Why does scanf function get automatically previous '\n' value and how can I escape from this event?

Question

I'm not new at writing code. But I'm just learning C language. I cannot understand this subject. Perhaps it is not an issue but now it is an issue for me. Would you please explain that?

Here is code, where I encounter with this:

#include <stdio.h>


int main(int argc, char *argv[])
{
    char letter;

    while (1)
    {
        printf("Enter a letter:\n");
        scanf("%c", &letter);

        switch (letter)
        {
            case 'a':
            case 'A':
            case 'e':
            case 'E':
            case 'i':
            case 'I':
            case 'u':
            case 'U':
            case 'o':
            case 'O':
                printf("%c is a vowel letter.\n", letter);
                break;
            case 'y':
            case 'Y':
                printf("%c is sometimes a vowel letter.\n", letter);
                break;
            default:
                printf("%c is not a vowel letter.\n", letter);
        }
    }

    return 0;
}

Output:

Enter a letter:
a
a is a vowel letter.
Enter a letter:

 is not a vowel letter.
Enter a letter:

There a multiple ways of doing this: [see here](http://stackoverflow.com/questions/3744776/simple-c-scanf-does-not-work), or [maybe here](http://stackoverflow.com/questions/1669821/scanf-skips-every-other-while-loop-in-c). — WhozCraig, Nov 01 '14 at 18:59

Deduplicator · Accepted Answer · 2014-11-01T19:05:18.773

2

Change the format from "%c" to " %c" to make scanf discard all whitespace (as determined by isspace(), " \v\f\r\n\t" in the POSIX and C locales) before assigning the next character, whatever it may be.

Maybe a better alternative, read a whole line with fgets and then use sscanf to parse it instead.

As an aside, take care that scanf can always fail. On success, it returns the number of assigned arguments.
Also, I strongly suggest you read the scanf-manpage I linked above (or even better, the C standard on scanf), because there are quite a lot of pitfalls you may not know yet.

edited Nov 01 '14 at 19:05

answered Nov 01 '14 at 18:59

Deduplicator

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You can also do `scanf("%s\n")` to consume the trailing newline. – fuz Nov 01 '14 at 19:23
@FUZxxl: That has a big disadvantage though: It waits until a non-whitespace it should not discard is entered. – Deduplicator Nov 01 '14 at 19:25
It does exactly the right thing for input from terminal which is usually line-buffered. You press the key you want and then hit enter. – fuz Nov 01 '14 at 19:27
You know `\n` matches any amount of whitespace, not only that single enter? – Deduplicator Nov 01 '14 at 19:28
I know. Which is the right thing, too, in most cases. – fuz Nov 01 '14 at 19:32

Why does scanf function get automatically previous '\n' value and how can I escape from this event?

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