how to copy and write a file from a specific line number - c

Question

I would like to copy a huge txt file and 'shrink' it. this is my code, but it seems it's still takes a lot of time reading the file. is there a way to read from a specific line number to EOF? for instance, the first 1 million lines are not useful to me, how to read from line 1 million. or anyway to read from EOF?
include<stdio.h>
include<stdlib.h>
void main() {
FILE *fp1, *fp2;
char ch;
int i = 1;
int n = 0;
int k;
fp1 = fopen("co.data", "r"); /* open a file to read*/
fp2 = fopen("Output.txt", "w"); /* open a file to write*/
printf("please enter how many lines do not need to be copied\n");
scanf ("%d", &k);
while (1) {
ch = fgetc(fp1); /* a loop to read/copy the file*/
if (ch == '\n') /* record the number of lines*/
i++;
if (ch == EOF)
break;
else if (i>k)
putc(ch, fp2);
}
printf("File copied Successfully!\n");
printf("number of lines read is %d\n",i-1);
printf("number of lines copied is %d\n",i-1-k);
fclose(fp1);
fclose(fp2);
}

Answer 1

There are two potential answers to your question, depending on if your file has known line lengths or not.
is there a way to read from a specific line number to EOF
In a file with line lengths are completely arbitrary (variable), no.
For example, if line 1 is 10 characters, and line 2 is 20 characters, then there is no way to calculate where line 3 is going to start without iterating through lines 1 and 2.
Operating systems aren't magic; if this kind of functionality was supported, they'd have to iterate through the file first as well. Either way, you're going to be looping through the contents.
Now, if the line lengths are guaranteed to be the same, that's a different story.
Say you have a text file like so:
AAAAAAA
BBBBBBB
CCCCCCC
Each line in the above text file is 7 characters. Assuming your line terminator is \n, each line takes up exactly 8 bytes.
In this case, you can safely fread() 8 bytes at a time and know that you're getting exactly one line. In order to jump to a particular byte in a file, you would use fseek().
Since you know the length of the lines in this scenario, you could jump to line N by simply doing
fseek(fp1, S * N, SEEK_SET);
where N is the line number (starting at 0) and S is the length of the line (as mentioned above, 8 bytes in our example file).
Note that the second solution will break if you're using a multi-byte encoding such as Unicode. Keep that in mind.

Answer 2

Using fgets() i made program, try it.
#include<stdio.h>
#include<stdlib.h>
int main()
{
FILE *fp1, *fp2;
char ch,*str,*r;
int i =0;
int n = 0;
int l;
fp1 = fopen("co.data", "r");
fp2 = fopen("Output.txt", "w+");
printf("please enter how many lines do not need to be copied\n");
scanf ("%d", &l);
while (1)
{
if(r=fgets(str, 500, fp1))
{ /* a loop to read/copy the file*/
i++;
}
if (r == NULL)
break;
else if (i > l)
fputs(str, fp2);
}
printf("File copied Successfully!\n");
printf("number of lines read is %d\n",i-1);
printf("number of lines copied is %d\n",i-1-l);
fclose(fp1);
fclose(fp2);
}