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.
Related
Say that I have a file with the below format
<records count="n">
record line 1
record line 2
.
.
.
record line n
</records>
I'll have to open this file and change the value of n to another value based on some logic. After change my file should look like.
<records count="m">
record line 1
record line 2
.
.
.
record line n
</records>
I can open the file in OPEN I-O mode and change the first line using the REWRITE option to replace the first line. But I don't want to use these methods. Is there a way to achieve the same logic using OPEN INPUT and OPEN OUTPUT mode and replace the line with WRITE method.
Is there a way to achieve the same logic using OPEN INPUT and OPEN
OUTPUT mode and replace the line with WRITE method[?]
No, that would leave you with only the <records count="m"> in the file. All other records would be lost!
As long as the length of the first record is the same, after changing n to m, REWRITE is the most straight forward way to update that record.
Perhaps, if you explain why you want to use WRITE, there may be something else that could be done.
If the file is not 'too' large, read all the records into memory, change the first record, then write all the records to the file.
If the file is 'too' large, copy the file changing the first record, delete the first file, then rename the copy.
Perhaps less efficient for 'too' large, sort the file by adding a sequence number and changing the first record. This simply uses the sort file to hold the data, temporarily. Possibly a poor choice for a program to be converted.
You need to define what the limit for 'too' is.
There are non-standard routines for file access in Micro Focus, but those might be more difficult to convert.
I wonder how can I update an existing file, and add a word in a given position.
so let say my file looks like:
this the first line in the file
and I want to add the word "is " in position 6 so the file will look like:
this is the first line in the file
what is the best method to achieve that?
what should be the fopen mode?
assume my file is to big to copy to memory, or create a temporary clone
thanks!
There is no magic "insert in the middle" open mode. You have to do that yourself.
If it can't fit in memory, and you don't want/can't create a temporary, you can rewrite it "from the bottom". (I.e. read the last "block", write it back shifted by the amount you want, repeat.)
Unfortunately, it is not possible to simply update a file this way. If it is a flat file, you will have to move the parts yourself.
I am reading info (numbers) from a txt file and after that I am adding to those numbers, others I had in another file, with the same structure.
At the start of each line in the file is a number, that identifies a specific product. That code will allow me to search for the same product in the other file. In my program I have to add the other "variables" from one file to the other, and then replace it, in the same place in one of those files.
I didn't open any of those files with a or a+, I did it with r and r+ because i want to replace the information in the lines that may be in the middle of the file, and not in the end of it.
The program compiles, and runs, but when it comes to replace the info in the file, it just doesn't do anything.
How should I resolve the problem?
A program can replace (overwrite) text in the middle of the file. But the question is whether or not this should be performed.
In order to insert larger text or smaller text (and close up the gap), a new text file must be written. This is assuming the file is not fixed width. The fundamental rule is to copy all original text before the insertion to a new file. Write the new text. Finally write the remaining original text. This is a lot of work and will slow down even the simplest programs.
I suggest you design your data layout before you go any further. Also consider using a database, see my post: At what point is it worth using a database?
Your objective is to design the data to minimize duplication and data fetching.
How can I go back to the very beginning of a csv file and add rows?
(I'm printing to a CSV file from C using fprintf(). At the end of printing thousands of rows (5 columns) of data, I would like to go back to the top of the file and insert some dynamic header data (based on how things went printing everything). )
Thank You.
Due to the way files are structured, this is more or less impossible. In order to accomplish what you want:
write csv data to file1
write header to file2
copy contents of file1 to file2
delete file1
Or you can hold the csv data in ram and write it to file after you're finished processing and know the header.
Another option is to set aside a certain number of bytes for the header, which will work much faster for large files at minimal space cost. Since the space is allocated in the file at the start of the write, there aren't any issues going back and filling it in. Reopen the file as random access ("r+"), which points to the top of the file by default, write header, and close.
The simplest way would be to simply store the entire contents of the file in memory until you are finished, write out the header, and then write out the rest of the file.
If memory is an issue and you can't safely store the entire file in memory, or just don't want to, then you could write out the bulk of the CSV data to a temporary file, then when you are finished, write the header out to the primary file, and copy the data from the temporary file to the primary file in a loop.
If you wanted to be fancy, after writing the main CSV data out to the primary file, you could loop through the file from the beginning, read into memory the data that you're about to overwrite with the header, then write the header over top of that data, and so forth, read each chunk into memory, overwrite it with the previous one until you reach the end and append the final chunk. In this way you "insert" data at the beginning, my moving the rest of the file down. I really wouldn't recommend this as it will mostly just add complexity without much benefit, unless there is a specific reason you can't do something simpler like using a temporary file.
I think that is not possible. Probably the easiest way would be to write the output to a temporary file, then create the data you need as the dynamic header, write them to the target file and append the previously created temporary file.
write enough blank spaces in the first line
write data
seek(0)
write header - last column will be padded with spaces
I've got a service which runs all the time and also keeps a log file. It basically adds new lines to the log file every few seconds. I'm written a small file which reads these lines and then parses them to various actions. The question I have is how can I delete the lines which I have already parsed from the log file without disrupting the writing of the log file by the service?
Usually when I need to delete a line in a file then I open the original one and a temporary one and then I just write all the lines to the temp file except the original which I want to delete. Obviously this method will not word here.
So how do I go about deleting them ?
In most commonly used file systems you can't delete a line from the beginning of a file without rewriting the entire file. I'd suggest instead of one large file, use lots of small files and rotate them for example once per day. The old files are deleted when you no longer need them.
Can't be done, unfortunately, without rewriting the file, either in-place or as a separate file.
One thing you may want to look at is to maintain a pointer in another file, specifying the position of the first unprocessed line.
Then your process simply opens the file and seeks to that location, processes some lines, then updates the pointer.
You'll still need to roll over the files at some point lest they continue to grow forever.
I'm not sure, but I'm thinking in this way:
New Line is a char, so you must delete chars for that line + New Line char
By the way, "moving" all characters back (to overwrite the old line), is like copying each character in a different position, and removing them from their old position
So no, I don't think you can just delete a line, you should rewrite all the file.
You can't, that just isn't how files work.
It sounds like you need some sort of message logging service / library that your program could connect to in order to log messages, which could then hide the underlying details of file opening / closing etc.
If each log line has a unique identifier (or even just line number), you could simply store in your log-parsing the identifier until which you got parsing. That way you don't have to change anything in the log file.
If the log file then starts to get too big, you could switch to a new one each day (for example).