c++ 数字类型和字符串类型互转详解

  int i=100.22; 
  //用 stringstream定义一个变量 str;
  stringstream s;
  s<<i;  //这不是输出语句！
  string str1=s.str();
  //str1即为转为的字符串

#include<iostream>
#include<sstream>
#include<string> 
using namespace std;
int NumberOf1Between1AndN_Solution(int n) 
{
        int count=0;
        for(int i=1;i<=n;i++)
        {
        	stringstream s;
            s << i;
             string str=s.str();
            for(int j=0;j<str.size();j++)
            {
               if(str[j]=='1')
               {
                     count++;
               }
            }
        }
        return count;
}
int main()
{
   int a=13;
   int count=NumberOf1Between1AndN_Solution(a);
   cout<<count;
   return 0;
 } 

string  str="3434";
int a=str[1]-'0'; //a=4;

string s="321";
int num=0;
for(int i=0; i<s.size() ;i++){
	//把单个字符变为数字
	int x= s[i]-'0';
	//每次都要进位，也就是*10
	num = num*10 + x;
}

#include<iostream>
#include<string> 
#include<sstream>
using namespace std;
int main()
{
	 string str="342324";
	 int a;
	 stringstream ss;
	 ss<<str;
	 ss>>a;
	 cout<<a-1;//输出342323
	 return 0;
}

//string  转为int 
//string 利用从   .c_str() 转  const char *
//利用atoi(const char * ) 转 int 
#include<iostream>
#include<stdlib.h>
#include<string> 
using namespace std;
int main()
{
	string str="3413414";
	int a=atoi(str.c_str());
	cout<<a-1;
	return 0; //输出3413413
}