C++ Primer Plus 学习笔记第十七章 cout

向右看齐 2021-12-23 17:49 540阅读 0赞

ios\_base类会存储输出格式状态的信息（比如 输出间隔，小数点位数等） ios继承于ios\_base ostream继承于ios

格式设置函数

hex()十六进制

oct()八进制

dec()十进制

abc()一万进制（骗你的）

使用的时候可以直接这样： cout << hex; 这样就设置输出为16进制的了

程序示例

#include <iostream>
    int main()
    {
      using namespace std;
      cout << "Enter an integer: ";
      int n;
      cin >> n;
    
      cout << "n    n*n\n";
      cout << n << "    " << n * n << "  (decimal)\n";
      cout << hex;
      cout << n << "    ";
      cout << n * n << "  (hexadecimal)\n";
    
      cout << oct << n << "    " << n * n << "  (octal)\n";
      dec(cout);
      cout << n << "    " << n * n << "  (decimal)\n";
    
      return 0;
    }

![20190626114808411.png][]

调整字段宽度

width（）返回当前字段宽度

width(int i ) 设置指端宽度（i个空格）并且返回以前的字段宽度

只影响下一个项目 也就是说 输出

cout.width(12)

cout << 12 << 22 << '\#';

这时候 宽度只有在12的前面显示。然后就1222\#了  它不会一直持续保持宽度 一次性的

width() 是成员函数

程序示例

#include <iostream>
    
    int main()
    {
      using std::cout;
      int w = cout.width(30);
      cout << "default field width = " << w << ":\n";
      cout.width(5);
      cout << "N" << ':';
      cout.width(8);
      cout << "N * N" << ":\n";
    
      for(long i = 1; i <= 100; i *= 10)
      {
        cout.width(5);
        cout << i << ':';
        cout.width(8);
        cout << i * i << ":\n";
      }
    
      return 0;
    }

![20190626130216777.png][]

字符填充

fill() 参数为指定用哪个字符填充未被使用的部分（只能是字符，不能使字符串）

#include <iostream>
    
    int main()
    {
      using std::cout;
      cout.fill('*');
      const char* staff[2] = {"waldo Whipsnade", "Wilmarie Wooper"};
      long bonus[2] = {900, 1350};
    
      for (int i = 0; i < 2; i++)
      {
        cout << staff[i] << ": $";
        cout.width(7);
        cout << bonus[i] << "\n";
      }
    
      return 0;
    }

![20190626130709421.png][]

fill()一直有效 不是一次性的

设置显示位数 precision（） 是整体位数不是小数点位数哦

#include <iostream>
    
    int main()
    {
      using std::cout;
      float price1 = 20.40;
      float price2 = 1.9 + 8.0 / 9.0;
    
      cout << "\"Furry Friends\" is $" << price1 << "!\n";
      cout << "\"Furry Friends\" is $" << price2 << "!\n";
    
      cout.precision(2);
      cout << "\"Furry Friends\" is $" << price1 << "!\n";
      cout << "\"Furry Friends\" is $" << price2 << "!\n";
    
      return 0;
    }

![2019062613110894.png][]

setf()设置末尾的0和小数点

定义了多个空值显示的多个常量

cout.setf(ios\_base::showpoint（静态常量）) 显示6位数（包含整数部分）并显示末尾的0

#include <iostream>
    int main()
    {
      using std::cout;
      using std::ios_base;
    
      float price1 = 20.40;
      float price2 = 1.9 + 8.0 / 9.0;
    
      cout.setf(ios_base::showpoint);
      cout << "\"Furry Friends\" is $" << price1 << "!\n";
      cout << "\"Furry Friends\" is $" << price2 << "!\n";
    
      cout.precision(2);
      cout << "\"Furry Friends\" is $" << price1 << "!\n";
      cout << "\"Furry Friends\" is $" << price2 << "!\n";
    
      return 0;
    }

![20190626131801774.png][]

第三行因为设置了位数 所以只能显示最前面的两位

![20190626132022179.png][]

每一位有特定的定义 不用去记哪一位的开关是啥作用的

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3NvdWx3eWI_size_16_color_FFFFFF_t_70][]

![20190626133132318.png][]

setf()有两个原型

第一个 fmtflags setf(fmtflags)

![20190626133220196.png][]

程序示例

#include <iostream>
    
    int main()
    {
      using std::cout;
      using std::endl;
      using std::ios_base;
    
      int temperature = 63;
      cout << "Today's water temperature: ";
    // 显示+号 10进制
      cout.setf(ios_base::showpos);
      cout << temperature << endl;
    
      cout << "Por our programming freinds, that's\n";
    // 显示十六进制
      cout << std::hex << temperature << endl;
    // 十六进制大写并且显示C风格标识
      cout.setf(ios_base::uppercase);
      cout.setf(ios_base::showbase);
      cout << "or\n";
      cout << temperature << endl;
      cout << "How " << true << "!  oops -- How ";
    // 显示布尔值
      cout.setf(ios_base::boolalpha);
      cout << true << "!\n";
      return 0;
    }

![2019062613441062.png][]

hex() 等于cout.setf(ios\_base::hex, ios\_base::basefield)

![20190626134638755.png][]

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![20190626135045794.png][]

