I've started using Lua for school, and I'm wondering if it is possible to save a string to a txt file?
i.e
I have a string called shroom and it equals mush.
I have a file in the same folder as the .lua file called output.txt.
I now want to save the string shroom to output.txt.
I then want to make a while loop and every 1 second save the string shroom to the output.txt file, overwriting the contents of output.txt each time i save the string.
Is this possible in Lua? I've never used Lua before and I'm really confused at the moment..
Thanks :)
You can read this chapter from Programming in Lua to see how you can work with files; you'll need to use "w" mode when you open file and then call f:write(shroom) to do the write.
For sleeping in a loop you may want to check suggestions in this SO question.
Related
I have opened a file using a and r+ but when I use fseek and ftell the file pointer is always 0.
My file looks like this:
1 -3
2 -8
And I want to add another line between the two but it is added in the end after the last line.
Someone in another forum said that when you open the file in append the pointer is always zero and you have to open it in r+ and if that doesn't work "you have to read the complete data and then insert the data in the variables and write it back." but I don't understand what they mean by that.
Can anyone help with inserting numbers in the middle of a file?
Thanks!
Would something like this work?
To transfer the data?
rewind(fp);
fscanf(fp,"%d",&ch);
fprintf(fp1,"%d",ch);
fseek(fp,1,0);
fscanf(fp,"%d",&ch);
fprintf(fp1,"%d",ch);
Like others already said, there's no easy way to insert data in the middle of a file. If you really want to do this, you can implement the following steps:
Create a second file
Copy all data before the place you want to insert to the second file
Insert the line you want to the second file
Copy the remaining data to the second file
Delete the original file
Rename the second file
Other approach is using binary files instead of text files. Although binary files are a bit harder to learn, once you understand how they work you'll see that working with them is much like working with arrays. To perform this task, for example, you'd not even need to use an auxiliary file.
There is no open mode that will allow you to "insert" data into a file at a random point. The only place you can add data without overwriting existing data is the end of the file (what you get opening with mode "a").
If you want to insert at a random position, you need to do it yourself.
One of the easier ways is to re-write the file completely (transfer the start of the old file to a new file, add your data to the new file, transfer the rest of the old file, and rename/overwrite at the end).
The hard way: you need to "shift" all the data from your insertion point to the end-of-file manually. That's not trivial to get right.
There isn't an easy way to insert data in the middle of the file. A file is basically an array of characters. To add a character in the middle, you need to copy everything following your insertion point down one location. With a file you need to read the data that follows and write it after your addition.
Generally, when you want to do something like this you create a new file. You copy the old file into it up to the point where you want to insert, then you write the data you want to insert, then you copy the rest of the old file. Finally, you rename the new file to the old file.
I am working with a text file, which contains a list of processes under my programs control, along with relevant data.
At some point, one of the processes will finish, and thus will need to be removed from the file (as its no longer under control).
Here is a sample of the file contents (which has enteries added "randomly"):
PID=25729 IDLE=0.200000 BUSY=0.300000 USER=-10.000000
PID=26416 IDLE=0.100000 BUSY=0.800000 USER=-20.000000
PID=26522 IDLE=0.400000 BUSY=0.700000 USER=-30.000000
So for example, if I wanted to remove the line that says PID=26416.... how could I do that, without writing the file over again?
I can use external unix commands, however I am not very familiar with them so please if that is your suggestion, give an example.
Thanks!
Either you keep the contents of the file in temporary memory and then rewrite the file. Or you could have a file for each of the PIDs with the relevant information in them. Then you simply delete the file when it's no longer running. Or you could use a database for this instead.
As others have already pointed out, your only real choice is to rewrite the file.
The obvious way to do that with "external UNIX commands" would be grep -v "PID=26416" (or whatever PID you want to remove, obviously).
Edit: It is probably worth mentioning that if the lines are all the same length (as you've shown here) and order doesn't matter, you could delete a line more efficiently by copying the last line into the space being vacated, then shorten the file so eliminate what had been the last line. This will only work if they really are all the same length though (e.g., if you got a PID of '1', you'd need to pad it to the same length as the others in the file).
The only way is by copying each character that comes after the deleted line down over the characters that are deleted.
It is far more efficient to simply rewrite the file.
how could I do that, without writing the file over again?
You cannot. Filesystems (perhaps besides more esoteric record based ones) does not support insertion or deletion.
So you'll have to write the lines to a temporary file up till the line you want to delete, skip over that line, and write the rest of the lines to the file. When done, rename/copy the temp file to the original filename
Why are you maintaining these in a text file? That's not the best model for such a task. But, if you're stuck with it ... if these lines are guaranteed to all be the same length (it appears that way from the sample), and if the order of the lines in the file doesn't matter, then you can write the last line over the line for the process that has died and then shorten the file by one line with the (f)truncate() call if you're on a POSIX system: see Jonathan Leffler's answer in How to truncate a file in C?
