Reading newline from previous input when reading from keyboard with scanf()

Question

This was supposed to be very simple, but I'm having trouble to read successive inputs from the keyboard.

Here's the code:

#include <string.h>
#include <stdio.h>

int main()
{
    char string[200];
    char character;
    printf ("write something: ");
    scanf ("%s", string);
    printf ("%s", string);
    printf ("\nwrite a character: ");
    scanf ("%c", &character);
    printf ("\nCharacter %c  Correspondent number: %d\n", character, character);

    return 0;
}

What is happening

When I enter a string (e.g.: computer), the program reads the newline ('\n') and puts it in character. Here is how the display looks like:

 write something: computer
 computer
 Character:
    Correspondent number: 10

Moreover, the program does not work for strings with more than one word. How could I overcome these problems?

Answer 1

First scanf read the entered string and left behind \n in the input buffer. Next call to scanf read that \n and store it to character.
Try this

scanf (" %c", &characte);   
     // ^A space before %c in scanf can skip any number of white space characters. 

Program will not work for strings more than one character because scanf stops reading once find a white space character. You can use fgets instead

 fgets(string, 200, stdin);

Answer 2

OP's first problem is typically solved by prepending a space to the format. This will consume white-space including the previous line's '\n'.

// scanf("%c", &character);
scanf(" %c", &character);

---

Moreover, the program does not work for strings with more than one word. How could I overcome these problems?

For the the 2nd issue, let us go for a more precise understanding of "string" and what "%s" does.

A string is a contiguous sequence of characters terminated by and including the first null character. 7.1.1 1

OP is not entering a string even though "I enter a string (e.g.: computer)," is reported. OP is entering a line of text. 8 characters "computer" followed by Enter. There is no "null character" here. Instead 9 char "computer\n".

"%s" in scanf("%s", string); does 3 things:

1) Scan, but not save any leading white-space.

2) Scan and save into string any number of non-white-space.

3) Stop scanning when white-space or EOF reached. That char is but back into stdin. A '\0' is appended to string making that char array a C string.

To read a line including spaces, do not use scanf("%s",.... Consider fgets().

fgets(string, sizeof string, stdin);
// remove potential trailing \r\n as needed
string[strcspn(string, "\n")] = 0;

Mixing scanf() and fgets() is a problem as calls like scanf("%s", string); fgets(...) leave the '\n' in stdin for fgets() to read as a line consisting of only "\n". Recommend instead to read all user input using fgets() (or getline() on *nix system). Then parse the line read.

fgets(string, sizeof string, stdin);
scanf(string, "%c", &character);

---

If code must user scanf() to read user input including spaces:

scanf("%*[\n]"); // read any number of \n and not save.
// Read up to 199 `char`, none of which are \n
if (scanf("%199[^\n]", string) != 1) Handle_EOF();

---

Lastly, code should employ error checking and input width limitations. Test the return values of all input functions.

Answer 3

What you're seeing is the correct behavior of the functions you call:

• scanf will read one word from the input, and leave the input pointer immediately after the word it reads. If you type computer<RETURN>, the next character to be read is the newline.

• To read a whole line, including the final newline, use fgets. Read the documentation carefully: fgets returns a string that includes the final newline it read. (gets, which shouldn't be used anyway for a number of reasons, reads and discards the final newline.)

I should add that while scanf has its uses, using it interactively leads to very confusing behavior, as I think you discovered. Even in cases where you want to read word by word, use another method if the intended use is interactive.

Answer 4

You can make use of %*c:

#include <string.h>
#include <stdio.h>

int main()
{
    char string[200];
    char character;
    printf ("write something: ");
    scanf ("%s%*c", string);
    printf ("%s", string);
    printf ("\nwrite a character: ");
    scanf ("%c%*c", &character);
    printf ("\nCharacter %c  Correspondent number: %d\n", character, character);

    return 0;
}

%*c will accept and ignore the newline or any white-spaces

Answer 5

You cal also put getchar() after the scanf line. It will do the job :)

Answer 6

The streams need to be flushed. When performing successive inputs, the standard input stream, stdin, buffers every key press on the keyboard. So, when you typed "computer" and pressed the enter key, the input stream absorbed the linefeed too, even though only the string "computer" was assigned to string. Hence when you scanned for a character later, the already loaded new line character was the one scanned and assigned to character.

Also the stdout streams need to be flushed. Consider this:

...
printf("foo");
while(1)
{}
...

If one tries to execute something like this then nothing is displayed on the console. The system buffered the stdout stream, the standard output stream, unaware of the fact it would be encounter an infinite loop next and once that happens, it never gets a chance to unload the stream to the console.

Apparently, in a similar manner whenever scanf blocks the program and waits on stdin, the standard input stream, it affects the other streams that are buffering. Anyway, whatsoever may be the case it's best to flush the streams properly if things start jumbling up.

The following modifications to your code seem to produce the desired output

#include <string.h>
#include <stdio.h>

int main()
{
char string[200];
char character;

printf ("write something: ");

fflush(stdout);

scanf ("%s", string);

fflush(stdin);

printf ("%s", string);
printf ("\nwrite a character: ");

fflush(stdout);

scanf ("%c", &character);
printf ("\nCharacter %c  Correspondent number: %d\n", character, character);

return 0;
} 

Output:
write something: computer
computer
write a character: a

Character a Correspondent number: 97