Sorting binary files

[TheVAL]

Hi!

I can't understand anymore why isn't my binary file sorting working.

I have created a binary file, e.g., like this:

fstream f("myFile.txt",ios::in|ios:ut|ios::binary);
int n=3, a=2;
f.write((char*) &a,sizeof(int));
a=1; f.write((char*) &a,sizeof(int));
a=3; f.write((char*) &a,sizeof(int));

Now I want to sort the numbers without taking all the contents of the file into the memory - just by manipulating the file. So I write:

f.seekg(0,ios::beg);
int a1,a2;
for (int j=0;j<n-2;j++) {
f.seekg(sizeof(int)*j,ios::beg);
for (int i=j;i<n-1;i++) {
f.read((char*) &a1,sizeof(int));
f.read((char*) &a2,sizeof(int));
if (a1>a2) {
f.seekp(sizeof(int)*(-2),ios::cur);
f.write((char*) &a2,sizeof(int));
f.write((char*) &a1,sizeof(int));
}
f.seekg(sizeof(int)*(-1),ios::cur);
}
}

If I do the job on paper for this trivial example with only 3 numbers, I get the right result - 1, 2, 3. However, the program prints 3 3 3, when performing the printing:
f.seekg(0,ios::beg);
for (int i=1;i<=n;i++) {
f.read((char*) &a,sizeof(int));
cout<<a<<' ';
}

And I just can not get, where the problem is.. I tried to write just `-2` instead of `sizeof(int)*(-2)` and several other things but nothing works.. Can somebody explain it to me?

[Philip Nicoletti]

It is difficult to follow your code without code tags. Please
see the FAQ concerning adding code tags so that there will
be proper indentation.

You are opening the file with the mode : in | out | binary

I think the file must already exist if you use that mode.
Does the file exist ? If not, all of the writes/reads failed.

[TheVAL]

Sorry for tags, I will see the FAQ.

Yes, the file exists, and reading and writing succeeds, just not the right numbers are written..

[Philip Nicoletti]

After the f.seekp() , try outputting the value of f.tellp() to
verify that it is correct.