숫자에 3자리마다 콤마 찍기
https://groups.google.com/forum/#!original/comp.lang.c/qdkgBJCgX9s/HjIy-MURXxYJ 참고 char * add_comma( int digit, char * buf ) { char *ptr, *result; int dp, sign; result = buf; ptr = fcvt( digit, 0, &dp, &sign ); while (dp--){ *buf++ = *ptr++; if (dp % 3 == 0) *buf++ = dp ? ',' : ' '; } *buf = '\0'; return result; }