Confusion when dealing with files - c

Question

I'm writing a program that takes a string from one file, a string from another file and writes them to a third file in separate columns. I really have two questions, If I use fgets to get the string from the files and it stops at the end of the line. Will it automatically know to start from the next line for the next string. Also, how can I format the input to make two columns. An example would be...
first string is "John" from the first file.
second string is "Appleseed" from the second file.
third file would have in it "John______________________Appleseed"
The second line of the third file would have "Benny__________________________ Backburner"
Just to format columns.

Answer 1

Do you want underscores, or will spaces suffice? It's much simpler with spaces. You can read the specification of printf()
to see the details of what the format strings do.
while (fgets(buffer1, sizeof(buffer1), fp1) != 0 &&
fgets(buffer2, sizeof(buffer2), fp2) != 0)
{
buffer1[strcspn(buffer1, "\n")] = '\0';
buffer2[strcspn(buffer2, "\n")] = '\0';
fprintf(fp3, "%-25s %s\n", buffer1, buffer2);
}
This reads one line from each of the first two files, removes the newlines from the buffer, and then formats them with the first column left-justified in a width of 25, and the second printed after 3 spaces.
If you must use underscores instead of spaces, then you need something like this:
char uscore[256];
memset(uscore, '_', sizeof(uscore)-1);
uscore[sizeof(uscore)-1] = '\0';
while (fgets(buffer1, sizeof(buffer1), fp1) != 0 &&
fgets(buffer2, sizeof(buffer2), fp2) != 0)
{
buffer1[strcspn(buffer1, "\n")] = '\0';
buffer2[strcspn(buffer2, "\n")] = '\0';
int len1 = max(0, 25 - strlen(buffer1));
fprintf(fp3, "%s%*.*s%s\n", buffer1, len1, len1, uscore, buffer2);
}
Putting this together, illustrating both techniques at once:
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv)
{
if (argc != 4)
{
fprintf(stderr, "Usage: %s infile-1 infile-2 outfile\n", argv[0]);
return 1;
}
FILE *fp1 = fopen(argv[1], "r");
FILE *fp2 = fopen(argv[2], "r");
FILE *fp3 = fopen(argv[3], "w");
if (fp1 == 0 || fp2 == 0 || fp3 == 0)
{
fprintf(stderr, "%s: failed to open one of the files %s, %s or %s\n",
argv[0], argv[1], argv[2], argv[3]);
return 1;
}
char uscore[256];
memset(uscore, '_', sizeof(uscore)-1);
uscore[sizeof(uscore)-1] = '\0';
char buffer1[1024];
char buffer2[1024];
while (fgets(buffer1, sizeof(buffer1), fp1) != 0 &&
fgets(buffer2, sizeof(buffer2), fp2) != 0)
{
buffer1[strcspn(buffer1, "\n")] = '\0';
buffer2[strcspn(buffer2, "\n")] = '\0';
fprintf(fp3, "%-25s %s\n", buffer1, buffer2);
int len1 = strlen(buffer1);
if (len1 < 28)
len1 = 28 - len1;
else
len1 = 0;
fprintf(fp3, "%s%*.*s%s\n", buffer1, len1, len1, uscore, buffer2);
}
fclose(fp1);
fclose(fp2);
fclose(fp3);
return 0;
}
Sample input file data.1:
California
Esoteric
Mismatch
Unexpected
Non-sequitur
Extra-long word list from file 1
Sample input file data.2:
Drought
Persecution
Preliminary
Adequate
Pusillanimous
Rather long word from file.2 too
Example output:
California Drought
California__________________Drought
Esoteric Persecution
Esoteric____________________Persecution
Mismatch Preliminary
Mismatch____________________Preliminary
Unexpected Adequate
Unexpected__________________Adequate
Non-sequitur Pusillanimous
Non-sequitur________________Pusillanimous
Extra-long word list from file 1 Rather long word from file.2 too
Extra-long word list from file 1Rather long word from file.2 too
There are endless tweaks you can make depending on a more precise definition of the format you want. Amongst other things, you can make sure there's a minimum of 3 underscores between the first and second words in the 'must have underscores' example. You could limit the lengths of the strings that are printed.
The code should check that it gets a newline within the first 1023 bytes; it doesn't.

Answer 2

If I use fgets to get the string from the files and it stops at the end of the line. Will it automatically know to start from the next line for the next string.
If the line can be completely stored in the buffer then Yes, it will (see below for further explanation).
However, there is not really something like lines in the data read from the file. It is more like a continuous stream of bytes. If you have a file in your editor that looks like:
a
b
c
the data that fgets see is like more like this byte stream:
a\nb\nc\n
The first call to fgets will read the a and the \n leaving the remaining input as
b\nc\n
The next call to fgets will read the b and the \n and thereby it works as if it starts from the "next line" but it really just continues from where the last call stopped.
Also notice what happens if the line is longer than your buffer. If the file is
abcd
efgh
and you do
fgets(buffer, 3, f)
then the first call to fgets will give ab\0 and the next call will continue reading cd\0.
In other words - if the line is too long to be completely stored in the buffer, fgets will not continue from "the next line". If you always want to continue from the next line, you must add code to read from the file until you read a \n
Also, how can I format the input to make two columns.
Well, your question doesn't include sufficient details to come up with exact code, e.g. what should be the spacing between columns, what to do if the input is larger than the spacing, etc.
In any case - see https://stackoverflow.com/a/45295262/4386427 (by Jonathan Leffler) which give you some good hints.