Weird printf & scanf behavior

Question

I'm pretty new to C programming and I'm wondering why I need to input the same value twice when using the following code?

#include <stdio.h>

int main(void)
{
  int ascii;

  printf("Enter an ASCII value: ");
  scanf(" %d\n", &ascii);
  printf("The %d ASCII code has the character value %c\n", ascii, ascii);

  return 0;
}

You can see that I've had to enter 89 twice in the image below.

Answer 1

You need 2 values because the format-string contains two format specifiers , eg:

" The %d ASCII code is character '%c'\n\n"

That specifies two conversion will take place, (eg %d and %c ). They each require a corresponding value in the argument list. eg:

printf ("\n The %d ASCII code is character '%c'\n\n", ascii, ascii);
                ^                           ^           1      2

No magic, just take a close look at man printf . You are simply printing two different conversion for the same value. Therefore, each conversion requires its own value.

---

If you are still having problems, feel free to ask further. Here is a short version of your code that works fine:

#include <stdio.h>

int main (void) {

    int ascii;
    printf ("Enter and ASCII code value: ");
    if (scanf ("%d%*c", &ascii) != 1) {
        fprintf (stderr, "error: invalid value entered.\n");
        return 1;
    }

    printf ("\n The %d ASCII code is character '%c'\n\n", ascii, ascii);

    return 0;
}

Output

$ ./bin/enterascii
Enter and ASCII code value: 89

 The 89 ASCII code is character 'Y'

---

Your 'quarts' Post

You do not include the '\\n' in the scanf format string:

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

should be something like

scanf("%d%*c", &quarts);

( note: the %*c just reads and discards the '\\n' that results from pressing the Enter key. It is not required, but it is good practice to remove it from the input buffer ( stdin ) or you will be surprised if you attempt to take character or string input with a subsequent scanf call in the same code)

Answer 2

You need to remove the whitespace (the space and \\n ) in your scanf pattern, ie "%d" instead of " %d\\n" .

#include <stdio.h>

int main(void)
{
  int ascii;

  printf("Enter an ASCII value: ");
  scanf("%d", &ascii);
  printf("The %d ASCII code has the character value %c\n", ascii, ascii);

  return 0;
}