Printing a float with commas in C

Question

Possible Duplicate:
Output 1000000 as 1,000,000 and so on

I have a float variable in the format xxxxxxxx.xx (Eg. 11526.99). I'd like to print it as 11,562.99 with a comma. How can I insert a comma in C?

Answer 1

Try:

#include <locale.h>
#include <stdio.h>

int main()
{
    float f = 12345.67;

    // obtain the existing locale name for numbers    
    char *oldLocale = setlocale(LC_NUMERIC, NULL);

    // inherit locale from environment
    setlocale(LC_NUMERIC, "");

    // print number
    printf("%'.2f\n", f);

    // set the locale back
    setlocale(LC_NUMERIC, oldLocale);
}

This depends on the current locale. The C and POSIX locales do not have a thousands separator. Instead of inheriting the locale from the environment you can set it yourself to a locale that you know uses a thousands separator. On my system, using "en_NZ" provides a thousands separator.

Answer 2

The below addcommas function is a version locale-less, that allows negative floats (doesn't work with exponent like 3.14E10 though)

#include <stdio.h>
#include <string.h>

#define DOT     '.'
#define COMMA   ','
#define MAX     50

static char commas[MAX]; // Where the result is

char *addcommas(float f) {
  char tmp[MAX];            // temp area
  sprintf(tmp, "%f", f);    // refine %f if you need
  char *dot = strchr(tmp, DOT); // do we have a DOT?
  char *src,*dst; // source, dest

  if (dot) {            // Yes
    dst = commas+MAX-strlen(dot)-1; // set dest to allow the fractional part to fit
    strcpy(dst, dot);               // copy that part
    *dot = 0;       // 'cut' that frac part in tmp
    src = --dot;    // point to last non frac char in tmp
    dst--;          // point to previous 'free' char in dest
  }
  else {                // No
    src = tmp+strlen(tmp)-1;    // src is last char of our float string
    dst = commas+MAX-1;         // dst is last char of commas
  }

  int len = strlen(tmp);        // len is the mantissa size
  int cnt = 0;                  // char counter

  do {
    if ( *src<='9' && *src>='0' ) {  // add comma is we added 3 digits already
      if (cnt && !(cnt % 3)) *dst-- = COMMA;
      cnt++; // mantissa digit count increment
    }
    *dst-- = *src--;
  } while (--len);

  return dst+1; // return pointer to result
}

How to call it, for instance (main example)

int main () {

   printf ("%s\n", addcommas(0.31415));
   printf ("%s\n", addcommas(3.1415));
   printf ("%s\n", addcommas(31.415));
   printf ("%s\n", addcommas(314.15));
   printf ("%s\n", addcommas(3141.5));
   printf ("%s\n", addcommas(31415));
   printf ("%s\n", addcommas(-0.31415));
   printf ("%s\n", addcommas(-3.1415));
   printf ("%s\n", addcommas(-31.415));
   printf ("%s\n", addcommas(-314.15));
   printf ("%s\n", addcommas(-3141.5));
   printf ("%s\n", addcommas(-31415));
   printf ("%s\n", addcommas(0));

  return 0;
}

Compilation instruction example

gcc -Wall comma.c -o comma

Doing

./comma

Should output

0.314150
3.141500
31.415001
314.149994
3,141.500000
31,415.000000
-0.314150
-3.141500
-31.415001
-314.149994
-3,141.500000
-31,415.000000
0.000000

• Set DOT to what is a dot
• Set COMMA to what is supposed to be a comma
• MAX set to 50 assumes the float converted as string will not be more than 49 characters (increase MAX in doubt)
• Returns a pointer to the commas added string from the float given as parameter, pointer to a static area, thus
  • addcommas is not reentrant, and the value pointed to by the returned pointer (usually) changes after each call, eg.
  • in char *a = addcommas(3.1415) ; char *b = addcommas(2.7182) ; a cannot be used safely anymore after the second call to addcommas