Thread: Accessing files.

you could switch, but you have to flush the file inbetween

Does it work?

Code:

// This should be changed someday.

Glad to see you're paying attention.

Like I think I said, these two calls

Code:

 fseek(fptr, 0, SEEK_SET);
 fflush(fptr);

are exactly the same as a single call to rewind(fptr). Why not use that?

I think it works, but I don't notice a difference.

The reason is that I'd like to learn one thing at a time, besides: I now know that it is always necesary to flush the buffer before switching between reading and writing.

And I took your advice:

Code:

 do    {
         puts("Press enter to quit.");
         getchar();
        }
 while (getchar() != '\n');

Thanks

Have you tested your code?

Code:

#include <stdio.h>
int main()
{
	do    {
		puts("Press enter to quit.");
		getchar();
	}
	while (getchar() != '\n');
}

You actually need to press Enter twice to exit this program

Originally Posted by omnificient

Yes, I tested it. The user only has to press enter once.

If that's the case, you probably had a left-over newline in the input buffer from a scanf() or some such - as it's quite clear that if there is NOTHING in the input buffer, the first getchar() will wait for a newline and consume it, the second one will have to wait for ANOTHER newline.

Have you actually tested the code posted above by vart?

--
Mats

Originally Posted by omnificient

Well matsp, as I said: I indeed tested the code. I did realise, after vart pointed it out, that there is some \n left from another scanf() or something.

It works, so is there a problem with the loop I have used here?

You are relying on the input buffer being "exactly what you expect", and for that reason, I would suggest that you "eat" the previous newline left-over before the loop, then wait for another newline.

--
Mats

Originally Posted by omnificient

So that would mean using another getchar()?

No, it means moving the gechar() from INSIDE the loop, to outside the loop.

Your current code expects the input to be '\n', '\n'. So your getchar() inside the loop "eats" the first newline, then the getchar() in the while-condition eats the final newline.

But consider that scanf() left an 'a' in the input buffer, as well as your expected two newlines. So your first getchar() eats the a, the while() one eats a newline and the user doesn't get to press enter before it exits. Other options exist.

--
Mats

Let me explain what is happening here:

Code:

 do    {
         puts("Press enter to quit.");
         getchar();
        }
 while (getchar() != '\n');

Every time the loop executes, you:

1. Print a message.
2. Discard a single character from the input stream.
3. Get another character from the input.
4. Start executing the loop at the beginning again if this character is not a '\n'.

A few problems to note with this approach:

• Only every other character is checked against a newline.
• A message is printed every two characters.

So you might try to fix it by using this . . .

Code:

 do    {
         puts("Press enter to quit.");
        }
 while (getchar() != '\n');

Right? Because now every character is checked against '\n'.

Not quite . . . what happens if the input stream contains "\nexit\n"? You'll get "Press enter to quit." printed six times!

So here's what you really want to do. You want to print the message once, and then eat as many characters as you need to until you get a '\n'. (Checking every character you read against '\n', of course.) In other words:

Code:

puts("Press enter to quit.");

do    {
        }
 while (getchar() != '\n');

Of course, that simplifies to:

Code:

puts("Press enter to quit.");
while(getchar() != '\n');

. . . which is what I had in the first place, I believe.