如何使用CStdioFile读取最后10行文本文件

Question

I would like to read last 10 lines of a big text file. Normally I loop through the file and get the last 10 lines. Please kindly tell me, are there other ways to read the last lines quicker.

Thanks.

Answer 1

You can get the file's length in bytes with ( CStdioFile::GetLength ) (inherited from CFile ):

http://msdn.microsoft.com/en-US/library/b569d0t4(v=VS.80).aspx

And you can use CStdioFile::Seek to jump to an arbitrary offset:

http://msdn.microsoft.com/en-US/library/8c5ccz0x(v=VS.80).aspx

If the line length is fixed, it's a pretty easy problem to read the last N lines. But there's no "read backwards" operation, and in the general case you don't know how long the lines in an arbitrary text file are. (You could build and maintain an index file that kept track of it, if you wanted to do this faster.)

Be aware that seeking and reading one character at a time may be inefficient. Exactly how inefficient depends on several factors. Although buffering at the operating system level might make it not as terrible as it could be if it went back to the disk each time, there's always going to be overhead each time you seek and each time you call a read operation.

So you would probably be better off picking a buffer size, and step through reading chunks. As a strong indication that this is useful, see some source code for the UNIX called tail . It gets the last N lines of a file, and although it's written in C it might give you some insight:

http://git.savannah.gnu.org/cgit/coreutils.git/tree/src/tail.c#n477

Answer 2

In binary files, you (normally) have a fixed record length. In text files, one possibility is to consider the text line as the record in which you divide the contents of the file. Unfortunately, lines are not of fixed size in regular text files, so you cannot rely in a formula such as:

lastTenthLine = ( LengthOfFile / LengthOfLine ) - 10;

The only way I can think of is to move the reading pointer to the end of the file. Tne start reading backwards, one char each time, and count the number of times you pass over a '\\n' char. Once you have counted ten times, you can return the next char position.

(Note that you can have "end line issues" depending on the origin (OS) of the files you are using as example. Since you are ussing MFC, you won't have any problem as long as the files used were generated in Windows)

Since I suppose this is homework, I won't post code. You have documentation about MFC here, explaining how to deal with files. You need the CFile::Read and CFile::Seek methods (apart from Open and Close). Note that CFile is the parent class of CStdioClass .

http://msdn.microsoft.com/en-us/library/ey6xh9bk(v=vs.80).aspx

Hope this helps.