Code Golf: Four is magic

Question

The puzzle

A little puzzle I heard while I was in high school went something like this...

• The questioner would ask me to give him a number;
• On hearing the number, the questioner would do some sort of transformation on it repeatedly (for example, he might say ten is three) until eventually arriving at the number 4 (at which point he would finish with four is magic).
• Any number seems to be transformable into four eventually, no matter what.

The goal was to try to figure out the transformation function and then be able to reliably proctor this puzzle yourself.

The solution

The transformation function at any step was to

• Take the number in question,
• Count the number of letters in its English word representation, ignoring a hyphen or spaces or "and" (e.g., "ten" has 3 letters in it, "thirty-four" has 10 letters in it, "one hundred forty-three" has 20 letters in it).
• Return that number of letters.

For all of the numbers I have ever cared to test, this converges to 4. Since "four" also has four letters in it, there would be an infinite loop here; instead it is merely referred to as magic by convention to end the sequence.

The challenge

Your challenge is to create a piece of code that will read a number from the user and then print lines showing the transformation function being repeatedly applied until "four is magic" is reached.

Specifically:

1. Solutions must be complete programs in and of themselves. They cannot merely be functions which take in a number-- factor in the input.
2. Input must be read from standard input. (Piping from "echo" or using input redirection is fine since that also goes from stdin)
3. The input should be in numeric form.
4. For every application of the transformation function, a line should be printed: a is b., where a and b are numeric forms of the numbers in the transformation.
5. Full stops (periods) ARE required!
6. The last line should naturally say, 4 is magic..
7. The code should produce correct output for all numbers from 0 to 99.

Examples:

> 4
4 is magic.

> 12
12 is 6.
6 is 3.
3 is 5.
5 is 4.
4 is magic.

> 42
42 is 8.
8 is 5.
5 is 4.
4 is magic.

> 0
0 is 4.
4 is magic.

> 99
99 is 10.
10 is 3.
3 is 5.
5 is 4.
4 is magic.

The winner is the shortest submission by source code character count which is also correct.

BONUS

You may also try to write a version of the code which prints out the ENGLISH NAMES for the numbers with each application of the transformation function. The original input is still numeric, but the output lines should have the word form of the number.

(Double bonus for drawing shapes with your code)

(EDIT) Some clarifications:

1. I do want the word to appear on both sides in all applicable cases, e.g. Nine is four. Four is magic.
2. I don't care about capitalization, though. And I don't care how you separate the word tokens, though they should be separated: ninety-nine is okay, ninety nine is okay, ninetynine is not okay.

I'm considering these a separate category for bonus competition with regard to the challenge, so if you go for this, don't worry about your code being longer than the numeric version.

Feel free to submit one solution for each version.

Answer 1

J, 107 112 characters

'4 is magic.',~}:('.',~":@{.,' is ',":@{:)"1]2&{.\.
(]{&(#.100 4$,#:3 u:ucp'䌵䐵吶梇禈榛ꪛ멩鮪鮺墊馊꥘誙誩墊馊ꥺ겻곋榛ꪛ멩鮪鮺'))^:a:

(Newline for readability only)

Usage and output:

    '4 is magic.',~}:('.',~":@{.,' is ',":@{:)"1]2&{.\.(]{&(#.100 4$,#:3 u:ucp'䌵䐵吶梇禈榛ꪛ멩鮪鮺墊馊꥘誙誩墊馊ꥺ겻곋榛ꪛ멩鮪鮺'))^:a:12
12 is 6.    
6 is 3.     
3 is 5.     
5 is 4.     
4 is magic. 

