Thread: file sorting strategy question..

Put the whole file into an array:

Code:

#include <stdio.h>
#include <stdlib.h>

#define LLEN 256

char **readfile(char *file, int *i) {
	FILE *fh = fopen(file,"r");
	char **lines = malloc(sizeof(char*));
	*i = 0;
	lines[0] = malloc(LLEN);
	while (fgets(lines[*i],LLEN,fh)) {
		(*i)++;
		lines = realloc(lines,((*i)+1)*sizeof(char*));
		lines[*i] = malloc(LLEN);
	}
	fclose(fh);
	return lines;
}
	

int main(int argc, char *argv[]) {
	int len, i;
	char **lines;
	if (!argv[1]) puts("No file!");
	lines=readfile(argv[1],&len);
	for (i=0;i<len;i++) printf("%s",lines[i]);
	return 0;
}

Originally Posted by cfan

i am not allowed to do that
i only can copy one line at a time
from each file
and i cant do rewind

If you cannot read each file more than once, and you cannot store more than one line, you cannot compare that file to another one in the way you describe. Period.

You have misunderstood something about your assignment.

Originally Posted by slingerland3g

If each entry within both arrays maintain a unique user ID

The OP claimed s/he is not allowed to use an array and may only work with one line at a time.

Originally Posted by cfan

then i do fscanf of the old file over and over till i get a line wich id is equal

You are not allowed to rewind, or store the file in memory, but you can read the file over and over again with fscanf?

Actually, you cannot read the file over and over again with fscanf unless you use rewind, or open a new handle on the file. Over and over again.

I really think you should make sure that you cannot read the file once, and store the entire file in memory -- either in an array, or a single string. Otherwise, what you want is simply not possible*, which is why I believe you have misinterpreted something about the assignment.

*unless you can re-open a file, but since you are not allowed to rewind, this seems unlikely.

Originally Posted by cfan

i am allowed to read the files line by line and store them in a temporary variable


i am not allowed to copy the whole file
i am not allowed to use rewind
i showed the general way of solution which i am being expected

Okay, as long as you understand that it is not possible to "fscanf" through a file over and over again without using rewind. Try it yourself.

Also, consider there is a difference between "copying a file" (making another copy of it on disk) and "storing an entire file in memory". So being told you cannot copy the file DOES NOT MEAN YOU CAN'T PUT IT IN AN ARRAY.

i dont need to copy the whole file at once (and i am not allowed to)
the lines in files are sorted.

do you understand the general strategy that i have being presented
(and which i am asked to do a solution in that manner)

??

Originally Posted by cfan

do you understand the general strategy that i have being presented
(and which i am asked to do a solution in that manner)

??a

If it is the way you describe, it is not possible, so it cannot be the way you describe:

Originally Posted by cfan

if the new id line is bigger
then i do fscanf of the old file over and over till i get a line wich id is equal
or bigger
or eof

That will mean you have to read the file more than once. fscanf picks up where the file pointer has read to so far. You cannot rewind fscanf to the beginning of the file without using rewind() on the file pointer. Or, you open the file again. If you are allowed to keep fopen()ing the file over and over again, that's fine.

I'm just warning you that is probably not what is meant. You could read the *entire* file into one string, of course. That may work for what you want, since all you have to do is check if a name was in the other file. But that means you will be saving the entire file into memory.