File parsing using ifstream

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Hi All

If you use the following to find the size of a file and then effectively read every character of the file until the end of the file:

long begin, end, size;
ifstream myfile ("example.txt");
begin = myfile.tellg();
myfile.seekg (0, ios::end);
end = myfile.tellg();
myfile.seekg (0, ios::beg);
size = end - begin;

do
{
  myfile.get();
  size--;
}while( !myfile.eof() );

After running this I was expecting size to equal 0 however it was approx 114000. Is this down to whitespace in the text file or something else??? I know it is getting to the end of the file as I temporarily put some extra code in to catch the last 20 characters and it matched the end of the file.

Any help appreciated.

Thanks
Andy

Update - Have now run this exact code as a seperate project as a console app and am having the same problem so it is not other code causing the problem.

Posted 5-May-10 1:19am

Andy Melville

Updated 5-May-10 2:21am

v2

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Richard MacCutchan 5-May-10 9:18am

Re your comment below: Yes, the original was ordinary text (contains CR/LF), and I think the reader senses EOF one character early. A binary file works as noted below and leaves the size variable set to -1. I don't know what's happening with your test, but you may want to use the debugger to get more information.

Richard MacCutchan 5-May-10 10:38am

Update: Using ios::binary gives -1 as the result, using text mode gives 1 if the file ends with CR/LF on a blank line.

2 solutions

Solution 1

I just tried this and got to the end with size being equal to 1 at the end, which seems correct. Perhaps you have some other code that is corrupting your size variable. I did also notice the line

C++

size = end - beg;

which does not compile on my system, I think the second variable should be begin.

[edit]My previous test was flawed. If I use a non text file then size counts down to -1 because you always decrement its value after the myfile.get() call, even if that was the end of file condition.[/edit]

Posted 5-May-10 1:53am

Richard MacCutchan

Updated 5-May-10 2:01am

v2

Comments

Andy Melville 5-May-10 8:04am

Yep should have been begin. Interesting it works on your implementation. Did you have carriage returns in your file???

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Sauro Viti · Accepted Answer · 2010-05-06T00:10:00

The reason is this: when you get characters from the text file using myfile.get(), the ifstream object read a new character from the file in translated mode, i.e. it replace CR-LF sequences from the file to a single CR character.
Another issue is that the ifstream::eof() method returns true not when there are no more characters to read, but only after that you have read a new character past the end of the file.
You can fix your problem this way:

C++

long begin, end, size;
ifstream myfile("example.txt");
begin = myfile.tellg();
myfile.seekg(0, ios::end);
end = myfile.tellg();
myfile.seekg(0, ios::beg);
size = end - begin;

ifstream::int_type character;
while ((character = myfile.get()) != EOF)
{
   if (character == '\n') size -= 2;
   else size -= 1;
}