Tuesday, April 16, 2013

Learning Perl5i #2

Continuing the exploration of Perl5i beyond part one, I decided to treat perl5i as a language and use it to implement tasks at Rosetta Code.

Since Moose is a complete OO solution, I use a lightweight approach to OO, where used.

The first RC task I implemented is 100 doors: see the full implementation.

In terms of documentation, I gave each routine a small header in which I show how the routine is invoked, specify the inputs and outputs, and summarize the primary actions of the routine.

Since the task is defined in terms of a number of doors, and operations on those doors, it seemed appropriate to use an object. "some object" + "operations on that object" ==> "segregate into an OO implementation".

I used a simple constructor with initialization performed in _init() to simplify subclassing. The constructor doesn't need to be modified, just a new _init() written to handle additional attributes or arguments, if any. The new implementation can handle the duties of the original routine, or better, invoke the original as $self->SUPER::_init( @_ ).

The initializer stores how many doors we are dealing with, then then creates an array of doors. We are interested in doors 1..N, but we actually create one more, 0..N, since that's the way Perl arrays work. We'll simply ignore door zero, it is permanently closed. One subtle detail about the x multiplier: $CLOSED x $N is 0000..., string repetition; list repetition uses parentheses: ($CLOSED) x $N to generate ( 0, 0, 0, ... ).

The instructions say there are N passes, in which you toggle every door, every other door, every Nth door. I created toggle() to toggle a single door, toggle_n() to process a single pass, and toggle_all() to handle all the passes.

Besides eliminating duplication and excessive typing, one advantage of the ternary operator ( ? : ) over an if-block is that it is clear a single l-value is the target.

Simple statement modifier loops like these are, I believe, quicker than more general loops, and they offer simpler optimization in the future into parallel processing, but they also communicate that we are dealing with a set of data. Too often, people coming to Perl from C and Java think in terms of indices: I get the first index; I get the first element in the array; I do something ... I prefer to think in terms of sets, like a Unix pipeline: Take this array of elements, do something to them, ...

Note in toggle_n(), that $n * int( N / $n ) is the largest multiple of $n les than N. Taking the sequence 1.. int($self->{N} / $n ) and multiplying each element by $n produces $n, 2 * $n, 3 * $n, ... $n * int( N / $n ), the list of doors to toggle on that pass.

The final requirement is to print which doors are open.  grep() takes a code block and a list of values. Values are passed if they generate a true value in the block, and barred if they generate a false value.

Putting the routines together into a demonstration of the 100 door task becomes a simple three-line sequence.

1 comment:

Darren Duncan said...

It seems to me that your perl5i example may have been counter-productive here. Doing that closed doors problem looks much more verbose and complicated in your example than in in any of the other languages nearby on Rosetta code, including vanilla Perl 5, nevermind Perl 6. Using objects you define yourself is the wrong way to attack this problem. Just use plain Perl numbers and arrays.