Getting char as input by scanf

[namandixit]

In GCC v4.6, I am encountering a strange problem. If I try to get a character input and a int input like this:

int main()
{
int a;
char b;
scanf("%d",&a);
scanf("%c",&b);
}

the compiled program asks for input only one time. On the other hand, if I do the char scanf earlier like this:

int main()
{
int a;
char b;
scanf("%c",&b);
scanf("%d",&a);
}

it asks for input twice. Any idea why this behaviour?

[Spork Schivago]

I think it has something to do with a whitespace character (the enter you hit after entering your integer is being stored in the char b). One way I would handle it would be to have something like:

 scanf("%d\n",&a);

[grumpy]

It is because %c format picks up whitespace as characters.

A common solution is to add a space to the format string to skip any whitespace (a space instead of the '\n' in Spork's post). A robust solution is to use fgets() to read a line of input into an array of char, and then check the contents of the complete line (using sscanf() if one wishes).

Incidentally, you could have worked this out by printing a and b (using the %d format for both, so a newline results in visible output).

[Kotik]

Scanf() is a function not really recommended for input because of the kind of issue you are having with your code. What you have to realize is that scanf sure is good for reading input but it has a side-effect when used with anything other than integers, that it also reads new line (\n) characters from the input stream (in this case stdin). So when you are doing a scanf of a character followed by an integer, it is leaving behind the new line character in the stdin buffer which is then read by the following integer scanf call. My code below will show 2 inputs.

    int a;
    char b;


    scanf("%c", &b);
    getchar();
    scanf("%d", &a);

This is because getchar(); in between the two scanf will read the rubbish new line character that we do not care about.

[namandixit]

@Kotik This by far is the most ingenious solution. Thanks.

[Click_here]

Scanf() is a function not really recommended for input because of the kind of issue you are having with your code. What you have to realize is that scanf sure is good for reading input but it has a side-effect when used with anything other than integers, that it also reads new line (\n) characters from the input stream (in this case stdin). So when you are doing a scanf of a character followed by an integer, it is leaving behind the new line character in the stdin buffer which is then read by the following integer scanf call. My code below will show 2 inputs.

    int a;
    char b;


    scanf("%c", &b);
    getchar();
    scanf("%d", &a);

This is because getchar(); in between the two scanf will read the rubbish new line character that we do not care about.

The second scanf will skip the new line anyway because it is waiting for an integer, not a char.

If you wanted to discard a character you can use the '*' in the scanf statement -> This discards the input

sscanf("%d%*c", &apple);

However, you are assuming that the user won't accidentally type the wrong thing.

Another thing you can do is:

int ch;

while ((ch = getchar()) != '\n' && ch != EOF)
{
  continue;
}

That way you can get rid of everything in the input buffer (most of the time) - For more info, FAQ > Flush the input buffer - Cprogramming.com

[grumpy]

@Kotik This by far is the most ingenious solution. Thanks.

It's also one that can confuse a user since the program behaviour will change depending on small changes of user behaviour.

getchar() and scanf() have different styles of input (read characters directly, versus interpret data in the stream according to a format string).

Generally speaking, mixing styles of input on one stream is a REALLY bad idea.