The other days somebody asked on IRC for help with this question:

He was looking for all 9 digit numbers that didn't contain a zero digit anywhere, and the first digit should be divisible by one, the number formed from the first two digits should be divisible by 2, the number formed from the first three digits should be divisible by 3 etc.

The first such number is 1232525616, because 1 can be divided by 1, 12 can be divided by 2, 123 can be divided by 3 etc.

Since finding one isn't really a challenge, let's focus on finding them all. The easiest approach is, of course, brute force. There are 109 numbers with 9 digits, that's quite a manageable number for a modern computer. On my laptop it takes perl about 45 seconds to loop over all 109 numbers, doing nothing else.

So all you've got to do is iterate over all numbers, test if they meet the criterion stated above, and print them out if they do:

use strict ; use warnings ; sub test { my $z = shift ; for ( 2..9 ) { return if ( substr ( $z , 0 , $_ ) % $_ != 0 ); } return 1 ; } for ( my $i = 1 x 9 ; length ( $i ) == 9 ; $i += 9 ) { next if $i =~ / 0 / ; if (test( $i )) { print $i , $/ ; } }

This uses a small trick: the first number that doesn't contain any zero is 111111111 , or short 1 x 9 .

It takes 4 minutes and 20 seconds, produces the right answer, and we're happy.

But it wastes a lot of resources, and wouldn't scale for larger numbers. So for the sake of fun I tried a few different optimizations.

The first one is quite simple: the second digit must always be even, otherwise the number consisting of the first two digits could not be even. Likewise the fifths digit must be 5 or 0 to ensure that it can be divided by 5. Since 0 is forbidden anyway, it has to be 5. So let's skip the expensive check if those conditions aren't fulfilled:

for ( my $i = 1 x 9 ; length ( $i ) == 9 ; $i += 9 ) { next if $i =~ / 0 / ; next if $i =~ / ^ .[13579] / ; next unless $i =~ / ^ .... 5 / ; if (test( $i )) { print $i , $/ ; } }

This speeds up the computation to roughly a minute. Since 40 seconds are minimum for the iteration alone, it's nearly as good as it gets with this approach.

But of course there's still room for improvement: when the second digit is odd, the program iterates over a hundred million of numbers without finding one. Instead of skipping them each time, it would be much more efficient to generate the number digit by digit, checking divisibility at each step of the way.

for my $a ( 1..9 ) { for my $b ( 2 , 4 , 6 , 8 ) { for my $c ( 1..9 ) { next if ( $a + $b + $c ) % 3 ; for my $d ( 2 , 4 , 6 , 8 ) { next if ( 10 * $c + $d ) % 4 ; my $e = 5 ; for my $f ( 2 , 4 , 6 , 8 ) { next if ( $a + $b + $c + $d + $e + $f ) % 3 ; for my $g ( 1..9 ) { my $so_far = " $a$b$c$d$e$f$g " ; next if $so_far % 7 ; for my $h ( 2 , 4 , 6 , 8 ) { next if ( $so_far . $h ) % 8 ; for my $i ( 1..9 ) { my $so_far = $so_far . " $h$i " ; next if $so_far % 9 ; print " $so_far$h

" ; } } } } } } } }

Wow, that's ugly code, but it works and it's fast. Very fast. 27 milliseconds, or more than 2000 times faster as the previous version.

Bug it contains lots of duplicated code, and again it wouldn't scale for finding larger numbers, this time because for digit a loop needs to be written.

Whenever I find myself repeating some piece of code a few times, but in slightly different forms, I try to pack the code into a data structure instead.

For each digit position there needs to a list of digit to try, and a test that determines if the newly added digit violates its divisibility rule.

use strict ; use warnings ; my @config = ( [[ 1..9 ], sub { 0 }], [[ 2 , 4 , 6 , 8 ], sub { 0 }], [[ 1..9 ], sub { $_[ 0 ] % 3 }], [[ 2 , 4 , 6 , 8 ], sub { $_[ 0 ] % 4 }], [[ 5 ], sub { 0 }], [[ 2 , 4 , 6 , 8 ], sub { $_[ 0 ] % 6 }], [[ 1..9 ], sub { $_[ 0 ] % 7 }], [[ 2 , 4 , 6 , 8 ], sub { $_[ 0 ] % 8 }], [[ 1..9 ], sub { $_[ 0 ] % 9 }], );

The nested loops from the previous script can be emulated by recursion, passing the previous digits along (and the configuration for all digit positions that still need to be tested).

sub f { my ( $so_far , $config ) = @_ ; $config = [ @$config ]; if (! @$config ) { print " $so_far

" ; } else { my $c = shift @$config ; for my $current ( @{$c->[ 0 ]} ) { next if $c->[ 1 ] ->( $so_far . $current ); f( $so_far . $current , $config ); } } } f( '' , \ @config );

With 46ms runtime it's still acceptably fast, and much leaner than the nested loops.

Actually a rough sketch of the configuration table can also be generated automatically, and then optimized manually:

my @config ; for my $n ( 1..9 ) { push @config , [ ( $n % 2 ? [ 1..9 ] : [ 2 , 4 , 6 , 8 ]), sub { $_[ 0 ] % $n }, ] } $config[ 4 ] = [[ 5 ], sub { 0 } ];

Now the script contains as little duplication as possible, is reasonably fast, and I'm happy.

(Just one final note: there's no scalability problem in this particular task, because it can't be extended to more than nine digits: if no zero is allowed, there won't be any numbers divisible by 10.)