Answer 2

Python:

#!/usr/bin/env python

# Number of letters in each part, we don't count spaces
Decades = ( 0, 3, 6, 6, 6, 5, 5, 7, 6, 6, 0 )
Smalls  = ( 0, 3, 3, 5, 4, 4, 3, 5, 5, 4 )
Teens  =  ( 6, 6, 8, 8, 7, 7, 9, 8, 8 )

def Count(n):
    if n > 10 and n < 20: return Teens[n-11]
    return   Smalls[n % 10 ] + Decades [ n / 10 ]

N = input()

while N-4:
    Cnt = Count(N)
    print "%d is %d" % ( N, Cnt)
    N = Cnt

print "4 is magic"

Answer 3

Python 2.x, 144 150 154 166 chars

This separates the number into tens and ones and sum them up. The undesirable property of the pseudo-ternary operator a and b or c that c is returned if b is 0 is being abused here.

n=input()
x=0x4d2d0f47815890bd2
while n-4:p=n<20and x/10**n%10or 44378/4**(n/10-2)%4+x/10**(n%10)%10+4;print n,"is %d."%p;n=p
print"4 is magic."

---

The previous naive version (150 chars). Just encode all lengths as an integer.

n=input()
while n-4:p=3+int('1yrof7i9b1lsi207bozyzg2m7sclycst0zsczde5oks6zt8pedmnup5omwfx56b29',36)/10**n%10;print n,"is %d."%p;n=p
print"4 is magic."

Answer 4

Java (with boilerplate), 308 290 286 282 280 characters

class A{public static void main(String[]a){int i=4,j=0;for(;;)System.out.printf("%d is %s.%n",i=i==4?new java.util.Scanner(System.in).nextInt():j,i!=4?j="43354435543668877988699;::9;;:699;::9;;:588:998::9588:998::9588:998::97::<;;:<<;699;::9;;:699;::9;;:".charAt(i)-48:"magic");}}

I'm sure Groovy would get rid of much of that.

Explanation and formatting (all comments, newlines and leading/trailing whitespace removed in count):

Reasonably straight forward, but

//boilerplate
class A{
   public static void main(String[]a){
      //i is current/left number, j right/next number.  i=4 signals to start
      //by reading input
      int i=4,j=0;
      for(;;)
         //print in the form "<left> is <right>."
         System.out.printf(
            "%d is %s.%n",
            i=i==4?
               //<left>: if i is 4 <left> will be a new starting number
               new java.util.Scanner(System.in).nextInt():
               //otherwise it's the next val
               j,
            i!=4?
               //use string to map number to its length (:;< come after 9 in ASCII)
               //48 is value of '0'.  store in j for next iteration
               j="43354435543668877988699;::9;;:699;::9;;:588:998::9588:998::9588:998::97::<;;:<<;699;::9;;:699;::9;;:".charAt(i)-48:
               //i==4 is special case for right; print "magic"
               "magic");
   }
}

Edit: No longer use hex, this is less keystrokes

Answer 5

Perl: 148 characters

(Perl: 233 181 212 206 200 199 198 185 179 149 148 characters)

• Moved exceptions hash into unit array. This resulted in my being able to cut a lot of characters :-)
• mobrule pointed out a nasty bug. Quick fix adds 31 characters, ouch!
• Refactored for zero special case, mild golfing done as well.
• Direct list access for single use rather than storing to array? Hell yes!
• SO MUCH REFACTORING for just ONE bloody character. This, truly, is the life of a golfer. :-(
• Oops, easy whitespace fix. 198 now.
• Refactored some redundant code.
• Last return keyword in r is unnecessary, shaved some more off.
• Massive refactoring per comments; unfortunately I could only get it to 149 because I had to fix a bug that was present in both my earlier code and the commenters' versions.
• Trying bareword "magic".

Let's get this ball rolling with a modest attempt in Perl.

@u=split'','4335443554366887798866555766';$_=<>;chop;print"$_ is ".($_=$_==4?0:$_<20?$u[$_]:($u[$_/10+18]+($_%10&&$u[$_%10]))or magic).".
"while$_

Tricks:

Too many!

