I have the text file with 2000 lines and each line contains up to 256 characters.
I need to detect EOF as I'm reading the file.
Should the code process a line as it is read or should it read the lines into an array and then process the array one element at a time?
If the data don't change in your file, you should stock the data in an array.
In the other case, you have to read your file each time you need to check a line.
Related
Is it possible to write at the middle of a file for example I want to insert some string at the 5th position in the 2nd line of a file in c ?
I'm not very familiar with some of C functions that are related to handling files , if someone could help me I would appreciate it
I tried using fputs but I couldn't insert characters at the desired location
open a new output file
read the input file line by line (fgets) writing each line out to a new file as you read.
When you hit the place you want to insert write the new line(s)
The carry on copy the old lines to the new file
close input and output
rename output file to input
Continuing from my comments above. Here's what I'd do:
Create two large, static char[] buffers of the same size--each large enough to store the largest file you could possibly ever need to read in (ex: 10 MiB). Ex:
#define MAX_FILE_SIZE_10_MIB (10*1024*1024)
static char buffer_file_in[MAX_FILE_SIZE_10_MIB];
static char buffer_file_out[MAX_FILE_SIZE_10_MIB];
Use fopen(filename, "r+") to open the file as read/update. See: https://cplusplus.com/reference/cstdio/fopen/. Read the chars one-by-one using fgetc() (see my file_load() function for how to use fgetc()) into the first large char buffer you created, buffer_file_in. Continue until you've read the whole file into that buffer.
Find the location of the place you'd like to do the insertion. Note: you could do this live as you read the file into buffer_file_in the first time by counting newline chars ('\n') to see what line you are on. Copy chars from buffer_file_in to buffer_file_out up to that point. Now, write your new contents into buffer_file_out at that point. Then, finish copying the rest of buffer_file_in into buffer_file_out after your inserted chars.
Seek to the beginning of the file with fseek(file_pointer, 0, SEEK_SET);
Write the buffer_file_out buffer contents into the file with fwrite().
Close the file with fclose().
There are some optimizations you could do here, such as storing the index where you want to begin your insertion, and not copying the chars up to that point into buffer_file_in, but rather, simply copying the remaining of the file after that into buffer_file_in, and then seeking to that point later and writing only your new contents plus the rest of the file. This avoids unnecessarily rewriting the very beginning of the fie prior to the insertion point is all.
(Probably preferred) you could also just copy the file and the changes you insert straight into buffer_file_out in one shot, then write that back to the file starting at the beginning of the file. This would be very similar to #pm100's approach, except using 1 file + 1 buffer rather than 2 files.
Look for other optimizations and reductions of redundancy as applicable.
My approach above uses 1 file and 1 or 2 buffers in RAM, depending on implementation. #pm100's approach uses 2 files and 0 buffers in RAM (very similar to what my 1 file and 1 buffer approach would look like), depending on implementation. Both approaches are valid.
Is there a way to get a specific line inside a text file without iterating line-by-line in C?
for example I have this text file names.txt it contains the following names below;
John
James
Julia
Jasmine
and I want to access 'Julia' right away without iterating through 'John' and 'James'?, Something like, just give the index value of '2' or '3' to access 'Julia' right away.
Is there a way to do this in C?
I just want to know how because I want to deal with a very large text file something like in about 3 billion lines and I want to access a specific line in there right away and iterating line-by-line is very slow
You have to at least once iterate thru all lines. In this iteration, before reading a line, you record the position in the file and save it to an array or to another file (Usually named an index file). The file shall have a fixed record size that is good for storing the position off the line in the text file.
Later, when you want to access a give line, you either use the array to get the position (Line number is the array index) or the file (You seek into the file to offset line number of record size) and read the position. Once you get the position, you can see into the text file to that position and read the line.
Each time the text file is updated, you must reconstruct the array or index file.
There are other way to do that, but you need to better explain the context.
This question already has answers here:
How to fgets() a specific line from a file in C?
(5 answers)
Closed 7 years ago.
I want to read a specific line from a text file without reading the whole file line by line. For Example, if I have 10 lines in a text file and I have to read 6th line, I will not read the first 5 lines but will directly read the 6th one. Can anyone help me??
This question is answered here
Quoting from above,
Unless you know something more about the file, you can't access specific lines at random. New lines are delimited by the presence of line end characters and they can, in general, occur anywhere. Text files do not come with a map or index that would allow you to skip to the nth line.
If you knew that, say, every line in the file was the same length, then you could use random access to jump to a particular line. Without extra knowledge of this sort you simply have no choice but to iterate through the entire file until you reach your desired line.
Credits : Quoted answered was by David Heffernan
You could 'index' the file. Please note that this is only worth the effort if your text file:
is big
is frequently read and rarely written
The easiest (and probably most efficient) way is to use a database engine. Just store your file in a table, one row for each line.
Alternatively, you could make your own indexing mechanism. Basically, this means:
create a new file (the index)
scan the entire text file once, storing the offset of each line in the index file
repeat the above each time the text file changes
Finding line n in the text file requires two seeks:
read the nth offset from the index
read a line from the text file, starting at the offset found in the index
I'm writing a small program in C and I want to have the option of saving data to file and then reading it from that file. The data is BIG, so I want to somehow dynamically write to a file without having to create a new file and copy modified old file into it.
Here's exactly what I want to do:
In the first line, I want to have "description" of the data in the form "%s %s %s ... %s \n" where %s is a string and the n'th string describes data in n+1'th line. I want to read the 1'st line of the file, scan for corresponding "description" string, and if it is not present, append it to the first line, and the data corresponding to it after the last line of the file.
The question is - is it possible to "jump" into lines in the file without scanning all the previous lines, and can I somehow read the first line of the file and append something to it after reading? Or maybe it is not the way to go in this situation and C offers some kind of different solution?
What you want can be done using stdio and fseek(). As long as you know at what byte offset you want to go, you can overwrite and/or append anywhere in the file without reading the data before, or the data you're overwriting. What you can not easily do is insert data, i.e., open the file, split it in half and put data in between.
Not too sure if that is what you mean though...
hello i got a problem with reading from a file, i am trying to read from a file using fscanf() and i cant seem to sort it out.
i try to read the file line by line and putting the string in a variable (buffer) each time but i cant understand how the while loop is suppose to be looking like
thanks in advance
the file that i want to read from is a txt file with this format: first line :"1234,abc,etc" second line : "2432,fjh,etc" and more lines like those i want to be able to use the fscanf method inorder to put in each loop the all line lets say "1234,abc,etc" in my string variable and so on till i dont have any more lines to read from
this is what i managed to gather so far (ofc its not the currect way to write it):
char* buffer[100];
while (fscanf(FILE *finput,"%s",buffer)!=something)
{
printf("%s",buffer);
}
i want this code to be able to print all of the lines in my code if you would be able to correct my errors i will greatly appriciate it
I feel like you should read some of these great topics first:
Trouble reading a line using fscanf()
Reading file using fscanf() in C
fscanf multiple lines [c++]
There are plenty of reasons why you should use fgets or something else instead.
Quoting from this place:
fscanf() is a field oriented function and is inappropriate for use in a robust, general-purpose text file reader. It has two major drawbacks:
You must know the exact data layout of the input file in advance and rewrite the function call for every different layout.
It's difficult to read text strings that contain spaces because fscanf() sees space characters as field delimiters.
If you know the size of file you're trying to read, you could use fread(), which is block oriented.