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

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++

Answer 2

Follow #πάντα ῥεῖ solution, you could using ifstream to read the contents in the specific file, then judge and change UPPER/lower for every character. At the end, using ofstream to overwrite the original file.
Wish is helpful.

Answer 3

You can use fstream to do in-place rewriting in C++. This will replace all lowercase with uppercase and vice-versa, character by character.
#include <iostream>
#include <fstream>
#include <locale>
int main()
{
std::locale loc;
std::fstream s("a.txt");
char next;
int pos = 0;
while((next = s.peek()) != EOF) {
if(islower(next,loc))
next = toupper(next,loc);
else if(isupper(next,loc))
next = tolower(next,loc);
s.seekp(pos);
s.write(&next,1);
pos++;
}
return 0;
}