sprintf format specifier replace by nothing

Question

I am currently wondering if there is a way to replace the format specifier %u by nothing using sprintf

My question is about the use of a ternary operator in sprintf which gonna replace %u by a value or by nothing.

Here is an example of what I am trying to do :

int main (void)
{
   char mytab[10]={'\0'};
   uint_32 i=0;

   scanf("%u",&i);

   sprintf(mytab, "\"%u"\",i>0?i:/*Here is the syntax I want to find if it exists*/);
   printf("%s\r\n",mytab);

   return 0;
}

The result of the code I am trying to get is for example "1" if the input is 1 (or "2" if the input is 2...) and "" if the input is 0.

Do you have any ideas or explantion about it ? Thanks by advance.

I would say nothing but the use of ternary operator will avoid multiple sprintf line (for example if have multiple variable to test...). — Joze, Oct 23 '13 at 12:32
No, you can't, and also your `scanf` format is wrong for that type. — interjay, Oct 23 '13 at 12:34
@Joze - the answer you accepted doesn't deal well with multiple variables to test in one statement. Since the accepted answer only works for a single conditional format you might as well go with using a straightforward `if` statement. — Michael Burr, Oct 24 '13 at 08:48

abelenky · Answer 1 · 2013-10-23T17:19:38.893

3

I think you should put the ternary operator on the format-string, choosing to use either "%u", or "", depending on the value of i.

sprintf(mytab, i? "\"%u\"" : "\"\"", i);

edited Oct 23 '13 at 17:19

answered Oct 23 '13 at 15:46

abelenky

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Nice, because "clean". However if this is about tightness: `sprintf(mytab, i ?"%"PRIu32 :"", i); printf("\"%s\"", mytab);` – alk Oct 23 '13 at 17:00
1

@abelenky: True in general, but the OP specified `i` to be unsigned. – alk Oct 23 '13 at 17:18
Ah... so he did. So his comparison `>0` doesn't make much sense. Good point. – abelenky Oct 23 '13 at 17:19
Why doesn't `>0` make sense? – Michael Burr Oct 23 '13 at 18:52
Another thing to keep in mind is that this will only work if the conditional format-spec is the last one. That problem can be handled if you're willing to rely on the POSIX extension of numbered argument conversion specifications (`"i? "\"%1$u\"" : "\"\""`). – Michael Burr Oct 23 '13 at 19:20
Comparing an UNSIGNED int > zero doesn't make sense since the value cannot be less than zero. You might as well just write `i?`. (`i>0` is legal... and it works. But to me, it implies that the other possibility is that i could be less than zero) – abelenky Oct 23 '13 at 19:20

Gangadhar · Answer 2 · 2013-10-23T15:31:56.587

2

Modify this statement

sprintf(mytab, "\"%u"\",i>0?i:/*Here is the syntax I want to find if it exists*/);

This is what you need.

 (i>0)? sprintf(mytab, "%u",i) : sprintf(mytab,"%s","") ;

EDIT

AS H2CO3 suggested

You can also Use in this way.

if (i > 0)  
sprintf(mytab, "%"PRIu32, i); 
else
sprintf(mytab,"%s","");

Also note that %u is not the appropriate format specifier for uint32_t, use "%"PRIu32

(i>0)? sprintf(mytab, "%"PRIu32,i) : sprintf(mytab,"%s","") ;

edited Oct 23 '13 at 15:31

answered Oct 23 '13 at 12:31

Gangadhar

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Doesn't work, this won't print the number `i`. And your use of `*` will cause undefined behavior because you don't pass a value for it. – interjay Oct 23 '13 at 12:33
Agreed with interjay I want to print the value of i. Sorry if my question wasn't clear. – Joze Oct 23 '13 at 12:34
@Joze modified answer. Check it now. – Gangadhar Oct 23 '13 at 13:18
1

