getchar, putchar & while is confusing me

[alter.ego]

I am in the process of learning C programming and K & R is the book of choice. While going through book examples on getchar & putchar; I decided to tweak the example a bit (just to have a better understanding). Now I am confused.

#include <stdio.h>
main()
{

int c;

c = getchar();

while (c != EOF)
{
printf ("I knew youd type: ");
putchar (c);
c=getchar();
}
}

Output:

4
Somehow I know youd type:4Somehow I know youd type:

My expected/desired output should have been:

4
Somehow I know youd type: 4

The question is why the comments within printf is getting printed again. The program should have stopped at getchar() which was within the while statement block? Why isn't that happening? Can anyone please help me in clearing my logic here?

[Salem]

"4\n" is two characters, so you go round the loop twice.

If you want to ignore the newline at the end of each line of input, you need to add that to the code.

[alter.ego]

"4\n" is two characters, so you go round the loop twice.

If you want to ignore the newline at the end of each line of input, you need to add that to the code.

Thanks! Ummmm...although a new line is not a consideration for me. All I want is program should pause for new input after executing the first getchar & putchar

<<input from keyboard>>4
Somehow I know youd type:4 <<wait now for new input>>

[tabstop]

Wishing C wouldn't treat enter-key as a character won't make it so; enter-key is a character and will be read in by getchar() every time you push it. If you want to ignore it, you'll have to do the ignoring in your code.

[CommonTater]

Thanks! Ummmm...although a new line is not a consideration for me. All I want is program should pause for new input after executing the first getchar & putchar

<<input from keyboard>>4
Somehow I know youd type:4 <<wait now for new input>>

I don't think you understood what Salem was telling you...
Ok... you're running the program...
Now you type the number 4 ... that's one character but it's still waiting for something
So now you have to press Enter ... that's another character.
So your input buffer now has 4 and \n...
getchar() takes off and your loop prints your first keystroke...
now the loop is in control....
getchar() now finds the \n left behind by the first getchar()
It decides you didn't enter anything and off it goes... your loop executes again
Now the buffer is empty and it will sit at getchar() waiting for new input.

EVERY keyboard entry you make produces this extra character... so you get to deal with it in your code...

[alter.ego]

Thanks Commontater, Tabstop & Salem. I got it now. So apparently the program always waited at first getchar() & not the 2nd getchar() as I initially suspected. A big big thanks again guys!

[Alexander.]

I believe you can put the getchar() in the while loop constraints (and removing the first one) so that it evaluates the second time that it is a newline before it even runs (also checking for EOF)

There are various methods to grab all remaining newlines and there is no real standard one function to do this, although you best keep things simple to prevent newlines from messing up your loops.

I am glad you have spotted this problem and had asked about it! Always good to understand these cases that sometimes are sparsely noted.

[alter.ego]

Correct. These little things are never captured in book. Either your tutor points them to you or the good folks at these forums.

[quzah]

Correct. These little things are never captured in book. Either your tutor points them to you or the good folks at these forums.

I don't know why it is so hard for people to grasp this concept. You have pressed two keys on your keyboard. You are calling a function that takes one key. Why is it so difficult to understand that 2-1 = 1?


Quzah.

[CommonTater]

I don't know why it is so hard for people to grasp this concept. You have pressed two keys on your keyboard. You are calling a function that takes one key. Why is it so difficult to understand that 2-1 = 1?


Quzah.

Yeah but we all know the Enter key isn't a real key...

[ssharish2005]

Perhaps we need this topic to be Sticky on the C Programming form. I would imagine, the sort of issue gets asked several times and something which the newbie will have to get use to these things by reading the sticky posts before asking.

ssharish