scanf issue

**cooper1200** · 04-14-2019

hi all complete newbie here im just trying to practice some stuff i have learnt so far and have come across an issue i have no clue as to why it is happening.

i have some code that asks the user a yes or no question and acts apon the answer ie

Code:

printf("are you male\n");
scanf("%c",&yn);
if (yn=='y')
{
      //do suff
}

printf("are you tall\n");
scanf("%c",&yn);
if (yn=='y')
{
      //do suff
}
return 0;

doesn't ask for input on the second scanf it just exits the program

however .....

Code:

scanf("%c",&yn); //why this extra scanf
printf("are you tall\n");
scanf("%c",&yn);
if (yn=='y')
{
      //do suff
}
return 0;

works as in a get asked to enter y or n a second time.

any help would be appreciated
many thanks
coop

**flp1969** · 04-14-2019

Did you notice the name of the funcion? scanf?!

When you ask to scanf() to get a single char ("%c") from the input stream (stdin) and you type 'y' followed by ENTER ('\r'). BOTH characters are in the stream... If you want ignore "space" chars (' ', '\r', '\n', '\t', '\f' and '\v' - as checked by isspace() macro on ctype.h), add a single space after %c.

**Click_here** · 04-14-2019

Here is more info on what is going wrong -
How do I avoid a "dangling" newline when reading single character user input? - Cprogramming.com

**rstanley** · 04-14-2019

Originally Posted by flp1969

Did you notice the name of the funcion? scanf?!

When you ask to scanf() to get a single char ("%c") from the input stream (stdin) and you type 'y' followed by ENTER ('\r'). BOTH characters are in the stream... If you want ignore "space" chars (' ', '\r', '\n', '\t', '\f' and '\v' - as checked by isspace() macro on ctype.h), add a single space after %c.

The "ENTER" char is '\n', (newline), NOT '\r', (carriage return)!

**flp1969** · 04-14-2019

Originally Posted by rstanley

The "ENTER" char is '\n', (newline), NOT '\r', (carriage return)!

Nope... '\r' (13) is the "enter" char... The system translate it to '\n' (10).

The symbol for "Enter" on the keyboard is ⏎ (carriage return).

**rstanley** · 04-14-2019

Originally Posted by flp1969

Nope... '\r' (13) is the "enter" char... The system translate it to '\n' (10).

The symbol for "Enter" on the keyboard is ⏎ (carriage return).

The subject of this thread is data received using "scanf(), NOT keyboard keycodes!!!

When using scanf(), and I enter 'y', then press the [Enter] key on the keyboard, the two characters in the input buffer are, "y\n", the '\n' character is a 0x0a hex, or the '\n' character.

It is the '\n' that needs to be removed after receiving the 'y'.

**bozware** · 04-14-2019

Code:

#include <stdio.h>

int main()
{
    char yn;

    printf("Are you male? ");
    scanf("\n%c", &yn);
    if (yn == 'y')
        printf("You're a boy!\n");
    else
        printf("You're a girl!\n");
    printf("\n");

    printf("Are you tall? ");
    scanf("\n%c", &yn);
    if (yn == 'y')
        printf("You're tall!\n");
    else
        printf("You're short!\n");

    return 0;
}

For some reason, typing scanf("\n%c", &yn); provides me the chance to input the value for char yn; once the second question is asked. I'm not too sure why it is like that, however. Of course, I could be wrong and/or there might be a better workaround for this so take my solution with a grain of salt for now. I'm a complete newbie when it comes to the C language.

If anyone wants to chime in as to why my solution appears to be working and/or can tell me what I can do to improve on this, let me know! I've only completed the introductory material on C's tutorial page, so I haven't quite got around to understanding why this works on my end or anything.

EDIT:

Originally Posted by Click_here

Here is more info on what is going wrong -
How do I avoid a "dangling" newline when reading single character user input? - Cprogramming.com

I am only now reading about this, I should have skimmed through the replies to this thread better before posting up my own.

