"..., and the easiest way to do this would be (if appending itself is impossible) to rewrite the line in its entirety by simply setting the current line to a different string. There have been a few posts that I've seen that say this is impossible, but I find this hard to believe."
Well, believe it in general.
Sequential text files are, well, "sequential" -- they're just a string of bytes -- the only way to replace text in line in a text file (that isn't record oriented as is supported by Fortran, but NOT supported by C/MATLAB) is if the new line is exactly the same number of characters as the line it is replacing. Otherwise it will overwirte subsequent characters if longer or leave old data that was part of the previous line if write fewer characters.
The way to do something like this is either
- read the file into memory and operate on it in memory to make the changes desired, then rewrite the modified file in its entirety, or
- as you're doing read a record at a time, make any modifications needed, then write the original/modified record to a new file.
With large files that may be too large for memory, the second is the obvious choice...I've written and posted several such filters on Answers in the past; unfortunately I don't have a link to any of them to point to a particular instance.