Well... why not just `if (i > 0) { sprintf(mytab, "%u", i); }`? – Oct 23 '13 at 14:43
@H2CO3 Yes OP can use as you suggested, if he wants to do with if-else.writing else statement is also important When OP need to repeats this in loop. – Gangadhar Oct 23 '13 at 15:25
@H2CO3 `if (i > 0) { sprintf(mytab, "%u", i); }` does not fulfill OP's: "" if the input is 0. – chux - Reinstate Monica Oct 23 '13 at 16:44
1

+1 for `(i>0)? sprintf(mytab, "%"PRIu32,i) : sprintf(mytab,"%s","")`. Although not used by OP, it returns a consistent value that would be expected from `sprint()`. – chux - Reinstate Monica Oct 23 '13 at 16:48
@H2CO3 If the user enters 0 via `scanf("%u",&i);`, a `if (i > 0) { sprintf(mytab, "%u", i); }` results in a `mytab` with a length of 0. OP wanted `\"%u\"` for non-zero input (example "1" if the input is 1) and `\"\"` for 0 input ("" if the input is 0.). – chux - Reinstate Monica Oct 23 '13 at 18:08
@chux So that would be the literal string `""`, not the empty string? – Oct 23 '13 at 18:09
@H2CO3 But this minor issue is about if `\"` in the output. – chux - Reinstate Monica Oct 23 '13 at 18:09
@chux Then he could just initialize `mytab` like `char mytab[10] = "\"\"";`... Much cleaner. – Oct 23 '13 at 18:10

score 2 · Accepted Answer · answered Oct 23 '13 at 18:12

2

I think a simple if statement is still the cleanest option:

char mytab[10] = "\"\"";

if (n > 0) {
    snprintf(mytab, sizeof mytab, "\"%" PRIu32 "\"", n);
}

answered Oct 23 '13 at 18:12

alk · Answer 4 · 2013-10-23T16:47:54.857

1

You might (not?) like to use this dirty trick^*1 (which makes gcc yell out warnings):

char mytab[11] = ""; 
uint32_t i = 0;
int result = scanf("%"SCNu32, &i);

if (1 != result)
{
  if (-1 == result)
  {
    if (errno)
    {
      perror("scanf() failed");
    }
    else
    {
      fprintf(stderr, "EOF reached\n");
    }
  }
  else
  {
    fprintf(stderr, "invalid input\n");
  }
}

sprintf(mytab, i>0 ?"%"PRIu32 :"%s", i>0 ?i :"");
printf("%s", mytab);

^{*1 To mention this explicitly: "dirty" in this context is a hint that this might invoke the fabulous Undefined Behaviour. So don't do this in prodcution!}

The sane apporach would be to do:

...

if (i)
{
  sprintf(mytab, "%"PRIu32, i);
}
printf("%s", mytab);

Please note that although the maximum number of digits for an unsigned 32bit number in decimal format is 10, you still need to reserve one character for the 0-terminator.

edited Oct 23 '13 at 16:47

answered Oct 23 '13 at 12:43

alk

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Also note that `%u` is not the appropriate format specifier for `uint32_t` (I think that's what OP meant). – Oct 23 '13 at 14:47
1

If you're going to use the `inttypes.h` format specifier macros, the one in the `scanf()` call should be `SCNu32`. – Michael Burr Oct 23 '13 at 15:15
1

`i>0 ?i :""` is invalid according to the standard because arguments 2 and 3 are `uint32_t` and `char*`. I'm not sure why both gcc and MSVC only generate a warning. Both compilers treat the type of the expression as `char*` so if you're building for a 64-bit target with 32-bit ints you'll get undefined behavior when `sprintf()` tries to format a 32-bit unsigned int value from a 64-bit pointer when `i>0`. This problem might not show up in the example, but will almost certainly show up when a more complex format is used with more arguments. It's pretty difficult to read as well. – Michael Burr Oct 23 '13 at 15:34

sprintf format specifier replace by nothing

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