How to rewrite/overwrite strings/lines in a text file?

Question

I'm totally new in c++ and having problem with a particular portion of my program. As no one to help me in real, I have dared to ask it here. If a text file includes this line "hELp mE", it should be rewritten/overwritten as "HelP Me" in the same exact text file. What i know is that, I might need to use ofstream for the overwriting but i'm very confused about how it should be done. I have tried for about 2 hours and failed. Here is my half completed code which is only able to read from the file.

 int main()
 {
   string sentence;
   ifstream firstfile;
   firstfile.open("alu.txt");
   while(getline(firstfile,sentence))
      {  
         cout<<sentence<<endl;
         for(int i=0;sentence[i] !='\0';i++)
           {
             if((sentence[i] >='a' && sentence[i] <='z') || (sentence[i] >='A' && sentence[i] <='Z'))
              {

                 if(isupper(sentence[i]))
                    sentence[i]=tolower(sentence[i]);
                 else if(islower(sentence[i]))
                    sentence[i]=toupper(sentence[i]);
              }
         }
      }


 return 0;
}

Answer 1

It's works good:

#include <iostream>
#include <fstream>
using namespace std;
int main()
{
    string sentence;
    ifstream firstfile;
    firstfile.open("alu.txt");
    while(getline(firstfile,sentence))
    {
        cout<<sentence<<endl;
        for(int i=0; sentence[i] !='\0'; i++)
        {
            if((sentence[i] >='a' && sentence[i] <='z') || (sentence[i] >='A' && sentence[i] <='Z'))
            {

                if(isupper(sentence[i]))
                    sentence[i]=tolower(sentence[i]);
                else if(islower(sentence[i]))
                    sentence[i]=toupper(sentence[i]);
            }
        }
    }
    firstfile.close();
    cout<<sentence<<endl;
    ofstream myfile;
    myfile.open ("alu.txt");
    myfile<<sentence;
    myfile.close();

    return 0;
}

according to your code, I simply just close() your read mode and open open() mode & take the sentence

this link is may be help to you Input/output with files in C++