inserting comma in a large int number - c

Question

i am trying to insert a comma in a large integer number eg, 870120000 and present it as: 870,120 and the last three digits are discarded. The format will allways be the same, ie., xxxxxxxxx number where i am only interested in the first 6 digits.
What i have been doing so fare is using snprintf:
#include <stdio.h>
#include <string.h>
int main()
{
char buffer[5];
unsigned int lNum= 870120000;
int cx;
memset(buffer,0,sizeof(buffer));
cx = snprintf(buffer, 4, "%d,", lNum);
buffer[3] = ',';
printf("%s\n",buffer);
cx = snprintf(buffer+4, 4, "321"); // Note 1 ???
printf("%s\n", buffer);
return 0;
}
My problem is that i am stuck at (comment // Note 1), how do I add the last next three digits in to buffer ?
in addition I would like to know if this approch is ideal or is there any better(easier) way to do this?

Answer 1

You can do that in two steps. First convert the lNum into a string, then cut the needed parts out of this string. Something like that.
#include <stdio.h>
#include <stdlib.h>
int main(void) {
char tmp[256], out[256];
unsigned int lNum= 870120000;
sprintf(tmp, "%u", lNum);
sprintf(out, "%3.3s,%3.3s", tmp, tmp+3);
printf("%s\n", out); // prints 870,120
return 0;
}
See http://ideone.com/bnQNou
Or you can convert it into a float and change the locale to have "," as delimiter:
#include <stdio.h>
#include <locale.h>
int main()
{
unsigned int lNum = 870120000;
setlocale(LC_NUMERIC, "de_DE");
printf("%3.3f\n", (float)(870120000 / 1000000)); // prints 870,120
return 0;
}

Answer 2

First of all to hold 870,120 you have to define a 8 chars buffer.
So you can use %f format specifier to do what you the job, e.g.:
#include <stdio.h>
#include <string.h>
int main()
{
char buffer[8] = {0};
unsigned int lNum= 870120000;
snprintf(buffer, sizeof(buffer), "%3.3f", (float)(lNum)/(1000000));
char *temp = strstr(buffer, ".");
if (temp != NULL)
{
*temp = ',';
}
printf("%s\n",buffer);
return 0;
}
or another example using double snprintf and a given number of integer part digits and decimal part digits.
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#define INT_DIGIT 3
#define DEC_DIGIT 3
int main()
{
char temp_buffer[128];
char buffer[INT_DIGIT+DEC_DIGIT+2] = {0};
unsigned int lNum= 87012000;
snprintf(temp_buffer, sizeof(temp_buffer), "%u",lNum);
snprintf(buffer, sizeof(buffer), "%.*s,%.*s", INT_DIGIT, temp_buffer, DEC_DIGIT, &temp_buffer[INT_DIGIT]);
printf("%s\n",buffer);
return 0;
}