Question 12.11

comp.lang.c FAQ list · Question 12.11

Q: How can I print numbers with commas separating the thousands?
What about currency formatted numbers?

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A: The functions in <locale.h> begin to provide some support for these operations, but there is no standard C function for doing either task. (In Standard C, the only thing printf does in response to a custom locale setting is to change its decimal-point character.)
POSIX specifies a strfmon function for formatting monetary quantities in a locale-appropriate way, and that the apostrophe flag in a numeric printf format specifier (e.g. %'d, %'f) requests comma-separated digits.
Here is a little routine for formatting comma-separated numbers, using the locale's thousands separator, if available:

#include <locale.h>

char *commaprint(unsigned long n)
{
 static int comma = '\0';
 static char retbuf[30];
 char *p = &retbuf[sizeof(retbuf)-1];
 int i = 0;

 if(comma == '\0') {
  struct lconv *lcp = localeconv();
  if(lcp != NULL) {
   if(lcp->thousands_sep != NULL &&
    *lcp->thousands_sep != '\0')
    comma = *lcp->thousands_sep;
   else comma = ',';
  }
 }

 *p = '\0';

 do {
  if(i%3 == 0 && i != 0)
   *--p = comma;
  *--p = '0' + n % 10;
  n /= 10;
  i++;
 } while(n != 0);

 return p;
}

(A better implementation would use the grouping field of the lconv structure, rather than assuming groups of three digits. A safer size for retbuf might be 4*(sizeof(long)*CHAR_BIT+2)/3/3+1; see question 12.21.)
References: ISO Sec. 7.4
H&S Sec. 11.6 pp. 301-4