words;  cout << "Enter the words until the end of the file: ";  string word;  while (cin >> word) {    words.push_back(word);    }  if(words.size()==0)  {  cout<<"Please enter at least 1 word."<<endl;  return 0;  }  sort(words.begin(),words.end());  vec_sz size=words.size();  vec_sz index = 0;  int count = 0;  word=words[0];  while(index<size)  {  if(words[index]!=word)  {  cout<<word<<": "<<count;  count=0;  }  count++;  index++;  }  cout<<word<<": "<<count;  return 0;}

下面同样附上github上前辈的代码，不过貌似没有我的简洁。。。但是更加规矩：

#include <iostream>#include <string>#include <vector>using namespace std;int main() {  typedef vector<string>::size_type vec_sz;    vector<string> words;  vector<int> counts;  cout << "Words: ";  string s;  while (cin >> s) {    bool found = false;    for (vec_sz i = 0; i < words.size(); ++i) {      if (s == words[i]) {++counts[i];found = true;      }    }    if (!found) {      words.push_back(s);      counts.push_back(1);    }  }  for (vec_sz i = 0; i < words.size(); ++i)    cout << words[i] << " appeared " << counts[ ] << " times" << endl;  return 0;}

3-4 源代码：（本题没什么好说的。。。）

//#include <algorithm>#include <iostream>//#include <ios>//#include <iomanip>#include <string>using namespace std;int main(int argc, char *argv[]) {cout << "Enter the words until the end of the file: "<<endl;string word="";string max_word, min_word;cin >> word;max_word = word;min_word = word;while (cin >> word) {if (word.size() > max_word.size()) {max_word = word;}if (word.size() < min_word.size()) {min_word = word;}}cout << max_word.size() << endl;cout << min_word.size() << endl;//system("PAUSE");//return EXIT_SUCCESS;return 1;}

3-5 源代码：（本题题意一开始没有搞清楚，先贴上前辈的代码，自己的明天有空再写。。。）

#include <iomanip>#ifndef __GNUC__#include <ios>#endif#include <iostream>#include <string>#include <vector>using std::cin;                  using std::setprecision;using std::cout;                 using std::string;using std::endl;                 using std::streamsize;#define NUM_HOMEWORK 2using std::vector;int main() {  vector<string> names;  vector<double> final_grades;  bool done = false;  while (!done) {    // ask for and read the student's name    cout << "Please enter your first name: ";    string name;    cin >> name;    cout << "Hello, " << name << "!" << endl;    names.push_back(name);    // ask for and read the midterm and final grades    cout << "Please enter your midterm and final exam grades: ";    double midterm, final;    cin >> midterm >> final;    // ask for the homework grades    cout << "Enter both your homework grades, "      "followed by end-of-file: ";    // the number and sum of grades read so far    int count = 0;    double sum = 0;    // a variable into which to read    double x;    // invariant:    //     we have read `count' grades so far, and    //     `sum' is the sum of the first `count' grades    while (count < NUM_HOMEWORK) {      ++count;      cin >> x;      sum += x;    }      double final_grade = 0.2 * midterm + 0.4 * final + 0.4 * sum / count;    final_grades.push_back(final_grade);    cout << "More? (Y/N) ";    string s;    cin >> s;    if (s != "Y")      done = true;  }  for (vector<string>::size_type i = 0; i < names.size(); ++i) {    // write the result    streamsize prec = cout.precision();    cout << names[i] << "'s final grade is " << setprecision(3)       << final_grades[i]       << setprecision(prec) << endl;  }  return 0;}

3-6 同上，先贴上前辈的代码，自己的晚上回去写。

#include <iomanip>#ifndef __GNUC__#include <ios>#endif#include <iostream>#include <string>using std::cin;                  using std::setprecision;using std::cout;                 using std::string;using std::endl;                 using std::streamsize;int main() {  // ask for and read the student's name  cout << "Please enter your first name: ";  string name;  cin >> name;  cout << "Hello, " << name << "!" << endl;  // ask for and read the midterm and final grades  cout << "Please enter your midterm and final exam grades: ";  double midterm, final;  cin >> midterm >> final;  // ask for the homework grades  cout << "Enter all your homework grades, "    "followed by end-of-file: ";  // the number and sum of grades read so far  int count = 0;  double sum = 0;  // a variable into which to read  double x;  // invariant:  //     we have read `count' grades so far, and  //     `sum' is the sum of the first `count' grades  while (cin >> x) {    ++count;    sum += x;  }  double homework_grade = (count > 0) ? sum / count : 0.0;  // write the result  streamsize prec = cout.precision();  cout << "Your final grade is " << setprecision(3)       << 0.2 * midterm + 0.4 * final + 0.4 * homework_grade       << setprecision(prec) << endl;  return 0;}