Answer 6

C++ Stdio version, minified: 196 characters

#include <cstdio>
#define P;printf(
char*o="43354435543668877988";main(int p){scanf("%d",&p)P"%d",p);while(p!=4){p=p<20?o[p]-48:"0366555966"[p/10]-96+o[p%10]P" is %d.\n%d",p,p);}P" is magic.\n");}

C++ Iostreams version, minified: 195 characters

#include <iostream>
#define O;std::cout<<
char*o="43354435543668877988";main(int p){std::cin>>p;O p;while(p!=4){p=p<20?o[p]-48:"0366555966"[p/10]-96+o[p%10]O" is "<<p<<".\n"<<p;}O" is magic.\n";}

Original, un-minified: 344 characters

#include <cstdio>

int ones[] = { 4, 3, 3, 5, 4, 4, 3, 5, 5, 4, 3, 6, 6, 8, 8, 7, 7, 9, 8, 8 };
int tens[] = { 0, 3, 6, 6, 5, 5, 5, 9, 6, 6 };

int n(int n) {
    return n<20 ? ones[n] : tens[n/10] + ones[n%10];
}

int main(int p) {
    scanf("%d", &p);
    while(p!=4) {
        int q = n(p);
        printf("%i is %i\n", p, q);
        p = q;
    }
    printf("%i is magic\n", p);
}

Answer 7

Ruby, 164 characters

n=gets.to_i;s="03354435543668877987";if n==0;puts"0 is 4.";else;puts"#{n} is #{n=(n<20)?s[n]-48:"0066555766"[n/10]-48+s[n%10]-48}." until n==4;end;puts"4 is magic."

decoded:

n = gets.to_i
s = "03354435543668877987"
if n == 0
  puts "0 is 4."
else
  puts "#{n} is #{n = (n < 20) ? s[n] - 48 : "0066555766"[n / 10] - 48 + s[n % 10] - 48}." until n == 4
end

puts "4 is magic."

Answer 8

C, 158 characters

main(n,c){char*d="03354435543668877988";for(scanf("%d",&n);n-4;n=c)printf("%d is %d.\n",n,c=n?n<19?d[n]-48:d[n%10]-"_,**+++)**"[n/10]:4);puts("4 is magic.");}

(originally based on Vlad's Python code, borrowed a trick from Tom Sirgedas' C++ solution to squeeze out a few more characters)

expanded version:

main(n, c) {
    char *d = "03354435543668877988";
    for (scanf("%d",&n); n-4; n = c)
        printf("%d is %d.\n", n, c = n ? n<19 ? d[n]-48 : d[n%10] - "_,**+++)**"[n/10]  : 4);
    puts("4 is magic.");
}

Answer 9

PhP Code

function get_num_name($num){  
    switch($num){  
        case 1:return 'one';  
    case 2:return 'two';  
    case 3:return 'three';  
    case 4:return 'four';  
    case 5:return 'five';  
    case 6:return 'six';  
    case 7:return 'seven';  
    case 8:return 'eight';  
    case 9:return 'nine';  
    }  
}  

function num_to_words($number, $real_name, $decimal_digit, $decimal_name){  
    $res = '';  
    $real = 0;  
    $decimal = 0;  

    if($number == 0)  
        return 'Zero'.(($real_name == '')?'':' '.$real_name);  
    if($number >= 0){  
        $real = floor($number);  
        $decimal = number_format($number - $real, $decimal_digit, '.', ',');  
    }else{  
        $real = ceil($number) * (-1);  
        $number = abs($number);  
        $decimal = number_format($number - $real, $decimal_digit, '.', ',');  
    }  
    $decimal = substr($decimal, strpos($decimal, '.') +1);  

    $unit_name[1] = 'thousand';  
    $unit_name[2] = 'million';  
    $unit_name[3] = 'billion';  
    $unit_name[4] = 'trillion';  

    $packet = array();    

