I wish to read and write to a .csv file using this format: ~83474\t>wed 19 march 2014\n
When reading, I need to ignore the ~, the tab and the >. They are just there to remind my program of what the values that follow are used for. So far I figured out how to write to file using that format, however, I do not know how to read from the file either. I wish to store the numbers after the ~ as an integer value and the characters after the > as a string. How can I read those two values from every line in the file if each line has the format stated above?
Read the whole line as a string using fgets and process it.
Related
I started to learn C and I am writing a code where the user needs to type a string he wants to search in the csv file , and if the string was found , it will print the number of line the string was found at .
and I am not exactly sure what is the best way to do that
You may read line by line using getline() until it returns -1 (EOF), keeping a counter on how many iterations you have done.
For each line you read, you can use strstr() to check if the string is present. If yes, just print the counter (that will correspond to the line).
I want to do the following:
open and read and ASCII file
locate a substring (geographical coordinates)
create its replacement (apply corrections to the original coordinates)
overwrite the original substring (write in the original file the corrected coordinates).
The format of the ASCII file is:
$GPGGA,091306.00,4548.17420,N,00905.47990,E,1,09,0.87,233.5,M,47.2,M,,*53
I will paste here only the part of the code that is responsible for this operation:
opnmea = fopen (argv[1], "r+");
if (fgets(row_nmea, ROW, opnmea)==NULL){
if (strstr(row_nmea,"$GPGGA")!=NULL) {
sscanf(row_nmea+17, "%10c", old_phi);
sscanf(row_nmea+30, "%11c", old_lam);
sscanf(row_nmea+54, "%5c", old_h);
fputs();
}
}
What I do till now is to extract in a variable the old coordinates and I was thinking to use fputs() for overwriting the old with new values. But I could not do it. The other part of the code that is not here is computing the correct coordinates. My idea is to correct the rows one by one, as the fgets() function reads each line.
I would appreciate very much any suggestion that can show me how to use fputs() or another function to complete my work. I am looking for something simple as I am beginner with C.
Thank you in advance.
Patching a text file in place is not a good solution for this problem, for multiple reasons:
the modified version might have a different length, hence patching cannot be done in place.
the read-write operation of standard streams is not so easy to handle correctly and defeats the buffering mechanism.
if you encounter an error during the patching phase, a partially modified file can be considered corrupted as one cannot tell which coordinates have been modified and which have not.
other programs might be reading from the same file as you are writing it. They will read invalid or inconsistent data.
I strongly recommend to write a program that reads the original file and writes a modified version to a different output file.
For this you need to:
open the original file for reading opnmea = fopen(argv[1], "r");
open the output file for writing: outfile = fopen(temporary_file_name, "w");
copy the lines that do not require modification: just call fputs(row_nmea, outfile).
parse relevant data in lines that require modification with whatever method you are comfortable with: sscanf, strtok, ...
compute the modified fields and write the modified line to outfile with fprintf.
Once the file has been completely and correctly handled, you can replace the original file with rename. The rename operation is usually atomic at the file-system level, so other programs will either finish reading from the previous version or open the new version.
Of course, if the file has only one line, you could simply rewind the stream and write back the line with fprintf, but this is a special case and it will fail if the new version is shorter than the original. Truncating the extra data is not easy. An alternative is to reopen the file in write mode ("w") before writing the modified line.
I would recommend strtok(), followed by your revision, followed by strcat().
strtok() will let you separate the line using the comma as a delimiter, so you will get the field you want reliably. You can break up the line into separate strings, revise the coordinates you wish, and reassemble the line, including the commas, with strcat().
These pages include nice usage examples, too:
http://www.cplusplus.com/reference/cstring/strtok/
http://www.cplusplus.com/reference/cstring/strcat/?kw=strcat
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.