Why the fscanf is used 2 times with: while( !feof(File)) - c

Question

why when using fscanf to acquire data from a file, is used 2 times, once before the "!feof(fin)" and later, as shown in the code below:
fscanf(fin, "%s", letta);
while(!feof(fin)){
fscanf(fin, "%s", letta);
}
the word read is not the same?

Answer 1

No the word read would not be the same since you read from the file twice, you would get different data each time.
The condition in the while tests to see if you are at the end of file, in order to do that a read attempt must have been made, which requires the fscanf() before the loop.
Once you are inside the loop you want to continue reading from the file. Presumably there would be more code inside the loop to process the data you are reading.
In the code you have posted, if you encountered the end of file with your first read attempt, you wouldn't enter the (while) loop.
Contrast this with a do-while construct where the test is at the bottom of the loop (ie the read occurs only once, at the "bottom" of the loop). There you will always enter the loop at least once.

Answer 2

There's probably no point in using feof() like this here, yet many people who are (I guess, apologies if I'm mistaken) learning C seem to "want to".
The fact is that fscanf() itself has a return value; if the file ends it will fail to do the requested scanning, and it will tell you so. It will even return the special constant EOF if the file ends. So just loop on the return value of fscanf():
while( fscanf(fin, "%s", letta) == 1 )
{
/* Process, parse, store or do whatever with letta. */
}

Answer 3

Because the EOF flag is only set after a call to fscanf. The condition while(!feof(fin)) doesn't make sense when fscanf hasn't been called yet.

Answer 4

feof doesn't return true until after you try to read past the end of file. Imagine a file like the following:
This is a test$
^
where ^ indicates the current location in the file and $ represents EOF. After the first call to fscanf(fin, "%s", letta);, the stream looks like this:
This is a test$
^
After three iterations of the loop (reading "is", "a", and "test"), you have this:
This is a test$
^
At this point, feof(fin) still returns false, because at this point all you know is that you've reached the end of the string "test". So your loop will execute one more time. Since you're at the end of the file, there's nothing to read, so the contents of letta will not be changed, so it will look like you've read "test" twice. Now feof(fin) will return true.
The moral of the story is that you should not use feof as your loop condition; rather, you should use the result of the read operation itself, like so:
errno = 0;
while (fscanf(fin, "%s", letta) == 1)
{
// process letta
}
if (feof(fin))
{
printf("Reached end of file\n");
}
else
{
perror("Error on read");
}