Why the program prints line twice?

details in the reply..

Hi all!

I cannot figure out why the output prints the printf line twice.. in this program.. thanks..

here is the code with the output:

Code:

---

#include<stdio.h>
main()
{
char ch;
while (ch!='0')
        {
        printf("enter a char other than 0\n");
        ch=getchar();
        }
}

---

enter a char other than 0
d
enter a char other than 0
enter a char other than 0

What happens after you press the d key?

Quzah.

well, it prints "enter a char other than 0" twice...

Quote:

---

Originally Posted by red463

Hi all!

I cannot figure out why the output prints the printf line twice.. in this program.. thanks..

here is the code with the output:

Code:

---

#include<stdio.h>
main()
{
char ch;
while (ch!='0')
        {
        printf("enter a char other than 0\n");
        ch=getchar();
        }
}

---

enter a char other than 0
d
enter a char other than 0
enter a char other than 0

---

Yes, but you are also probably pressing another button after 'd'.

This is where I believe quzah is trying to lead you.

Also, on another note, initialize your variables before use. -- although this is not the reason you are getting that behavior.

Quote:

---

Originally Posted by red463

well, it prints "enter a char other than 0" twice...

---

No. I asked what happens after you press the D key. The answer was:

"I press the enter key."

Which should have given you the reason it is printing the line twice.


Quzah.

what is the solution to this problem?

ok teacher, what is the right way to deal with this then?

Quote:

---

Originally Posted by red463

ok teacher, what is the right way to deal with this then?

---

The answer is either "recognize that this is the right answer" or "also check if the read character is enter-key". (EDIT TO ADD: For instance, think about what happens when they type "flibbertigibbet" at the prompt.)

Quote:

---

Originally Posted by tabstop

The answer is either "recognize that this is the right answer" or "also check if the read character is enter-key". (EDIT TO ADD: For instance, think about what happens when they type "flibbertigibbet" at the prompt.)

---

when you do that, it would print the command n+1 times (n=number of chars in flibbertigibbet).
I would please like to know how to deal with this, I am very new to programming, and cryptic answers aren't helping me here...

Quote:

---

Originally Posted by red463

when you do that, it would print the command n+1 times (n=number of chars in flibbertigibbet).
I would please like to know how to deal with this, I am very new to programming, and cryptic answers aren't helping me here...

---

It really depends on what the specs of the program are. If the specs are something like "print a prompt and read a character; if the character is not '0', then repeat" then we will say it for the fourth time in this thread: there is no error in your code and therefore there is nothing to fix. If the specs are something other than that, then you need to tell us what they are.

allright, my bad, the spec follows-

write a program, which allows users to enter characters until the input is '0'.

Without using system specific functions (kbdhit, getch), you can't do what your program specs, as written in your last post, request. There's nothing keeping you from entering: "1230321" and hitting enter. So unless you read keystrokes - not characters - you can't do what you want.


Quzah.

well, thanks for all the help guys...

Your program fits the requirement as given by you, if you change the wording slightly. It will allow you to enter characters, and it will stop the loop when it hits '0'.

All the rest has to do with buffering, and the way input is actually typed versus how it is being handled.


Quzah.

1. You prompt for input.
2. You press 'd' followed by the "Enter" key -> the buffer contains two characters 'd', '/n'.
3. getchar reads 'd' from the buffer -> buffer contains single character '/n'.
4. Since 'd' is not '0' you reprompt for input, but the buffer already contains '/n' and program doesn't wait for you to input anything.
5. getchar reads '/n' from the buffer -> buffer is now empty.
6. Since '/n' is not '0' you reprompt for input, program waits for user input.

The double print happens because of the green bits above.