spacing in %d for inputs for scanf

Question

1. If I give the input 1234 5.7 elephant will give the resulting output 12 34 eleph I know that * means it ignores the input field, %2d is up to 2 character will be considered. but why 34 is applied to the second conversion character %d it belongs to the first %d?
2. Can I include space, tab, newline in the scanf? what happens? scanf("% d % d%s",var1, var2, var3);//is this fine?

/* input: 1234  5.7  elephant */
/* output: 12  34  eleph */

#include <stdio.h>

int main() {
    int x, y;
    char text[20];

    scanf("%2d %d %*f %5s", &x, &y, text);
    /* input: 1234  5.7  elephant */
    printf("%d  %d  %s", x, y, text);
    /* output: 12  34  eleph */

    return 0;
}

result expected: 12 elephant

@user3121023 It is invalid for scanf. It is valid for printf. — Vlad from Moscow, Jul 31 '19 at 09:49
@KamilCuk Yes, '*' is an assignment suppressing character. I did not see it.:) — Vlad from Moscow, Jul 31 '19 at 09:52
The `34` input does not belong to the first input, becuase it was halted at two characters. That's the point of the restriction `%2d` - so that the 3rd and 4th characters do not belong with it. No input is **forgotten**: if it is in the input buffer, it remains there until it is read. — Weather Vane, Jul 31 '19 at 09:53
vijay_ky Tip: post text as text, not as a picture. Text as pictures attracts down-votes. — chux - Reinstate Monica, Jul 31 '19 at 10:17
@chux yes %2d upto 2 characters and thanks for information on not to post text as image — vijay_ky, Jul 31 '19 at 10:17
Note (1) [Trailing white space in a format string](https://stackoverflow.com/questions/19499060/what-is-difference-between-scanfd-and-scanfd) and (2) [`scanf()` leaves the newline in the input buffer](https://stackoverflow.com/questions/5240789/scanf-leaves-the-new-line-char-in-buffer). — Jonathan Leffler, Jul 31 '19 at 15:24

chux - Reinstate Monica · Answer 1 · 2019-07-31T10:22:01.333

spacing in %d for inputs for scanf

In scanf() format a " " never fails. It scans for optional white-space.

why 34 is applied to the second conversion character ?

"%2d" consumes the 12 of input. The 2 in the format limits input to 2 characters: 1 and 2. @Weather Vane
" " scans for spaces and sees none in the input. Scanning continues. "%d" scans in 34.

can I include space, tab, newline in the scanf? what happens?

Yes. A whites-space in the scan format matches 0 or more input white-space.¹

scanf("% d % d%s",var1, var2, var3); is this fine?

No. "% d" is not a valid scanf() conversion specifier.

result expected: 12 elephant

This is curious. printf("%d %d %s", x, y, text); would print something out for y. This is no nothing value for an int.

Tip: good code checks the return value from scanf().

if (scanf("%2d %d %*f %5s", &x, &y, text) == 3) {
  printf("%d  %d  %s", x, y, text);
} else {
  puts("Bad input");
}

Better code does not use scanf(), but fgets(). Far easier to handle errant input.

¹White-space in a "%[]" specifier is handled different though. It needs an exact match.

It might be worth noting that `"%d %d"` and `"%d%d"` (`scanf` format specifier) behave exactly the same. — pmg, Jul 31 '19 at 11:10
@pmg True. Surprising, to me, `"%d"` will consume leading white-space even if the conversion fails. — chux - Reinstate Monica, Jul 31 '19 at 11:13

score 0 · Answer 2 · answered Jul 31 '19 at 09:52

%2d will get 2 lengths decimal when you in put.

If you try this...

int x,y;
/* type  : 1234 */
scanf("%2d %d", &x &y);
/* var x : 12   */
printf("%d %d", x,y);
/* input : 12 34  */

Or this?

int x,y;
/* type  : 1 234 */
scanf("%2d %d", &x &y);
/* var x : 1   */
printf("%d %d", x,y);
/* input : 1 234   */

Then if you get limited length It will record as next conversion character

and it depent on your space to!

spacing in %d for inputs for scanf

2 Answers2