    $number = strrev($real);  
    $packet = str_split($number,3);  

    for($i=0;$i<count($packet);$i++){  
        $tmp = strrev($packet[$i]);  
        $unit = $unit_name[$i];  
        if((int)$tmp == 0)  
            continue;  
        $tmp_res = '';  
        if(strlen($tmp) >= 2){  
            $tmp_proc = substr($tmp,-2);  
            switch($tmp_proc){  
                case '10':  
                    $tmp_res = 'ten';  
                    break;  
                case '11':  
                    $tmp_res = 'eleven';  
                    break;  
                case '12':  
                    $tmp_res = 'twelve';  
                    break;  
                case '13':  
                    $tmp_res = 'thirteen';  
                    break;  
                case '15':  
                    $tmp_res = 'fifteen';  
                    break;  
                case '20':  
                    $tmp_res = 'twenty';  
                    break;  
                case '30':  
                    $tmp_res = 'thirty';  
                    break;  
                case '40':  
                    $tmp_res = 'forty';  
                    break;  
                case '50':  
                    $tmp_res = 'fifty';  
                    break;  
                case '70':  
                    $tmp_res = 'seventy';  
                    break;  
                case '80':  
                    $tmp_res = 'eighty';  
                    break;  
                default:  
                    $tmp_begin = substr($tmp_proc,0,1);  
                    $tmp_end = substr($tmp_proc,1,1);  

                    if($tmp_begin == '1')  
                        $tmp_res = get_num_name($tmp_end).'teen';  
                    elseif($tmp_begin == '0')  
                        $tmp_res = get_num_name($tmp_end);  
                    elseif($tmp_end == '0')  
                        $tmp_res = get_num_name($tmp_begin).'ty';  
                    else{  
                        if($tmp_begin == '2')  
                            $tmp_res = 'twenty';  
                        elseif($tmp_begin == '3')  
                            $tmp_res = 'thirty';  
                        elseif($tmp_begin == '4')  
                            $tmp_res = 'forty';  
                        elseif($tmp_begin == '5')  
                            $tmp_res = 'fifty';  
                        elseif($tmp_begin == '6')  
                            $tmp_res = 'sixty';  
                        elseif($tmp_begin == '7')  
                            $tmp_res = 'seventy';  
                        elseif($tmp_begin == '8')  
                            $tmp_res = 'eighty';  
                        elseif($tmp_begin == '9')  
                            $tmp_res = 'ninety';  

                        $tmp_res = $tmp_res.' '.get_num_name($tmp_end);  
                    }  
                    break;  
            }  

            if(strlen($tmp) == 3){  
                $tmp_begin = substr($tmp,0,1);  

                $space = '';  
                if(substr($tmp_res,0,1) != ' ' && $tmp_res != '')  
                    $space = ' ';  

                if($tmp_begin != 0){  
                    if($tmp_begin != '0'){  
                        if($tmp_res != '')  
                            $tmp_res = 'and'.$space.$tmp_res;  
                    }  
                    $tmp_res = get_num_name($tmp_begin).' hundred'.$space.$tmp_res;  
                }  
            }  
        }else  
            $tmp_res = get_num_name($tmp);  
        $space = '';  
        if(substr($res,0,1) != ' ' && $res != '')  
            $space = ' ';  
        $res = $tmp_res.' '.$unit.$space.$res;  
    }  

    $space = '';  
    if(substr($res,-1) != ' ' && $res != '')  
        $space = ' ';  

    if($res)  
        $res .= $space.$real_name.(($real > 1 && $real_name != '')?'s':'');  

    if($decimal > 0)  
        $res .= ' '.num_to_words($decimal, '', 0, '').' '.$decimal_name.(($decimal > 1 && $decimal_name != '')?'s':'');  
    return ucfirst($res);  
}  

//////////// testing ////////////////

 $str2num = 12;
    while($str2num!=4){
        $str = num_to_words($str2num, '', 0, '');  
        $str2num = strlen($str)-1;
        echo $str . '=' . $str2num .'<br/>';
        if ($str2num == 4)
            echo 'four is magic';
    }

