Opening multiple files for writing

[Peds]

Hi,

I am new to C++, I am using it for a small project where I need to sort data and output it to multiple different files for further processing.

I am trying to create and open a number of files, which may change between runs. The files don't need clever names, just something like "dump001.dat", "dump002.dat", etc.

I was hoping to do something like:

std::string filename;
ifstream myfile;
for (int i=0; i!=totalnumber; ++i)
{
    filename = "dumpfile" + i+1 + ".dat" ;
    myfile.open(filename);
}
// code to read in data
// more code to put data into the relevant output file 

This gives me a world of errors and I don't really know why.
Any help would be greatly appreciated!!!
Thanks in advance.

Edit: I missed the semicolons, there were there in my code though!

[mutexe]

it's line 5. You can't just concantanate strings and ints together like that.

try
filename = "dumpfile" + std::to_string(i) + ".dat";

if you want "001", "002" you'll have to format it as well.

[Peds]

Thanks mutexe!
how do I then pickout a specific file later on? is it as simple as:

dump_file1 << "blah blah blah" 

?

[mutexe]

If i understand what you're asking (and i'm not sure I am!), you could, after line6, add the file name to a std::vector<std::string> collection.
That way you can do what you want with them, iterate over the whole collection or search for a specific filename string.

[Peds]

I the code i am basically writing is to bin large amounts of data into smaller files that can be loaded into memory, with each bin being handled separately.
for example with 2 files, 'file1.dat' & 'file2.dat' and read in int 'x'

if (x < 10) {
  file1.dat << x;
} else { 
  file2.dat << x;
}

These files will then be read in and post processed by an old code we've already got written.

I hope that's a little clearer without going into all the detail?
Thanks,

[mutexe]

You'll need one stream object per file you want to write to then (i think).
perhaps it would be better to keep an array of ostreams rather than filename strings then..

[Peds]

Thanks mutexe, I don't really understand what you mean by that, could you explain it a tad more?

[mutexe]

well this from your first post:

std::string filename;
ifstream myfile;
for (int i=0; i!=totalnumber; ++i)
{
    filename = "dumpfile" + i+1 + ".dat" ;
    myfile.open(filename);
}

will indeed open a number of files. but the way you'd write to a file normally would be something like:

myfile << "This is a line.\n";

i.e. you get to the file via the stream object. So i'm guessing (although if someone could clarify/confirm/correct that would be good) that your 'myFile' stream would be associated with the last file created in your for loop.

You would effectively have a bunch of "dangling" files with no stream to get to them through, apart from the last one you open.