Skipping scanf for character input with %d

Question

  int valid = 0;
  while (!valid) {

    printf("\nEnter number of characters (1-30)> ");
    scanf("%d", & n);
    printf("\nInclude numbers (Y/N)> ");
    scanf(" %c", & nf);
    printf("\nInclude symbols (Y/N)> ");
    scanf(" %c", & sf);
    //cond1 and cond2 initialized here
    if (cond1 && cond2)
      valid = 1;
    else
      printf("Invalid input");
  }

I need to implement a faulty input detection functionality for the first integer scan. If the user enters a character instead of an integer the second scanf is skipped and it goes directly to the 3rd scanf. How can I stop this from happening in case of a character input on %d? I want to ask for the input again if the user enters a character instead of a number

You haven't clarified if you want your program to terminate or ask for input again. — alex01011, May 10 '21 at 12:52
Please re-work your question to include a [mcve]. Consider using more specific words in your instructions to user, such as _"enter from 1-30 digits"_ if that is what you want. Or _"enter from 1 to 30 alpha characters"_ if that is what you want. _characters_ is ambiguous. The entire table of ASCII characters are _characters_. — ryyker, May 10 '21 at 12:59
@alex01011 I want to ask for the input again if the user enters a character instead of a number — zerb, May 10 '21 at 13:02
Your last comment should be part of your post, not a comment. Edit your post please. — ryyker, May 10 '21 at 13:04
Where are `cond1` and `cond2` created/initialized/set in your code? — ryyker, May 10 '21 at 13:07
the `scanf()` function returns a value. That value is the number of successful 'input conversion specifiers' (or EOF) You should use the returned value to determine if a `%d` is successful or not — user3629249, May 11 '21 at 14:55

alex01011 · Accepted Answer · 2021-05-10T15:07:43.713

Just check the return value of scanf(), in your case:

if ((scanf("%d", &n) != 1) /* should return 1 if 1 int is read */
{ 
  fprintf(stderr,"Input not a number\n");
  exit(EXIT_FAILURE) /* include stdlib or just use return */
}

Note that scanf() doesn't provide any protection from arithmetic overflow, in the case of a big number. You might want to use fgets() and later parse that string with a function such as strtol() for safety.

Example: https://godbolt.org/z/oaMhac983

If you want to ask for input again, I would suggest that you use fgets() instead and later check that string for non-digit characters.

You can use strspn(),it returns the number of characters from the first argument that are present in the second argument.

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

#define MAX_N 13 /* save space \0 and \n */

int main(void) {
  int num = 0;
  char n[MAX_N] = {0};
  char *numbers = "-+0123456789";

  while (1) {
    if (fgets(n, MAX_N, stdin) == NULL) /* check return value of fgets() */
    {
      exit(EXIT_FAILURE);
    }
    if (strspn(n, numbers) ==
        strlen(n) - 1) /* only numbers were read (exclude '\n')*/
    {
      if (sscanf(n, "%d", &num) != 1) /* check return value of scanf() */
      {
        exit(EXIT_FAILURE);
      }
      break;
    }
  }

  printf("%d\n", num);

  return 0;
}

Example: https://godbolt.org/z/d5KTrTGGE

In my code if I try: if ((scanf("%d", &n) != 1)) continue; it ends up being an infinite loop instead of asking for the input again in the next iteration. — zerb, May 10 '21 at 12:58
@zerb I updated my answer. You can go with that, or with Vlad's, you can modify mine to make use of the `isdigit()` is function if you want it simpler. — alex01011, May 10 '21 at 13:21
`char *numbers = "0123456789";` is insufficient for inputs like `"-123\n", "+456\n", "0x789\n", "123"`. — chux - Reinstate Monica, May 10 '21 at 14:50
@chux-ReinstateMonica I've included the prefix option, as for the hex option, it would require a modification in the `scanf()` format specifier. — alex01011, May 10 '21 at 15:08

score 1 · Answer 2 · answered May 10 '21 at 12:53

It seems you mean something like the following

#include <stdio.h>

int main(void) 
{
    int n;
    const int MIN = 1, MAX = 30;
    
    do
    {
        printf( "\nEnter number of characters (%d-%d)> ", MIN, MAX );
        
        if ( scanf( "%d", &n ) != 1 )
        {
            clearerr( stdin );
            scanf( "%*[^\n]" );
            scanf( "%*c" );
            n = 0;
        }
    } while ( n < MIN || MAX < n );
    
    printf( "n = %d\n", n );
    
    return 0;
}

The program output might look like

Enter number of characters (1-30)> Hello
Enter number of characters (1-30)> 0
Enter number of characters (1-30)> 31
Enter number of characters (1-30)> Bye
Enter number of characters (1-30)> 10
n = 10

Or something like

#include <stdio.h>

int main(void) 
{
    int n = 0;
    const int MIN = 1, MAX = 30;
    
    printf( "\nEnter number of characters (%d-%d)> ", MIN, MAX );
        
    if ( scanf( "%d", &n ) != 1 )
    {
        clearerr( stdin );
        scanf( "%*[^\n]" );
        scanf( "%*c" );
        n = 0;
    }

    char nf, sf;
    
    if ( n != 0 )
    {
        printf("\nInclude numbers (Y/N)> ");
        scanf( " %c", &nf );
    }
    else
    {
        printf("\nInclude symbols (Y/N)> ");
        scanf(" %c", &sf);
    }
    
    return 0;
}

chux - Reinstate Monica · Answer 3 · 2021-05-10T17:24:53.040

How can I stop this from happening in case of a character input on %d?

Check the return value of scanf().

scanf("%d", &n); returns 1 of 3 values under various conditions:

1 Scanning succeed. Scanned int value in n.
0 Scanning failed due to non numeric input (the case OP is interested in). Value in n is unchanged.
EOF Scanning failed due to no more input (End-of-file). Value in n is unchanged.
EOF Scanning failed due to input error (rare). Value in n is indeterminate.

do { 
  printf("\nEnter number of characters (1-30)> ");
  int retval = scanf("%d", &n);
  if (retval == EOF) {
    printf("End of file or input error.  Quitting\n");
    exit(-1);
  }
  if (retval == 0) {
    // read in rest of line (except \n) and toss it.
    scanf("%*[^\n]");
    continue; 
  }
  // If here, valid `int` input read, now testfor range.
} while (n < 1 || n > 30);

....

Consider coding a helper function to write a prompt and read an int.

Skipping scanf for character input with %d

3 Answers3