Member Avatar for monkey_king

Hey,
does anyone know if it possible to have something like

#DEFINE NUM_DEC 4

printf("This is your value:%.NUM_DECf\n");

thanks in advance

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  • Latest Post Latest Post by Prabakar

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Short answer: a macro name that appears inside a string literal is not treated as source tokens to be expanded, so the literal "%.NUM_DECf" will not become "%.4f" at preprocess time. and were right about that, and /’s runtime approach (passing the precision as an argument to the formatter) is the usual, simple fix — see the printf spec for the runtime-precision * feature.

If a compile-time format is preferred, you can generate a format literal using the preprocessor stringizing operator. The common pattern uses a two-step macro so the numeric macro is expanded before it gets turned into a string:

#define DECIMALS 4
#define _STR(x) #x
#define STR(x) _STR(x)
#define FMT "%." STR(DECIMALS) "f"

double value = 3.141592653589793;
printf("This is your value: " FMT "\n", value);

Why the two steps? The stringize operator (#) will not expand its macro argument, so the indirection forces expansion of DECIMALS into 4 before stringizing. This yields a compile-time literal like "%.4f" without runtime overhead.

Cautions:

  • Make sure the generated format matches the argument type (mismatched printf formats cause undefined behavior).
  • If you need runtime variability, use the runtime-precision approach mentioned by / instead.
  • Be careful if DECIMALS is a complex expression (you’ll get the source text stringified, not its evaluated value).

References: cppreference on preprocessing and macro stringizing (preprocessor overview) and the POSIX printf description for runtime * width/precision (printf).

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Member Avatar for ArkM

Hey,
does anyone know if it possible to have something like

#DEFINE NUM_DEC 4
printf("This is your value:%.NUM_DECf\n");

Something like?.. No, it's impossible: no printed value presented ;)

printf("This is your value: %.*f\n",NUM_DEC,3.14159265358);
Member Avatar for csurfer

Hey,
does anyone know if it possible to have something like

#DEFINE NUM_DEC 4

printf("This is your value:%.NUM_DECf\n");

thanks in advance

Its impossible because anything other than data type specifiers and escape characters within double quotes of printf function is treated as string or character constant and is printed as it is in output.

Member Avatar for death_oclock

The real question here is whether or not macros are evaluated within strings. The answer: nope. ArkM's example is the correct approach.

Member Avatar for Prabakar

I believe, your real requirement, is to set variable precision for your floating point number.
If so
The bad news is you can't do it that way.
The good news is there is . printf("This is your value:%.*f", 4, 22./7); Follow the link to know how it works

Or are you looking for something like this?

#include<stdio.h>
#define st "String 2"

#include<math.h>
int main ()
{
  printf("Str 1 "st);//equivalent to printf("str 1 %s", st)
  return 0;
}
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