C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  compare 2 files          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Siemel Naran Guest Posted: Sun Dec 19, 2004 10:13 am    Post subject: compare 2 files How to compare if two files are identical? I wrote the following: bool comparefiles(const std::string& lhs, const std::string& rhs) { std::ifstream lhsfile(lhs.c_str()); std::ifstream rhsfile(rhs.c_str()); typedef std::istreambuf_iterator<char> istreambuf_iterator; return std::equal( istreambuf_iterator(lhsfile), istreambuf_iterator(), istreambuf_iterator(rhsfile) ); } But I don't think it will work becuase: (1) we only compare the first N chars where N is the number of chars in lhsfile, so if rhsfile has more chars the function will return true if the first N are equal which is incorrect, (2) the standard says that calling operator* on an end of stream is undefined (24.5.3.3), so if lhsfile has more chars then we will at some point call operator* on rhsfile when it is at EOF, and the result is undefined (though I think it should always return EOF). So what else can we do? I could use the stat function to check if lhsfile and rhsfile have the same size, but I want to keep my code ANSI compatible. So I came up with the following function, which looks very much like strcmp. bool comparefiles(const std::string& lhs, const std::string& rhs) { using namespace std; const streambuf::int_type eof = streambuf::traits_type::eof(); ifstream lhsfile(lhs.c_str()); ifstream rhsfile(rhs.c_str()); streambuf * lhsbuf = lhsfile.rdbuf(); streambuf * rhsbuf = rhsfile.rdbuf(); char lhschar, rhschar; while (true) { lhschar = lhsbuf->sbumpc(); rhschar = rhsbuf->sbumpc(); if (lhschar == eof && rhschar == eof) return true; if (lhschar == eof || rhschar == eof) break; if (lhschar != rhschar) break; } cout << "compare "" << lhs << "" and "" << rhs << "" failedn"; return false; } Any comments? Back to top Ivan Vecerina Guest Posted: Sun Dec 19, 2004 10:29 am    Post subject: Re: compare 2 files "Siemel Naran" <SiemelNaran (AT) REMOVE (DOT) att.net> wrote Quote: How to compare if two files are identical? I wrote the following: .... So I came up with the following function, which looks very much like strcmp. bool comparefiles(const std::string& lhs, const std::string& rhs) { using namespace std; const streambuf::int_type eof = streambuf::traits_type::eof(); ifstream lhsfile(lhs.c_str()); ifstream rhsfile(rhs.c_str()); streambuf * lhsbuf = lhsfile.rdbuf(); streambuf * rhsbuf = rhsfile.rdbuf(); Since only the stream buffer interface is used, you can directly create instances of std::filebuf instead of an ifstream. Quote: char lhschar, rhschar; These two variables should be of type int_type. char may be unable to represent eof (or be equal to eof when it should not, e.g. when reading 0xFF on an implementation where char is signed). Quote: while (true) { lhschar = lhsbuf->sbumpc(); rhschar = rhsbuf->sbumpc(); if (lhschar == eof && rhschar == eof) return true; if (lhschar == eof || rhschar == eof) break; if (lhschar != rhschar) break; } or: do { lhschar = lhsbuf.sbumpc(); rhschar = rhsbuf.sbumpc(); if( lhschar != rhschar ) return false; } while( lhschar != eof ); return true; Cheers, Ivan -- http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form Back to top Howard Guest Posted: Mon Dec 20, 2004 4:31 pm    Post subject: Re: compare 2 files I think the first thing I'd do it check if the file sizes are the same. No need to read tthrough the file looking for differences if they're different sizes. I'm not familiar with how to check file size, but if that's easy enough to do, you might want to throw in a check for that equality before bothering to check the contents. Just a thought... -Howard Back to top Siemel Naran Guest Posted: Mon Dec 20, 2004 6:48 pm    Post subject: Re: compare 2 files "Howard" <alicebt (AT) hotmail (DOT) com> wrote Quote: I think the first thing I'd do it check if the file sizes are the same. No need to read tthrough the file looking for differences if they're different sizes. I'm not familiar with how to check file size, but if that's easy enough to do, you might want to throw in a check for that equality before bothering to check the contents. Just a thought... This is the ideal solution, then I can continue to use std::equal as in my original code. However, the standard does not provide a way to find the file size without opening it and scanning to the last character. Opening the file, calling file.seekg(ios::end) followed by file.tellp() is allowed to return 0 rather than the actual byte position though my implementation does in fact return the file size. There is a function stat, and it's on Windows and Linux, but it's not ANSI standard (though maybe it should be). I know that boost also has some way to get the file size, and I imagine the implementation calls stat on Windows and Linux, etc. Back to top Jeff Flinn Guest Posted: Mon Dec 20, 2004 7:11 pm    Post subject: Re: compare 2 files Siemel Naran wrote: Quote: "Howard" <alicebt (AT) hotmail (DOT) com> wrote I think the first thing I'd do it check if the file sizes are the same. No need to read tthrough the file looking for differences if they're different sizes. I'm not familiar with how to check file size, but if that's easy enough to do, you might want to throw in a check for that equality before bothering to check the contents. Just a thought... This is the ideal solution, then I can continue to use std::equal as in my original code. However, the standard does not provide a way to find the file size without opening it and scanning to the last character. Opening the file, calling file.seekg(ios::end) followed by file.tellp() is allowed to return 0 rather than the actual byte position though my implementation does in fact return the file size. There is a function stat, and it's on Windows and Linux, but it's not ANSI standard (though maybe it should be). I know that boost also has some way to get the file size, and Yes, at: http://www.boost.org/libs/filesystem/doc/operations.htm#file_size Quote: I imagine the implementation calls stat on Windows and Linux, etc. Windows: GetFileAttributes() POSIX: stat() Jeff Flinn Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.