Thanks Dennis Lee and Thomas On Thu, Oct 11, 2018 at 6:32 PM Dennis Lee Bieber <wlfr...@ix.netcom.com> wrote:
> On Thu, 11 Oct 2018 15:14:49 +0530, Iranna Mathapati > <iranna.gan...@gmail.com> declaimed the following: > > >Hi Team, > > > >How to replace particular line text with new text on a file > >i have below code but its writing whole code. > > > <SNIP> > > For C-style streams (which seem to have taken over every operating > system) in which "lines" are determined by specific characters in the > stream, you can only "replace" a line IF THE REPLACEMENT IS THE EXACT SAME > SIZE! If there is any difference in length of the input and output lines, > you have to read/write everything from the line to the end of the file. > > I emphasize "C-style streams" as there were operating systems that > did > support other file organizations natively. Xerox CP-V offered > "consecutive", "keyed", and "random" file organizations. > > "Consecutive" being the closest to streams -- pure sequentially > written > data. > > "Keyed" files used a multi-level ISAM structure -- which is what > was > used by the text editor and by the FORTRAN direct-access mode (line/record > numbers were used as the ISAM key). In these, replacing a line meant > writing a new line <somewhere> and updating the ISAM headers to point to > the new line. Periodically one would have to pack the file to reclaim > unused/freed space. > > "Random" files were an organization in which the OS did /nothing/. > These files were created as a predetermined maximum size, and were > allocated as a contiguous section of disk space -- the OS didn't even > bother tracking blocks. An application basically was given two pieces of > information: the starting address on disk, and the total length of the > allocation. ALL data management had to be done by the application. > > Past that, there is the concept of a fixed-length record. ALL > records > (lines) in the file having the same predefined maximum length. Internally, > each record would either have a length datum at the beginning, or use some > special character to indicate where the end of the record's data really is > -- but all I/O was performed on the full <maximum length>. Even > TRSDOS/LS-DOS (TRS-80 Model III/4) supported fixed-length I/O in the OS, > though the maximum record length was one disk sector (256 bytes; shorter > records definitions were permitted and the system /did/ pack them into disk > space rather than wasting the tail of the sector). Fixed length schemes > typically permit direct access > > x = read(rec=123) > > write(y, rec=123) > > > > -- > Wulfraed Dennis Lee Bieber AF6VN > wlfr...@ix.netcom.com HTTP://wlfraed.home.netcom.com/ > > -- > https://mail.python.org/mailman/listinfo/python-list > -- https://mail.python.org/mailman/listinfo/python-list
