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How do I insert and delete some characters in the middle of a file?
(4 answers)
Closed 9 years ago.
I'm writing a program to edit a txt file.
But I found that the windows API WriteFile can only add data/characters to a file, but not deleting data from files.
The only solution I've come up is to read the whole file into a buffer using ReadFile, and then use a loop to shift the data one by one, then replace the old file with the new file. But I think this will probably make my program really slow.
Can anyone help please
thanks.
If you're trying to delete from the end of the file it can be very fast with truncate() and ftruncate().
Where are you trying to delete the data from? If it's from the middle, you'll have to use fseek(): If the file contains "ABCDEFG", and you want to delete "DEF", use fseek() to get to G, copy "G" into a buffer, fseek to where "C" is, then write() what's there. Then truncate the file to the correct size with ftruncate().
If this really becomes a performance issue for you, you'll want to either design your file in a way that accounts for this or use a database of some kind. You may also want to use memory-mapped files, but usually this is better done by a database that someone else wrote instead of reinventing the wheel.
Files are linear streams of data. If you want to remove content from a file, you must re-write all the content of the file that follows the part that you have remove. So, unless the content to be removed is at the end of the file, you will need to perform some writing. In the worst case scenario, in order to remove the first byte of a file, you need to re-write the entire file apart from the byte that you removed.
FWIW, Raymond Chen wrote a nice article on this subject: How do I delete bytes from the beginning of a file?
Related
I have a file that can be opened thru notepad application.
Basically, this file(which can be opened thru notepad) is created by a software and that software uses the values inside that file to run. You can edit the values inside the file using its software.
I just want specific lines(values) to be restricted from being edited because I am implementing strict values inside that file that no one will be able to edit except me.
Is there any clever way to restrict specific lines inside that file from being edited?
I tried the basic way - I used the change permission read/write on that file but I can't change ANY values inside the file which is undesirable.
Note: I have very little to no experience about python, c++, or java but any suggestion will give me idea to learn from it.
Edit:
Here's an example inside the file:
[Type Data]
Comment=Standard Dispense
[Shared_A]
802=1
807=750
11=0
12=0
.
What I want is restrict the value from row/column "807" which is equal to number 750.
I want this number 750 not to be edited even from the software so that other people will not mess it up. I want to set this value as standard value.
Is there any program that you can write inside that file so that it cannot be edited from the software unless I open that file and edit it?
I work from a production/manufacturing company that uses the software that is used for dispensing.
A text file is simply a sequence of bytes that represent code units to encode code points in any given character set. Every byte value is a potentially legal character encoding, leaving no values to encode additional semantics (like guard regions).
With that it should be obvious that there is nothing you can do to partially limit editing of a file using a standard text editor. Whatever problem you are trying to solve, this is not a solution. Next time around you might want to ask about the problem you are trying to solve rather than your proposed solution.
I am a novice Go lang programmer,trying to learn Go lang features.I wanted to split a large csv file into multiple files in GO lang, each file containing the header.How do i do this? I have searched everywhere but couldnt get the right solution.Any help in this regard will be greatly appreciated.
Also please suggest me a good book for reference.
Thanking You
Depending on your shell fu this problem might be better suited for common shell utilities but you specifically mentioned go.
Let's think through the problem.
How big is this csv file? Are we talking 100 lines or is it 5G ?
If it's smallish I typically use this:
http://golang.org/pkg/io/ioutil/#ReadFile
However, this package also exists:
http://golang.org/pkg/encoding/csv/
Regardless - let's return to the abstraction of the problem. You have a header (which is the first line) and then the rest of the document.
So what we probably want to do (if ignoring csv for the moment) is to read in our file.
Then we want to split the file body by all the newlines in it.
You can use this to do so:
http://golang.org/pkg/strings/#Split
You didn't mention but do you know how many files you want to split by or would you rather split by the line count or byte count? What's the actual limitation here?
Generally it's not going to be file count but if we pretend it is we simply want to divide our line count by our expected file count to give lines/file.
Now we can take slices of the appropriate size and write the file back out via:
http://golang.org/pkg/io/ioutil/#WriteFile
A trick I use sometime to help think me threw these things is to write down our mission statement.
"I want to split a large csv file into multiple files in go"
Then I start breaking that up into pieces but take the divide/conquer approach - don't try to solve the entire problem in one go - just break it up to where you can think about it.
Also - make gratiutious use of pseudo-code until you can comfortably write the real code itself. Sometimes it helps to just write a short comment inline with how you think the code should flow and then get it down to the smallest portion that you can code and work from there.
By the way - many of the golang.org packages have example links where you can literally run in your browser the example code and cut/paste that to your own local environment.
Also, I know I'll catch some haters with this - but as for books - imo - you are going to learn a lot faster just by trying to get things working rather than reading. Action trumps passivity always. Don't be afraid to fail.