程序示例

#include <iostream>
    #include <cmath>
    
    int main()
    {
      using namespace std;
    // 左对齐
      cout.setf(ios_base::left, ios_base::adjustfield);
    // 显示+号
      cout.setf(ios_base::showpos);
    // 显示小数点
      cout.setf(ios_base::showpoint);
    // 三位数
      cout.precision(3);
    // 科学计数法 
      ios_base::fmtflags old = cout.setf(ios_base::scientific, ios_base::floatfield);
      cout << "Left Justification:\n";
      long n;
      for(n = 1; n <= 41; n += 10)
      {
        cout.width(4);
        cout << n << "|";
        cout.width(12);
        cout << sqrt(double(n)) << "|\n";
      }
    //符号或前缀左对齐，值右对齐
      cout.setf(ios_base::internal, ios_base::adjustfield);
    // 科学计数法
      cout.setf(old, ios_base::floatfield);
      
      cout << "Internal Justification:\n";
      for ( n = 1; n <41; n +=10)
      {
        cout.width(4);
        cout << n << "|";
        cout.width(12);
        cout << sqrt(double(n)) << "|\n";
      }
    // 右对齐
      cout.setf(ios_base::right, ios_base::adjustfield);
    // 定点计数 非科学计数
      cout.setf(ios_base::fixed, ios_base::floatfield);
      cout << "Right Justification:\n";
        for ( n = 1; n <41; n +=10)
      {
        cout.width(4);
        cout << n << "|";
        cout.width(12);
        cout << sqrt(double(n)) << "|\n";
      }
    
      return 0;
    }

结果

![20190626141644166.png][]

可以使用void unsetf(fmtflags mask)消除setf()的效果 mask位模式 指定哪个位恢复为0

浮点数默认的位模式是 cout.setf(0, ios\_base::floatfield) 将floatfield中的两个位都设置成0

或者用cout.unsetf(ios\_base::floatfield)

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使用头文件 iomanip

3个常用的控制符

setprecision() 设置精度，接受1个整形参数定位精度数

setfill()接受一个char类型字符指定填充符

setw()接受一个指定字段宽度的整数参数

#include <iostream>
    #include <cmath>
    #include <iomanip>
    
    int main()
    {
      using namespace std;
      cout << fixed << right;
    // 设置宽度
      cout << setw(6) << "N" << setw(14) << "square root"
           << setw(15) << "fourth root\n";
      double root;
      for (int n = 10; n <= 100; n += 10)
      {
        root = sqrt(double(n));
    // . ' ' 填充字符
        cout << setw(6) << setfill('.') << n << setfill(' ')
    // 指定精度
             << setw(12) << setprecision(3) << root
             << setw(14) << setprecision(4) << sqrt(root)
             << endl;
      }
    
      return 0;
    }

完结  卧槽 最后一小节这么多内容的么

#include <iostream>
    #include <fstream>
    #include <iomanip>
    #include <cstdlib>
    
    const int LIM = 20;
    struct planet
    {
      char name[LIM];
      double population;
      double g;
    };
    
    const char* file = "planets.dat";
    inline void eatline() {while (std::cin.get() != '\n') continue;}
    
    int main()
    {
      using namespace std;
      planet pl;
      cout << fixed;
    
      fstream finout;
    // 读写
      finout.open(file, ios_base::in | ios_base::out | ios_base::binary);
      int ct = 0;
      if (finout.is_open())
      {
    // 输出流指针移到文件首
        finout.seekg(0);
        cout << "Here are the current contents of the "
             << file << " file:\n";
    // 如果有内容就先读取出来
        while (finout.read((char*) &pl, sizeof pl))
        {
          cout << ct++ << ": " << setw(LIM) << pl.name << ": "
               << setprecision(0) << setw(12) << pl.population
               << setprecision(2) << setw(12) << pl.g << endl;
        }
    // 判断是否到文件尾
        if (finout.eof())
          finout.clear();
        else{
          cerr << "Error in reading " << file << ".\n";
          exit(EXIT_FAILURE);
        }
      }
      else
      {
        cerr << file << " could not be opened -- bye.\n";
        exit(EXIT_FAILURE);
      }
      cout << "Enter the record number you wish to change: ";
      long rec;
      cin >> rec;
      eatline();
      if (rec < 0 || rec >= ct)
      {
        cerr << "Invalid record number -- bye\n";
        exit(EXIT_FAILURE);
      }
    // 将输出指针移动到指定位置
      streampos place = rec * sizeof pl;
      finout.seekg(place);
      if (finout.fail())
      {
        cerr << "Error on attempted seek\n";
        exit(EXIT_FAILURE);
      }
    // 读取出指定位置的内容
      finout.read((char*) &pl, sizeof pl);
      cout << "Your selection:\n";
      cout << rec << ": " << setw(LIM) <<pl.name << ": "
           << setprecision(0) << setw(12) << pl.population
           << setprecision(2) << setw(6) << pl.g << endl;
      if(finout.eof())
        finout.clear();
    
      cout << "Enter planet name: ";
      cin.get(pl.name, LIM);
      eatline();
      cout << "Enter planetary population: ";
      cin >> pl.population;
      cout << "Enter planet's acceleration of gravity: ";
      cin >> pl.g;
    // 将数据写入指定位置
      finout.seekp(place);
      finout.write((char *) &pl, sizeof pl) << flush;
      if(finout.fail())
      {
        cerr << "Error on attempted write\n";
        exit(EXIT_FAILURE);
      }
    
      ct = 0;
    // 输出指针移动到文件首 然后把文件全部读取出来
      finout.seekg(0);
      cout << "Here are the new contents of the " << file
           << " file:\n";
      while(finout.read((char *) &pl, sizeof pl))
      {
        cout << ct++ << ": " << setw(LIM) << pl.name << ": "
             << setprecision(0) << setw(12) << pl.population
             << setprecision(2) << setw(6) << pl.g << endl;
      }
      finout.close();
      cout << "Done.\n";
      return 0;
    }

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