字符串与数字格式转换__字符串转换成数字格式转换的函数（1）

背景小知识：
例如 int、long 和 double 等，有时也需要转换为字符串形式，这样结果字符串才能立即输出到文件或其他输入输出设备，或者存入内存中的某个字符串对象，供以后使用。

当用户在键盘上输入一个数字时，该数字以字符串的形式输入，就好像用户输入的一系列字符（数字）。在 C++ 中，这样的数字通常通过流提取运算符 >> 来读取。在存储到数字类型的变量之前，该运算符会根据需要自动执行转换。在输出期间，数字到字符串的反向转换由流输出运算符 << 执行。

简单的可以参考我另一篇博客​​C++实现数据反式转换为字符串格式​​

与本篇博客相对应的是下一篇​​字符串与数字格式转换__数字格式转换成字符串转换的函数（2）​​

介绍4个方法：

1 利用sscanf() 将字符串转化为数字

#include <iostream>
#include <string>
using namespace std;
// sscanf() 用于将字符串转化为数字
int main()
{
    char str[]="1234321"; 
    int a; 
    sscanf(str,"%d",&a); 
    cout<<a<<endl;
    char str1[]="908.456"; 
    double b; 
    sscanf(str1,"%lf",&b); 
    cout<<b<<endl;
}

​​详细请看百度百科​​

2. 利用stringstream

字符串格式转换 ss >>
过程：string -> stringstream对象 -> 数字

#include <string>
#include <sstream>
#include<iostream>
using namespace std;
int main(){
    double d ;
    string res= "-1";
    stringstream ss; 
    //string->double
    ss << res;         
     ss >> d;
     cout<<"d="<<d<<endl;
     
     //string->int
     ss.clear();//stringstream对象清零 
     int i;
     ss<<res;
     ss>>i;
     cout<<"i="<<i<<endl;
     
     //string->unsigned int
      ss.clear();//stringstream对象清零 
     unsigned int ui;
     ss<<res;
     ss>>ui;
     cout<<"ui="<<ui<<endl;
    return 0;
}

3、 atof()

是C 语言标准库中的一个字符串处理函数，功能是把字符串转换成浮点数，所使用的头文件为<stdlib.h>。该函数名是 “ascii to floating point numbers” 的缩写。语法格式为：double atof(const char *nptr)。

功能： 把字符串转换成浮点数

#include<iostream>
#include<stdlib.h>
using namespace std; 
int main()
{
  double d;
  char str[] = "123.456";
  d=atof(str);
  printf("string=%sdouble=%lf\n",str,d);
  cout<<str<<endl;
  cout<<d<<endl;
  return 0;
}

4.stoi(str, &pos); 转换函数

/数字转换函数1.// This program demonstrates the use of the stoXXX()
// numeric conversion functions.
#include <string>
#include <iostream>
using namespace std;
int main()
{
  string str; // string to convert
  size_t pos; // Hold position of stopping character
  //Convert string to double
  str = "-342.57is a number";
  cout << "The string is " << str << endl;
  double d = stod(str, &pos);
  cout << "The converted double is " << d << endl;
  cout << "The stopping character is " << str[pos] << " at position " << pos << endl;

  // Convert string to int (default to decimal)
  str = "-342.57is a number";
  cout << "\nThe string is " << str << endl;
  int i = stoi(str, &pos);
  cout << "The converted integer is " << i << endl;
  cout << "The stopping character is " << str[pos] << " at position " << pos << endl;

  // Convert string to int (base is binary)
  str = "01110binary number";
  cout << "\nThe string is " << str << endl;
  i = stoi(str, &pos, 2);
  cout << "The converted binary integer is " << i << endl;
  cout << "The stopping character is " << str[pos] << " at position " << pos << endl;
  return 0;
}

string str; // string to convert
size_t pos; // Hold position of stopping character

stoi(str, &pos); 转换函数

5.istringstream和ostringstream

//字符串向数字转化
//This program illustrates the use of sstream objects
#include <sstream>
#include <iostream>
#include <string>
using namespace std;
int main()
{
    string str = "John 20 50"; // string to read from
    const char *cstr = "Amy 30 42"; // Cstring to read from
    istringstream istr1 (str);    // istr1 will read from str
    istringstream istr2; // istr2 will read from cstr
  ostringstream ostr; // The ostringstream object to write to
  string name;
  int score1, score2, average_score;

   // Read name and scores and compute average then write to ostr
   istr1 >> name >> score1 >> score2;
   average_score = (score1 + score2)/2;
   ostr << name << " has average score" << average_score << "\n";
   
    // Set istr2 to read from the C string and repeat the above
    istr2.str(cstr);
    istr2 >> name >> score1 >> score2;
   average_score = (score1 + score2)/2;
   ostr << name << " has average score" << average_score << "\n";
   
    // Switch to hexadeximal output on ostr
    ostr << hex;
    // Write Amy's scores in hexadecimal
  ostr << name << "'s scores in hexadecimal are: " << score1 << " and " << score2 << "\n";
    // Extract the string from ostr and print it to the screen
   cout << ostr.str();
  return 0;
}