Task

Show how to spell out a number in English.

You can use a preexisting implementation or roll your own, but you should support inputs up to at least one million (or the maximum value of your language's default bounded integer type, if that's less).

Support for inputs other than positive integers (like zero, negative integers, and floating-point numbers) is optional.



Related task

   Spelling of ordinal numbers.


#include <stdio.h>
#include <string.h>
 
const char *ones[] = { 0, "one", "two", "three", "four",
	"five", "six", "seven", "eight", "nine", 
	"ten", "eleven", "twelve", "thirteen", "fourteen",
	"fifteen", "sixteen", "seventeen", "eighteen", "nineteen" };
const char *tens[] = { 0, "ten", "twenty", "thirty", "forty",
	"fifty", "sixty", "seventy", "eighty", "ninety" };
const char *llions[] = { 0, "thousand", "million", "billion", "trillion",
//	"quadrillion", "quintillion", "sextillion", "septillion",
//	"octillion", "nonillion", "decillion"
	};
const int maxillion = sizeof(llions) / sizeof(llions[0]) * 3 - 3;
 
int say_hundred(const char *s, int len, int depth, int has_lead)
{
	int c[3], i;
	for (i = -3; i < 0; i++) {
		if (len + i >= 0) c[i + 3] = s[len + i] - '0';
		else c[i + 3] = 0;
	}
	if (!(c[0] + c[1] + c[2])) return 0;
 
	if (c[0]) {
		printf("%s hundred", ones[c[0]]);
		has_lead = 1;
	}
 
	if (has_lead && (c[1] || c[2]))
		printf((!depth || c[0]) && (!c[0] || !c[1]) ? "and " :
			c[0] ? " " : "");
 
	if (c[1] < 2) {
		if (c[1] || c[2]) printf("%s", ones[c[1] * 10 + c[2]]);
	} else {
		if (c[1]) {
			printf("%s", tens[c[1]]);
			if (c[2]) putchar('-');
		}
		if (c[2]) printf("%s", ones[c[2]]);
	}
 
	return 1;
}
 
int say_maxillion(const char *s, int len, int depth, int has_lead)
{
	int n = len / 3, r = len % 3;
	if (!r) {
		n--;
		r = 3;
	}
	const char *e = s + r;
	do {
		if (say_hundred(s, r, n, has_lead) && n) {
			has_lead = 1;
			printf(" %s", llions[n]);
			if (!depth) printf(", ");
			else printf(" ");
		}
		s = e; e += 3;
	} while (r = 3, n--);
 
	return 1;
}
 
void say_number(const char *s)
{
	int len, i, got_sign = 0;
 
	while (*s == ' ') s++;
	if (*s < '0' || *s > '9') {
		if (*s == '-') got_sign = -1;
		else if (*s == '+') got_sign = 1;
		else goto nan;
		s++;
	} else
		got_sign = 1;
 
	while (*s == '0') {
		s++;
		if (*s == '\0') {
			printf("zero\n");
			return;
		}
	}
 
	len = strlen(s);
	if (!len) goto nan;
 
	for (i = 0; i < len; i++) {
		if (s[i] < '0' || s[i] > '9') {
			printf("(not a number)");
			return;
		}
	}
	if (got_sign == -1) printf("minus ");
 
	int n = len / maxillion;
	int r = len % maxillion;
	if (!r) {
		r = maxillion;
		n--;
	}
 
	const char *end = s + len - n * maxillion;
	int has_lead = 0;
	do {
		if ((has_lead = say_maxillion(s, r, n, has_lead))) {
			for (i = 0; i < n; i++)
				printf(" %s", llions[maxillion / 3]);
			if (n) printf(", ");
		}
		n--;
		r = maxillion;
		s = end;
		end += r;
	} while (n >= 0);
 
	printf("\n");
	return;
 
nan:	printf("not a number\n");
	return;
}
 
int main()
{
	say_number("-42");
	say_number("1984");
	say_number("10000");
	say_number("1024");
	say_number("1001001001001");
	say_number("123456789012345678901234567890123456789012345678900000001");
	return 0;
}