C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  deleting one char in ascii file          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Bernhard Hidding Guest Posted: Tue Jun 29, 2004 10:48 am    Post subject: deleting one char in ascii file Hi, my program writes chars to an ascii file via ofstream. You can use "n" for newline and "t" for tab there, but is there any command that deletes the last char in the current ofstream? Thanks in advance, Bernhard Back to top John Harrison Guest Posted: Tue Jun 29, 2004 11:07 am    Post subject: Re: deleting one char in ascii file "Bernhard Hidding" <hidding (AT) uni-duesseldorf (DOT) de> wrote Quote: Hi, my program writes chars to an ascii file via ofstream. You can use "n" for newline and "t" for tab there, but is there any command that deletes the last char in the current ofstream? No, there is a command that will move you back one position in the stream so you can overwrite the last char with something different, but no command that actually deletes the last char. If you need to do this sort of thing a lot then a file is the wrong thing to use. Do all your processing in a string (its easy to delete the last char in a string) and only when you are finished write out the whole string to a file. john Back to top Bernhard Hidding Guest Posted: Tue Jun 29, 2004 11:42 am    Post subject: Re: deleting one char in ascii file Quote: No, there is a command that will move you back one position in the stream so you can overwrite the last char with something different, but no command that actually deletes the last char. What would that command be? I could overwrite the char with a space then. Regards, Bernhard Back to top osmium Guest Posted: Tue Jun 29, 2004 11:56 am    Post subject: Re: deleting one char in ascii file Bernhard Hidding writes: Quote: No, there is a command that will move you back one position in the stream so you can overwrite the last char with something different, but no command that actually deletes the last char. What would that command be? I could overwrite the char with a space then. seekp(). There is also a seekg(). p for output streams and g for input streams, put and get. Back to top tom_usenet Guest Posted: Tue Jun 29, 2004 12:07 pm    Post subject: Re: deleting one char in ascii file On Tue, 29 Jun 2004 13:42:18 +0200, "Bernhard Hidding" <hidding (AT) uni-duesseldorf (DOT) de> wrote: Quote: No, there is a command that will move you back one position in the stream so you can overwrite the last char with something different, but no command that actually deletes the last char. What would that command be? I could overwrite the char with a space then. Assuming you're using iostreams, I think you want: myfstream.seekp(-1, std::ios_base::cur); //seeks back 1 char from current position Tom -- C++ FAQ: http://www.parashift.com/c++-faq-lite/ C FAQ: http://www.eskimo.com/~scs/C-faq/top.html Back to top John Harrison Guest Posted: Tue Jun 29, 2004 12:32 pm    Post subject: Re: deleting one char in ascii file "tom_usenet" <tom_usenet (AT) hotmail (DOT) com> wrote Quote: On Tue, 29 Jun 2004 13:42:18 +0200, "Bernhard Hidding" [email]hidding (AT) uni-duesseldorf (DOT) de[/email]> wrote: No, there is a command that will move you back one position in the stream so you can overwrite the last char with something different, but no command that actually deletes the last char. What would that command be? I could overwrite the char with a space then. Assuming you're using iostreams, I think you want: myfstream.seekp(-1, std::ios_base::cur); //seeks back 1 char from current position Note this is only guaranteed to work with streams opened in binary mode std::ofstream myfstream("some_file", std::ios_base::binary); Opening a file in binary mode may have other consequences, on line endings for instance. john Back to top Bernhard Hidding Guest Posted: Tue Jun 29, 2004 4:02 pm    Post subject: Re: deleting one char in ascii file Quote: Assuming you're using iostreams, I think you want: myfstream.seekp(-1, std::ios_base::cur); //seeks back 1 char from current position Note this is only guaranteed to work with streams opened in binary mode std::ofstream myfstream("some_file", std::ios_base::binary); Opening a file in binary mode may have other consequences, on line endings for instance. Now this seems to be complicated. I am doing ofstream scriptfile; scriptfile.open(script.c_str(),ios::trunc); /* here comes my ascii output */ scriptfile.seekp(-1, std::ios_base::cur); scriptfile.close(); This works fine. However, I have opened an ascii output, not a binary output, have I? What could go wrong when I am not in binary mode? You know I want my code to be robust. Regards, Bernhard Back to top Mike Wahler Guest Posted: Tue Jun 29, 2004 4:21 pm    Post subject: Re: deleting one char in ascii file "Bernhard Hidding" <hidding (AT) uni-duesseldorf (DOT) de> wrote Quote: Assuming you're using iostreams, I think you want: myfstream.seekp(-1, std::ios_base::cur); //seeks back 1 char from current position Note this is only guaranteed to work with streams opened in binary mode std::ofstream myfstream("some_file", std::ios_base::binary); Opening a file in binary mode may have other consequences, on line endings for instance. Now this seems to be complicated. I am doing ofstream scriptfile; scriptfile.open(script.c_str(),ios::trunc); /* here comes my ascii output */ scriptfile.seekp(-1, std::ios_base::cur); scriptfile.close(); This works fine. However, I have opened an ascii output, not a binary output, have I? First, note that C++ does not define 'ASCII'. C++ i/o can be done in one of two 'modes': 'text' or 'binary'. Quote: What could go wrong when I am not in binary mode? Unwanted character translations, or not getting a guarantee of a given behavior which requires binary mode (such as with 'seeking'). 'Seeking' is done using 'character positions'. On those implementations where 'text' mode translates e.g. a 'n' from a single character to more than one (such as Windows), this will skew the actual seek position within a physical file. Quote: You know I want my code to be robust. 'binary' mode will guarantee that the byte values you write will not be modified by the OS. (whereas with 'text' mode this can happen, e.g. with 'n'). Since all ASCII values are a subset of the range of type 'char', 'binary' mode will preserve them (but then you'll be responsible for ensuring that proper translation of e.g. 'n' (if necessary) does occur. -Mike Back to top John Harrison Guest Posted: Tue Jun 29, 2004 6:07 pm    Post subject: Re: deleting one char in ascii file "Bernhard Hidding" <hidding (AT) uni-duesseldorf (DOT) de> wrote Quote: Assuming you're using iostreams, I think you want: myfstream.seekp(-1, std::ios_base::cur); //seeks back 1 char from current position Note this is only guaranteed to work with streams opened in binary mode std::ofstream myfstream("some_file", std::ios_base::binary); Opening a file in binary mode may have other consequences, on line endings for instance. Now this seems to be complicated. I am doing ofstream scriptfile; scriptfile.open(script.c_str(),ios::trunc); /* here comes my ascii output */ scriptfile.seekp(-1, std::ios_base::cur); scriptfile.close(); This works fine. However, I have opened an ascii output, not a binary output, have I? What could go wrong when I am not in binary mode? Unless you open in binary mode it is not guaranteed that one character output by you results in one character output to the file. Therefore any attempt to count characters is dangerous. Seekp will count characters in the file, which might not tally with the characters you have actually output. The usual real world example is on Windows systems where outputting one character, 'n', results in two characters, 'r' and 'n', being output to the file. In text mode only three types of seeks are guaranteed to work, a seek to the beginning of the file, a seek to the end of the file, or a seek to a position previously saved from a call to tellp (or tellg). Quote: You know I want my code to be robust. Good for you. john 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.