////// Results /////////

Twelve =6
Six =3
Three =5
Five =4
four is magic

Answer 10

C++, 171 characters (#include omitted)

void main(){char x,y,*a="03354435543668877988";scanf("%d",&x);for(;x-4;x=y)y=x?x<19?a[x]-48:"_466555766"[x/10]+a[x%10]-96:4,printf("%d is %d.\n",x,y);puts("4 is magic.");}

Answer 11

C# 314 286 283 274 289 273 252 chars.

Squished:

252 

Normal:

using C = System.Console;
class P
{
    static void Main()
    {
        var x = "4335443554366877798866555766";
        int m, o, v = int.Parse(C.ReadLine());
        do {
            C.Write("{0} is {1}.\n", o = v, v == 4 ? (object)"magic" : v = v < 20 ? x[v] - 48 : x[17 + v / 10] - 96 + ((m = v % 10) > 0 ? x[m] : 48));
        } while (o != 4);
        C.ReadLine();
    }
}

Edit Dykam: Did quite some carefull insertions and changes:

• Changed the l.ToString() into a cast to object of the string "magic".
• Created a temporary variable o, so I could move the break outside the for loop, that is, resulting in a do-while.
• Inlined the o assignment, aswell the v assignment, continueing in inserting the calculation of l in the function arguments altogether, removing the need for l. Also inlined the assignment of m.
• Removed a space in int[] x, int[]x is legit too.
• Tried to transform the array into a string transformation, but the using System.Linq was too much to make this an improvement.

Edit 2 Dykam Changed the int array to a char array/string, added proper arithmics to correct this.

Answer 12

C#: 210 Characters.

Squished:

using C=System.Console;class B{static void Main(){int
x=0,y=int.Parse(C.ReadLine());while(x!=4)C.Write((x=y)+" is {0}.\n",x==4?"magic":""+(y=x==0?4:"03354435543668877988"[x<20?x:x%10]+"0066555766"[x/10]-96));}}

Expanded:

using C=System.Console;
class B
{
    static void Main()
    {
        int x=0,y=int.Parse(C.ReadLine());
        while(x!=4)
            C.Write((x=y)+" is {0}.\n",
                x==4?
                     "magic":
                     ""+(y= x==0?
                                4:
                                "03354435543668877988"[x<20?x:x%10]+
                                "0066555766"[x/10]-96)
                   );
    }
}

Tricks this approach uses:

• Create a lookup table for number name lengths based on digits that appear in the number.
• Use character array lookup on a string, and char arithmetic instead of a numeric array.
• Use class name aliasing to short Console. to C.
• Use the conditional (ternary) operator (?:) instead of if/else.
• Use the \n with Write escape code instead of WriteLine
• Use the fact that C# has a defined order of evaluation to allow assignments inside the Write function call
• Use the assignment expressions to eliminate extra statements, and thus extra braces

Answer 13

Common Lisp 157 Chars

New more conforming version, now reading form standard input and ignoring spaces and hyphens:

(labels((g (x)(if(= x 4)(princ"4 is magic.")(let((n(length(remove-if(lambda(x)(find x" -"))(format nil"~r"x)))))(format t"~a is ~a.~%"x n)(g n)))))(g(read)))

In human-readable form:

 (labels ((g (x)
           (if (= x 4)
            (princ "4 is magic.")
            (let ((n (length (remove-if (lambda(x) (find x " -"))
                                        (format nil "~r" x)))))
               (format t"~a is ~a.~%" x n)
               (g n)))))
    (g (read)))

And some test runs:

>24
24 is 10.
10 is 3.
3 is 5.
5 is 4.
4 is magic.

>23152436
23152436 is 64.
64 is 9.
9 is 4.
4 is magic.

