Convert a Number to Words

// spell out an integer number in words, keep it below a billion
// tested with Pelles C and Dev-CPP     vegaseat     08sep2005

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

char *make_words(char *s, int ncomma);
char *insert_comma(long n, int *ncomma);
char *int2words(int n);

int main(void)
{

  printf("100000000 = %s\n", int2words(100000000));
  printf("10000000 = %s\n", int2words(10000000));
  printf("1000000 = %s\n", int2words(1000000));
  printf("100000 = %s\n", int2words(100000));
  printf("10000 = %s\n", int2words(10000));
  printf("1000 = %s\n", int2words(1000));
  printf("100 = %s\n", int2words(100));
  printf("10 = %s\n", int2words(10));
  printf("1 = %s\n", int2words(1));
  printf("\n");
  printf("123456789 = %s\n\n", int2words(123456789));
  printf("12345678 = %s\n\n", int2words(12345678));
  printf("777777 = %s\n", int2words(777777));
  printf("2005 = %s\n", int2words(2005));
  printf("-273 = %s\n", int2words(-273));
  
  getchar();  // wait
  return 0;
}

char *make_words(char *s, int ncomma)
{
  int i, len, rest = 0;
  char *p = NULL;
  static char zzz[256];

  static char *ones[] = {"one ","two ","three ","four ",
    "five ","six ","seven ","eight ","nine "};
  // the odd balls
  static char *tens[] = {"ten ","eleven ","twelve ","thirteen ",
    "fourteen ","fifteen ","sixteen ","seventeen ","eighteen ","nineteen "};
  // from here on a more logic order sets in
  static char *twenties[] = {"","twenty ","thirty ","forty ",
    "fifty ","sixty ","seventy ","eighty ","ninety "};
  static char *hundreds[] = {
    "hundred ","thousand ","million "};
  
  memset(zzz, '\0', 256);  // fill with nulls
  len = strlen(s);
  for(i = 0; i < len; i++)
  {

// for testing
//printf("i = %d rest = %d  ncomma = %d  s[%d] = %c  len = %d  zzz = %s\n", i, rest, ncomma, i, s[i], len, zzz);

    // skip the comma
    if ((p = strchr((s[i] == ',') ? &s[++i] : &s[i], ',')) == NULL)
    {
      p = &s[strlen(s)];
    }
    if (s[i] == '0')
    {
      continue;  // skip one iteration
    }
    if ((rest = (p - &s[i])) != 0)
    {
      if (rest == 3)
      {
        strcat(zzz, ones[s[i] - '0' - 1]);
        strcat(zzz, hundreds[0]);
        // special cases
        if (len == 7 && s[2] == '0')  strcat(zzz, hundreds[1]);
        if (len == 11 && s[2] == '0')  strcat(zzz, hundreds[2]);
      }
      else if (rest == 2) 
      {
        if (s[i] == '1')
        {
          strcat(zzz, tens[s[++i] - '0']);
          rest--;
        }
        else
        {
          strcat(zzz, twenties[s[i] - '0' - 1]);
        }
      }
      else
        strcat(zzz, ones[s[i] - '0' - 1]);
    }
    if (rest == 1 && ncomma != 0)
    {
      strcat(zzz, hundreds[ncomma--]);
    }
  }
  return zzz;
}

// insert a comma every third place from right to help in make_words
// returns the modified numeric string and the number of commas
char *insert_comma(long n, int *ncomma)
{
  static char zzz[30];
  int i = 0;
  char *p = &zzz[sizeof(zzz)-1];
  
  *p = '\0';
  *ncomma = 0;
  do 
  {
    if (i % 3 == 0 && i != 0) 
    {
      *--p = ',';
      ++*ncomma;
    }
     *--p = (char)('0' + n % 10);
    n /= 10;
    i++;
  } while(n != 0);
  return p;
}

char *int2words(int n)
{
  int nc;
  char *ps, *zzz, *minus;
  char *buffer;
  buffer = (char *) malloc(256);

  // save any - sign
  if (n < 0)
  {
    minus = "minus";
    n = abs(n);
  }
  else
  {
    minus = "";
  }

  ps = insert_comma(n, &nc);
  // ps is the comma modified numeric string, nc is the number of commas
  zzz = make_words(ps, nc);

  // if there was a - sign add it back
  sprintf(buffer,"%s %s", minus, zzz);

  return buffer;
}