C++对string进行大小写转换操作方法

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string s = "ABCDEFG";

    for( int i = 0; i < s.size(); i++ )
    {
        s[i] = tolower(s[i]);
    }

    cout<<s<<endl;
    return 0;
}

#include <iostream>
#include <algorithm>
#include <string>

using namespace std;

int main()
{
    string s = "ABCDEFG";
    string result;

    transform(s.begin(),s.end(),s.begin(),::tolower);
    cout<<s<<endl;
    return 0;
}

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main(){
    string str = "ancdANDG";
    cout << "转换前的字符串: " << str << endl;
    
    for(auto &i : str){
        i = toupper(i);//i = tolower(i);
    }    
    cout << "转换后的字符串: " << str << endl;
    
    //或者
    for(int i = 0;i < str.size();++i){
		str[i] = toupper(s[i]);//str[i] = toupper(s[i]);
	}
	cout << "转换后的字符串： " << str << endl;
	
	return 0;
}

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main(){
    string str = "helloWORLD";
    cout << "转换前：" << str << endl;
    
    //全部转换为大写
    transform(str.begin(), str.end(), str.begin(), ::toupper);    
    cout << "转换为大写：" << str << endl;    
    
    //全部转换为小写
    transform(str.begin(), str.end(), str.begin(), ::tolower);    
    cout << "转换为小写：" << str << endl; 
    
    //前五个字符转换为大写
    transform(str.begin(), str.begin()+5, str.begin(), ::toupper);
    cout << "前五个字符转换为大写：" << str << endl; 
    
    //后五个字符转换为大写
    transform(str.begin()+5, str.end(), str.begin()+5, ::toupper);
    cout << "前五个字符转换为大写：" << str << endl; 
    
    return 0;
}