And the bonus version, at 165 chars:

 (labels((g(x)(if(= x 4)(princ"four is magic.")(let*((f(format nil"~r"x))(n(length(remove-if(lambda(x)(find x" -"))f))))(format t"~a is ~r.~%"f n)(g n)))))(g(read)))

Giving

>24
twenty-four is ten.
ten is three.
three is five.
five is four.
four is magic.

>234235
two hundred thirty-four thousand two hundred thirty-five is forty-eight.
forty-eight is ten.
ten is three.
three is five.
five is four.
four is magic.

Answer 14

Windows PowerShell: 152 153 184 bytes

based on the previous solution, with more influence from other solutions

$o="03354435543668877988"
for($input|sv b;($a=$b)-4){if(!($b=$o[$a])){$b=$o[$a%10]-48+"66555766"[($a-$a%10)/10-2]}$b-=48-4*!$a
"$a is $b."}'4 is magic.'

Answer 15

Lua 185 190 199

added periods, added io.read, removed ()'s on last print

 n=io.read();while(n~=4)do m=('43354435543668877988699;::9;;:699;::9;;:588:998::9588:998::9588:998::97::<;;:<<;699;::9;;:699;::9;;:'):sub(n+1):byte()-48;print(n,' is ',m,'.')n=m;end print'4 is magic.'

with line breaks

 n=io.read()
 while (n~=4) do
    m=('43354435543668877988699;::9;;:699;::9;;:588:998::9588:998::9588:998::97::<;;:<<;699;::9;;:699;::9;;:'):sub(n+1):byte()-48;
    print(n,' is ',m,'.')
    n=m;
 end 
 print'4 is magic.'

Answer 16

Haskell, 224 270 characters

o="43354435543668877988"
x!i=read[x!!i]
n x|x<20=o!x|0<1="0066555766"!div x 10+o!mod x 10
f x=zipWith(\a b->a++" is "++b++".")l(tail l)where l=map show(takeWhile(/=4)$iterate n x)++["4","magic"]
main=readLn>>=mapM putStrLn.f

And little more readable -

ones = [4,3,3,5,4,4,3,5,5,4,3,6,6,8,8,7,7,9,8,8]
tens = [0,0,6,6,5,5,5,7,6,6]

n x = if x < 20 then ones !! x else (tens !! div x 10) + (ones !! mod x 10)

f x = zipWith (\a b -> a ++ " is " ++ b ++ ".") l (tail l)
    where l = map show (takeWhile (/=4) (iterate n x)) ++ ["4", "magic"]

main = readLn >>= mapM putStrLn . f

Answer 17

GolfScript - 101 96 93 92 91 90 94 86 bytes

90 → 94: Fixed output for multiples of 10.
94 → 86: Restructured code. Using base 100 to remove non-printable characters.
86 → 85: Shorter cast to string.

{n+~."+#,#6$DWOXB79Bd")base`1/10/~{~2${~1$+}%(;+~}%++=" is "\".
"1$4$4-}do;;;"magic."

Answer 18

Delphi: 329 characters

Single Line Version:

program P;{$APPTYPE CONSOLE}uses SysUtils;const S=65;A='EDDFEEDFFEDGGIIHHJII';B='DGGFFFJGG';function Z(X:Byte):Byte;begin if X<20 then Z:=Ord(A[X+1])-S else Z:=(Ord(B[X DIV 10])-S)+Z(X MOD 10)end;var X,Y:Byte;begin Write('> ');ReadLn(X);repeat Y:=Z(X);WriteLn(Format('%d is %d.',[X,Y]));X:=Y;until X=4;WriteLn('4 is magic.');end.

