CODEWITHSUNDEEP
c
Kaaum
Kaaum

Reputation: 11

Passing a non-digit char after an integer to scanf

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

int main(void)
{
  int n, k;
  scanf("%d %d", &n, &k);
  printf("%d %d\n", n, k);

  return 0;
}

In the above sample code passing to the input

1 2

Would produce an expected output of

1 2

However passing any char in-between these two digits such as

1. 2 or 1 d 3

Will result in a strange output of the following

1 32766

I would like to know why this occurs, as it was to my belief that scanf would skip over any non-digit input.

Upvotes: 1

Views: 249

Answers (2)

Girish
Girish

Reputation: 376

You should check the return value of scanf, which tells you the number of data that are read into the passed arguments, here it is k. In your case, the return value will be zero as %d cannot be used to read in a char in C. If the first input is a char it will be 0, 1 if the first value is int and the second value is a char, 2 if both of the values are int.

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

int main(void)
{
    int n, k, rc;
    rc = scanf("%d %d", &n, &k);
    if (rc != 2) 
    {
        printf("scanf failed to set the values for n and k\n");
    }
    else
    {
        printf("valid input for n and k\n");
    }
    printf("%d %d\n", n, k);
    return 0;
}

So the int k is uninitialized and thus it will store some random value as scanf failed to set the value for this variable.

Upvotes: 3

dxiv
dxiv

Reputation: 17638

(Too long for a comment.)   The following answers OP's followup question from a comment.

is there anyway to pass over the character that stops the input stream?

scanf may not be the best or easiest way to do that, but it's still possible to do it using the %n format specifier which returns the offset into the original string during parsing. Following is an example, which uses sscanf but can be easily adapted to scanf.

#include <stdio.h>

void read_three_ints(const char *str) {
    int a, n1, b, n2, c;
    int ret = sscanf(str, "%d %n%d %n%d", &a, &n1, &b, &n2, &c);
    switch(ret)
    {
    case 3:
        printf("'%s': a = %d, b = %d, c = %d\n", str, a, b, c);
        break;
    case 2:
        printf("'%s': a = %d, b = %d, error parsing c = `%s`\n", str, a, b, str + n2);
        break;
    case 1:
        printf("'%s': a = %d, error parsing b = `%s`\n", str, a, str + n1);
        break;
    case 0:
        printf("'%s': error parsing a\n", str);
        break;
    default:
        printf("'%s': scanf error %d\n", str, ret);
        break;
    }
}

int main()
{
    read_three_ints("1 2 3");
    read_three_ints("1 2 x");
    read_three_ints("1, 2 ");
    read_three_ints(";1   ");
    read_three_ints("");
    return 0;
}

Output:

'1 2 3': a = 1, b = 2, c = 3
'1 2 x': a = 1, b = 2, error parsing c = `x`
'1, 2 ': a = 1, error parsing b = `, 2 `
';1   ': error parsing a
'': scanf error -1

Upvotes: 2

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