mhx
Enthusiast
/ Moderator
Oct 26, 2001, 2:44 PM
Post #5 of 10
(61187 views)
|
Jim, I'm really sorry if my solution sickens you. I didn't want that.
So here's my explanation for the code, I hope that makes it a bit clearer. First of all, I've added some line breaks and whitespace to the code to make it a bit more readable:
sub _
{
my $a = shift;
local $_ = shift;
defined || return print "$a\n";
$_ < 2 ? _( $a.$_, @_ )
: do {
$_=3*$_+91;
_( $a.chr, @_ ) for $_..$_+2
}
}
_( '', <> =~ /(\d)/g )
sub phone
{
my $a = shift;
local $_ = shift;
defined || return print "$a\n";
if( $_ < 2 ) {
phone( $a.$_, @_ )
}
else {
$_=3*$_+91;
phone( $a.chr, @_ ) for $_..$_+2
}
}
phone( '', <> =~ /(\d)/g )
The heart of that algorithm is obviously the phone() subroutine. In that subroutine, $a is always the string that is assembled to contain the phonobulized version of the phone number. In the first call, $a will be the empty string and $_ will be the first of the digits:
my $a = shift;
local $_ = shift;
if( $_ < 2 ) {
phone( $a.$_, @_ )
}
else {
$_=3*$_+91;
phone( $a.chr, @_ ) for $_..$_+2
}
Now, when does the recursion stop? As we've seen, with each level of recursion there's one digit shifted off the list. So at a certain level, there's no more digits and thus $_ will become undef. That's when the following line becomes interesting:
defined || return print "$a\n";
If you're familiar with recursion, this shouln't be too hard to understand. If you're not it may be, uh, quite hard.
To answer your other questions: I read your post shortly after I got up. Then I went shopping with my mom and that's where I made up most of the algorithm. It was clear to me from the beginning that it had to be recursive. I also had the idea of using $_=3*$_+91 to get the letters corresponding to a digit while being in town. Back home, coding those ideas took about 15 minutes and compressing the code took another 15 minutes. So, considering that I spent a total of 30 minutes thinking about the solution, you can say that it took me about an hour to come up with the above code.
I'm pretty sure there's still room for improvements. I'd guess about 120 characters should be possible. However, I hope you find the above explanations helpful and I also hope you won't be crying to much as you'll see that there's really nothing special about the code.
-- Marcus
s$$ab21b8d15c3d97bd6317286d$;$"=547269736;split'i',join$,,map{chr(($*+=
($">>=1)&1?-hex:hex)+0140)}/./g;$"=chr$";s;.;\u$&;for@_[0,2];print"@_," |
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