Formated:

program P;

{$APPTYPE CONSOLE}

uses
  SysUtils;

const
  S = 65;
  A = 'EDDFEEDFFEDGGIIHHJII';
  B = 'DGGFFFJGG';

function Z(X:Byte):Byte;
begin
  if X<20
  then Z := Ord(A[X+1])-S
  else Z := (Ord(B[X DIV 10])-S) + Z(X MOD 10);
end;

var
  X,Y: Byte;

begin
  Write('> ');
  ReadLn(X);

  repeat
    Y:=Z(X);
    WriteLn(Format('%d is %d.' , [X,Y]));
    X:=Y;
  until X=4;

  WriteLn('4 is magic.');
end.

Probably room for some more squeezing... :-P

Answer 19

JavaScript 1.8 (SpiderMonkey) - 153 Chars

l='4335443554366887798866555766'.split('')
for(b=readline();(a=+b)-4;print(a,'is '+b+'.'))b=a<20?l[a]:+l[18+a/10|0]+(a%10&&+l[a%10])
print('4 is magic.')

Usage: echo 42 | js golf.js

Output:

42 is 8.
8 is 5.
5 is 4.
4 is magic.

With bonus - 364 chars

l='zero one two three four five six seven eight nine ten eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen nineteen twenty thirty fourty fifty sixty seventy eighty ninety'.split(' ')
z=function(a)a<20?l[a]:l[18+a/10|0]+(a%10?' '+l[a%10]:'')
for(b=+readline();(a=b)-4;print(z(a),'is '+z(b)+'.'))b=z(a).replace(' ','').length
print('four is magic.')

Output:

ninety nine is ten.
ten is three.
three is five.
five is four.
four is magic.

Answer 20

T-SQL, 413 451 499 chars

CREATE FUNCTION d(@N int) RETURNS int AS BEGIN
Declare @l char(50), @s char(50)
Select @l='0066555766',@s='03354435543668877987'
if @N<20 return 0+substring(@s,@N+1,1) return 0+substring(@l,(@N/10)+1,1) + 0+(substring(@s,@N%10+1,1))END
GO
CREATE proc M(@x int) as BEGIN
WITH r(p,n)AS(SELECT p=@x,n=dbo.d(@x) UNION ALL SELECT p=n,n=dbo.d(n) FROM r where n<>4)Select p,'is',n,'.' from r print '4 is magic.'END

(Not that I'm seriously suggesting you'd do this... really I just wanted to write a CTE)

To use:

M 95

Returns

p                n
----------- ---- -----------
95          is   10.
10          is   3.
3           is   5.
5           is   4.
4 is magic.

Answer 21

C - without number words

180 175^* 172 167 characters

All newlines are for readability and can be removed:

i;V(x){return"\3#,#6$:WOXB79B"[x/2]/(x%2?1:10)%10;}main(c){for(scanf("%d",&c);
c-4;)i=c,printf("%d is %d.\n",i,c=c?c>19?V(c/10+19)+V(c%10):V(c):4);puts(
"4 is magic.");}

Slightly unminified:

i;
V(x){return"\3#,#6$:WOXB79B"[x/2]/(x%2?1:10)%10;}
main(c){
    for(scanf("%d",&c);c-4;)
        i=c,
        printf("%d is %d.\n",i,c=c?c>19?V(c/10+19)+V(c%10):V(c):4);
    puts("4 is magic.");
}

_{^* The previous version missed the mark on two parts of the spec: it didn't handle zero, and it took input on the command line instead of stdin. Handling zero added characters, but using stdin instead of command line args saved even more, resulting in a net savings.}

Answer 22

Perl, about 147 char

Loosely based on Platinum Azure's solution:

               chop
              ($_.=
              <>);@
             u="433
            5443554
           366  887
          798   866
         555    766
        "=~     /\d
       /gx      ;#4
      sub       r{4
     -$_        ?$_
    <20         ?$u
   [$_          ]:(
  $'?           $u[
 $']            :0)
+$u[18+$&]:magic}print"
$_ is ",$_=r(),'.'while
                /\d
                /x;
                444

Answer 23

Python, 129 133 137 148 chars

As a warm-up, here is my first version (improves couple of chars over previous best Python).