But note carefully netrom's answer, which gives three different better ways to maintain this info.
Also, if you stick with a text file (preferably written from scratch each time from data structures you maintain, as per netrom's first suggestion), and you want to be sure that the file is always well formed, then write the new data into a temp file on the same device (putting it in the same directory is easiest) and then do a rename() call, which is an atomic operation.
You can use sed:
sed -i.bak -e '/PID=26416/d' test
-i is for editing in place. It also creates a back-up file with the new extension .bak
-e is for specifying the pattern. The /d indicates all lines matching the pattern should be deleted.
test is the filename
The unix command for it is:
grep -v "PID=26416" myfile > myfile.tmp
mv myfile.tmp myfile
The grep -v part outputs the file without the rows with the search term.
The > myfile.tmp part creates a new temp file for this output.
The mv part renames the temp file to the original file.
Note that we are rewriting the file here, and moreover, we can lose data if someone write something to file between the two commands.
How do I replace a word in a file with another word using C?
For example, I have a file which contains:
my friend name is sajid
I want to replace the word friend with grandfather, such that the file is changed to:
my grandfather name is sajid
(I am developing on Ubuntu Linux.)
Update:
I am doing filing in C. I have created a .txt file and write some data into it, but as my program progresses I have to search some text and replace it with the other words. The problem I am facing is that suppose in my file I wrote
"I bought apple from the market"
If i replace apple with pineapples as apple has 5 char and pineapple has 9 char it will write it as
"I bought pineapple m the market"
It also has affected the words written after apple.
I have to do it using C, not a command line script.
How about using the exiting Linux sed program?
sed -i 's/friend/grandfather/' filename
That will replace friend with grandfather in the existing file. Make a copy first if you want to keep the original!
Edit:
Alternatively, load the file into an STL string, replace 'friend' with 'grandfather' using a technique such as this, and save the new string into a file.
As you've realized, you won't be able to make the change in place in the original file.
The easy solution is to read each string from the original file and write it to standard output, making the replacement as necessary. In pseudocode:
open original file
while not at end of original file
get next string from original file
if string == "friend"
write "grandfather" to standard output
else
write string to standard output
end while
You can then redirect the output to another file when you execute it, similar to how sed and awk work.
Alternately, you can create a destination file in the code and write to it directly. If you need to replace the original file with the new file, you can use the remove() and rename() library functions in stdio.
How can I delete few characters from a file using C program?
I could not find any predefined functions for it.
To understand the purpose, I am trying to send a file through a socket, if N bytes are sent successfully, I want to delete those bytes from the file. At the end, the file will be empty.
Any other way to do this efficiently?
Thanks
Pradeep
If they're at the end, truncate the file at the appropriate length. If they're not then you'll need to rewrite the file.
Your way is pretty inefficient for large files, since you would have to copy "the rest of the file" some bytes further to the beginning, which costs much. I would rather record the "current sending position" somewhere outside of the file and update that information. That way, you don't have to copy the rest of the file so often.
There is no straightforward way to delete bytes from the beginning of a file. You will have to start from where you want to trim the file, and read from there to the end of the file, writing to the start of the file.
It might make more sense to just track how many bytes you have already written to the file in some other file.
you should use an index which points to the beginning of the data you haven't sent yet.
It is not necessary to delete what you have sent, just pass them, when you send the whole file delete it.
If the char's are one after the other than why dont you give a try to fseek();
When you open a .txt file with fopen
Is there any way to delete some strings in a file without rewriting.
For example this is the txt file that i will open with fopen() ;
-------------
1 some string
2 SOME string
3 some STRING
-------------
i want to delete the line which's first character is 2 and change it into
-------------
1 some string
3 some STRING
-------------
My solution is;
First read all data and keep them in string variables. Then fopen the same file with w mode. And write the data again except line 2. (But this is not logical i am searching for an easier way in C ...)
(i hope my english wasn't problem)
The easiest way might be to memory-map the whole file using mmap. With mmap you get access to the file as a long memory buffer that you can modify with changes being reflected on disk. Then you can find the offset of that line and move the whole tail of the file that many bytes back to overwrite the line.
you should not overwrite the file, better is to open another (temp)-file, write contents inside and then delete old file and rename the file. So it is safer if problems occur.
I think the easiest way is to
read whole file
modify contents in memory
write back to a temp file
delete original file
rename temp file to original file
Sounds not too illogical to me..
For sequential files, no matter what technique you use to delete line 2, you still have to write the file back to disk.