Here is a package that might help. You can set a necessary chunk size in bytes and a file will be split on an appropriate amount of chunks.
Seems like it must be easy, but I just can't figure it out. How do you delete the very last character of a file using Ruby IO?
I took a look at the answer for deleting the last line of a file with Ruby but didn't fully understand it, and there must be a simpler way.
Any help?
There is File.truncate:
truncate(file_name, integer) → 0
Truncates the file file_name to be at most integer bytes long. Not available on all platforms.
So you can say things like:
File.truncate(file_name, File.size(file_name) - 1)
That should truncate the file with a single system call to adjust the file's size in the file system without copying anything.
Note that not available on all platforms caveat though. File.truncate should be available on anything unixy (such as Linux or OSX), I can't say anything useful about Windows support.
I assume you are referring to a text file. The usual way of changing such is to read it, make the changes, then write a new file:
text = File.read(in_fname)
File.write(out_fname, text[0..-2])
Insert the name of the file you are reading from for in_fname and the name of the file you are writing to for 'out_fname'. They can be the same file, but if that's the intent it's safer to write to a temporary file, copy the temporary file to the original file then delete the temporary file. That way, if something goes wrong before the operations are completed, you will probably still have either the original or temporary file. text[0..-2] is a string comprised of all characters read except for the last one. You could alternatively do this:
File.write(out_fname, File.read(in_fname, File.stat(in_fname).size-1))
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
How to read a file from bottom to top in Ruby?
In the course of working on my Ruby program, I had the Eureka Moment that it would be much simpler to write if I were able to parse the text files backwards, rather than forward.
It seems like it would be simple to simply read the text file, line by line, into an array, then write the lines backwards into a text file, parse this temp file forwards (which would now effectively be going backwards) make any necessary changes, re-catalog the resulting lines into an array, and write them backwards a second time, restoring the original direction, before saving the modifications as a new file.
While feasible in theory, I see several problems with it in practice, the biggest of which is that if the size of the text file is very large, a single array will not be able to hold the entirety of the document at once.
Is there a more elegant way to accomplish reading a text file backwards?
If you are not using lots of UTF-8 characters you can use Elif library which work just like File.open. just load Elif and replace File.open with Elif.open
Elif.open('read.txt', "r").each_line{ |s|
puts s
}
This is a great library, but the only problem I am experiencing right now is that it have several problems with line ending in UTF-8. I now have to re-think a way to iterate my files
Additional Details
As I google a way to answer this problem for UTF-8 reverse file reading. I found a way that already implemented by File library:
To read a file backward you can try the ff code:
File.readlines('manga_search.test.txt').reverse_each{ |s|
puts s
}
This can do a good job as well
There's no software limit to Ruby array. There are some memory limitations though: Array size too big - ruby
Your approach would work much faster if you can read everything into memory, operate there and write it back to disk. Assuming the file fits in memory of course.
Let's say your lines are 80 chars wide on average, and you want to read 100 lines. If you want it efficient (as opposed to implemented with the least amount of code), then go back 80*100 bytes from the end (using seek with the "relative to end" option), then read ONE line (this is likely a partial one, so throw it away). Remember your current position via tell, then read everything up until the end.
You now have either more or less than a 100 lines in memory. If less, go back (100+1.5*no_of_missing_lines)*80, and repeat the above steps, but only reading lines until you reach your remembered position from before. Rinse and repeat.
How about just going to the end of the file and iterating backwards over each char until you reach a newline, read the line, and so on? Not elegant, but certainly effective.
Example: https://gist.github.com/1117141
I can't think of an elegant way to do something so unusual as this, but you could probably do it using the file-tail library. It uses random access files in Ruby to read it backwards (and you could even do it yourself, look for random access at this link).
You could go throughout the file once forward, storing only the byte offset of each \n instead of storing the full string for each line. Then you traverse your offset array backward and can use ios.sysseek and ios.sysread to get lines out of the file. Unless your file is truly enormous, that should alleviate the memory issue.
Admitedly, this absolutely fails the elegance test.
I have a program that is reading a text file, and owing to the vagaries of the file definition and the definitions of the objects the data has to be shovelled in to, I appear to have a need to move the read pointer of the file back up the file for a line, in a manner roughly analagous to the FORTRAN BACKSPACE statement.
Is there any method of doing this, either with native VB6 statements or with VB6 FileSystem objects?
I'm pretty sure VB6 provides a seek() function to do this.
Otherwise, if the file is relatively small you could read it all into memory and use the split() function to separate it into lines. These could then be accessed however you want. Obviously if the file is large this is not a good idea though.
The FSO only lets you read forwards.
There isn't a way to do this in VB6. What you could do is to either read the whole file, a line at a time, into an array and then iterate through the array as needed. Or if that caused memory issues, then create a data structure and use Input to read a line into an instance of the structure based upon the line number.