PS. After a few redactions now it is about twenty char's shorter:

n=input()
while n-4:p=(922148248>>n/10*3&7)+(632179416>>n%10*3&7)+(737280>>n&1)+4*(n<1);print n,'is %d.'%p;n=p
print'4 is magic.'

Answer 24

while(true)
{
    string a;
    ReadLine(a)
    WriteLine(4);

}

Answer 25

Shameless Perl with Number Words (329 characters)

Adapted fairly directly from P Daddy's C code, with some tweaks to p() to make it do the same thing using Perl primitives instead of C ones, and a mostly-rewritten mainloop. See his for an explanation. Newlines are all optional.

@t=(qw(zero one two three four five six sM eight nine
tL elM twelve NP 4P fifP 6P 7P 8O 9P twLQ NQ forQ fifQ
6Q 7Q 8y 9Q en evL thir eL tO ty 4SmagicT)," is ",".\n");
sub p{local$_=$t[pop];1while s/[0-Z]/$t[-48+ord$&]/e;
print;length}$_=<>;chop;while($_-4){
$_=($_>19?(p($_/10+18),$_&&print("-"),$_%=10)[0]:0)+p$_;
p 35;p$_;p 36}p 34

Side note: it's too bad that perl print just returns true/false; if it returned a count it would save me 7 strokes.

Answer 26

Lua, 176 Characters

o={[0]=4,3,3,5,4,4,3,5,5,4,3,6,6,8,8,7,7,9,8,8}t={3,6,6,5,5,5,7,6,6}n=0+io.read()while n~=4 do a=o[n]or o[n%10]+t[(n-n%10)/10]print(n.." is "..a..".")n=a end print"4 is magic."

or

  o={[0]=4,3,3,5,4,4
  ,3,5,5,4,3,6,6,8,8
  ,7,7,9,8,8}t={3,6,
   6,5,5,5,7,6,6}n=
   0+io.read()while
   n ~= 4 do a= o[n
   ]or o[n%10]+t[(n
   -n%10)/10]print(
n.." is "..a.."." )n=a
end print"4 is magic."

Answer 27

perl, 123 122 characters

Just realized that there is no requirement to output to STDOUT, so output to STDERR instead and knock off another character.

@u='0335443554366887798866555766'=~/./g;$_+=<>;warn"$_ is ",$_=$_-4?$_<20?$u[$_]||4:$u[chop]+$u[$_+18]:magic,".\n"until/g/

And, a version that returns spelled out numbers:

279 278 276 280 characters

@p=(Thir,Four,Fif,Six,Seven,Eigh,Nine);@n=("",One,Two,Three,Four,Five,@p[3..6],Ten,Eleven,Twelve,map$_.teen,@p);s/u//for@m=map$_.ty,Twen,@p;$n[8].=t;sub n{$n=shift;$n?$n<20?$n[$n]:"$m[$n/10-2] $n[$n%10]":Zero}$p+=<>;warnt$m=n($p)," is ",$_=$p-4?n$p=()=$m=~/\w/g:magic,".\n"until/c/

While that meets the spec, it is not 100% well formatted. It returns an extra space after numbers ending in zero. The spec does say:

"I don't care how you separate the word tokens, though they should be separated"

That's kind of weaselly though. A more correct version at

282 281 279 283 characters

@p=(Thir,Four,Fif,Six,Seven,Eigh,Nine);@n=("\x8",One,Two,Three,Four,Five,@p[3..6],Ten,Eleven,Twelve,map$_.teen,@p);s/u//for@m=map$_.ty,Twen,@p;$n[8].=t;sub n{$n=shift;$n?$n<20?$n[$n]:"$m[$n/10-2]-$n[$n%10]":Zero}$p+=<>;warn$m=n($p)," is ",$_=$p-4?n$p=()=$m=~/\w/g:magic,".\n"until/c/