Overriting previous entries in a c++ text file

leaprachanus

I am a novice programmer trying to increment a counter and write it to a file, when i do this it adds it to the end of the text file. Is there any way i can overrite the previous entry with the new one

daymobrew

I gave this a go. I assumed that the counter file existed and the number was the first item on the first line.

The basic algorithm I used was:
- fopen counter file, "r+" mode
- Read line and convert to an integer.
- Increment integer
- Rewind file pointer and write new number back to counter file.
- Close file.

That should get you going. I won't post the bit of code I wrote unless you get nowhere with the above pseudo code.

leaprachanus

In a text file it still doesnt overwrite the previous entry it just adds it in before the first entry

daymobrew

leaprachanus wrote:

In a text file it still doesnt overwrite the previous entry it just adds it in before the first entry

Not in my code.
Post your code so we can point out where it might be going wrong.

leaprachanus

This is the origional code


int main()
{
int reference=0;
fstream hotreftxtfile;

if(reference==0)
{
hotreftxtfile.open("hotref.dat",ios::in|ios::out);
if (!hotreftxtfile)
{
cout << "Error: Can't open the file named data.txt.\n";
exit(1);
}

reference=1;
hotreftxtfile << reference;
hotreftxtfile.close();
}

hotreftxtfile.open("hotref.dat",ios::in|ios::out);
hotreftxtfile >> reference;
reference+=1;
hotreftxtfile << flush;
hotreftxtfile.flush();
hotreftxtfile << endl;
hotreftxtfile << reference;
hotreftxtfile.close();

return 0;
}

daymobrew

if(reference==0)

This is always true because you initialise reference to 0.

hotreftxtfile << endl;
hotreftxtfile << reference;

You are putting the incremented reference on a new line, but only the first line of the file ls read. Lose the endl bit.

You also need to move the file pointer back to the start of the file before writing reference to the file.

hotreftxtfile.seekp( 0, ios::beg );

cout << "Error: Can't open the file named data.txt.\n";

Minor: filename in message is not what you are trying to open.

You should check for read/write errors with the rdstate() function.

daymobrew

I implemented a solution by opening the file for reading and getting the value for 'reference'. If there was a read error I assumed that the file did not exist and set 'reference' to 0 (it is later incremented to 1). I close the file.

Then I reopen the file, for writing (and truncate it too) and write 'reference'.

I was opening it for read/write but rdstate() indicated errors (== 3) when I wrote to the file.