**rstanley** · 04-14-2019

Change:

Code:

scanf("\n%c", &yn);

with

Code:

scanf(" %c", &yn);

Using a space instead of the explicit '\n' will make scanf() ignore any possible whitespace characters before inputting the legitimate char you want.

Now you need to handle inputting uppercase 'Y' or 'N', or characters other than 'y' or 'n'.

I would also recommend a good book on the C Programming language. The tutorials on this site are limited.

**flp1969** · 04-14-2019

Originally Posted by rstanley

The subject of this thread is data received using "scanf(), NOT keyboard keycodes!!!

When using scanf(), and I enter 'y', then press the [Enter] key on the keyboard, the two characters in the input buffer are, "y\n", the '\n' character is a 0x0a hex, or the '\n' character.

It is the '\n' that needs to be removed after receiving the 'y'.

It doesn't matter if it is '\r' o '\n', a single space following %c will ignore both... AND, it is not a "key code". Technically speaking, the scan code from the keyboard for the ENTER key is 0x5A or 0xED5A:

scanf issue-scancodes-png

My point: ENTER is '\r', translated to '\n' from the keyboard scan codes...

**rstanley** · 04-14-2019

Once again!!!

This thread is about scanf(), NOT "keyboard scan codes..."!

Please stick to the subject of the OP. Please do not confuse the OP who is a beginner!

**bozware** · 04-14-2019

Originally Posted by rstanley

Change:

Code:

scanf("\n%c", &yn);

with

Code:

scanf(" %c", &yn);

Using a space instead of the explicit '\n' will make scanf() ignore any possible whitespace characters before inputting the legitimate char you want.

Now you need to handle inputting uppercase 'Y' or 'N', or characters other than 'y' or 'n'.

I would also recommend a good book on the C Programming language. The tutorials on this site are limited.

Code:

    char yn;

    printf("Are you male? ");
    scanf(" %c", &yn);
    do {
        switch (yn) {
        case ('y'):
            printf("You're a boy!\n");
            yn = ' ';
            break;
        case ('Y'):
            printf("You're a boy!\n");
            yn = ' ';
            break;
        case ('n'):
            printf("You're a girl!\n");
            yn = ' ';
            break;
        case ('N'):
            printf("You're a girl!\n");
            yn = ' ';
            break;
        default:
            printf("Are you male? ");
            scanf(" %c", &yn);
        }
    } while (yn != ' ');

This is only assuming that the end-user types in a single character, though I suppose I'll need to learn how to either flush the input buffer or parse additional values as mentioned from this link at some point.

And I agree, I'll definitely be looking into one of the recommended books soon for this language.

**Click_here** · 04-14-2019

A good way of handling input is by using fgets and then using sscanf on the buffer.
After that you can also do things like handle double presses...

Code:

if ((inputBuffer[0] == 'y') && (inputBuffer[1] == '\n'))
{
  ...
}

Code:

        printf("Are you male? ");
        scanf(" %c", &yn);

Not a big issue, but the output stream isn't guaranteed to write the data out immediately.
fflush(stdout)
c - Why does printf not flush after the call unless a newline is in the format string? - Stack Overflow
The fix to that is to use fflush(stdout)

You may also want to work on your variable names: 'yn' is not a very good one. You may want to call the variable 'userInput' / 'userResponse' / 'genderInput' / ect...

Hope this helps

**Click_here** · 04-14-2019

though I suppose I'll need to learn how to either flush the input buffer ...

Flush the input buffer - Cprogramming.com

**flp1969** · 04-15-2019

Originally Posted by rstanley

Once again!!!

This thread is about scanf(), NOT "keyboard scan codes..."!

Please stick to the subject of the OP. Please do not confuse the OP who is a beginner!

Ok... but why did YOU bring the subject on #3?

**rstanley** · 04-15-2019

Originally Posted by flp1969

Ok... but why did YOU bring the subject on #3?

I was originally responding to posting #2, by you, then you got into an off-topic discussion about keyboard scancodes.

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