I don’t really have a better name for this. It’s also not completely clean, but it works. I had, almost a year ago (362 days ago), written a blog post about lexicographic permutations. That was about permutations of elements within one array.
Someone on ruby-forum asked about permutations between multiple arrays. I found something in C#, which I was happy to transcribe to Ruby and tweak a little.

```def array_permutations array, index=0
# index is 0 by default : start at the beginning, more elegant.
return array[-1] if index == array.size - 1 # Return last element if at end.
result = []
array[index].each do |element| # For each array
array_permutations(array, index + 1).each do |x| # Permute permute permute
result << "#{element}, #{x}"
end
end
return result
end```

So, we get this:

```first = ['one', 'two']
second = ['three', 'four']
third = 'five', 'six']
result = array_permutations [first, second, third]
=> ["one, three, five", "one, three, six", "one, four, five", "one, four, six", "two, three, five", "two, thre
e, six", "two, four, five", "two, four, six"]```

Magic!

——
Edit – of course, my solution is hackish, and someone came up with a quicker and more elegant solution:

```def fancy_array_permutation array
return array[0] if array.size == 1
first = array.shift
return first.product( fancy_array_permutation(array) ).map {|x| x.flatten.join(" ")}
end```

This gives the same result as above.

I came across this beauty:

```def same_modality? list
check = list[0][0]
list.each do |i|
return false if check != i[0]
end
return true
end```

This was “necessary” because I got an array of one-element arrays back, and I wanted to check whether or not that one element was the same across the array.

Three seconds of thinking made me realize that just maybe, I could do this:

`list.uniq.size == 1`

I -like- Ruby.

An auto-vivifying hash is a hash that lets you create sub-hashes automatically. This means that the following code becomes possible:

```def cnh # silly name "create nested hash"
Hash.new {|h,k| h[k] = Hash.new(&h.default_proc)}
end
my_hash = cnh
my_hash[1][2][3] = 4
my_hash # => { 1 => { 2 => { 3 =>4 } } }
```

This is useful because it reduces the amount of logic in the code. No more “If sub-hash doesn’t exist, create it!”

Courtesy of Robert Klemme on www.ruby-forum.com

_____

Update:

Facets has a way of doing an auto-vivifying hash, too! Thanks to Stackoverflow.

```# Monkey patching Hash class:
# File lib/core/facets/hash/autonew.rb, line 19
def self.autonew(*args)
leet = lambda { |hsh, key| hsh[key] = new( &leet ) }
new(*args,&leet)
end
```

The standard new method for Hash accepts a block. This block is called in the event of trying to access a key in the Hash which does not exist. The block is passed the Hash itself and the key that was requested (the two parameters) and should return the value that should be returned for the requested key.

You will notice that the `leet` lambda does 2 things. It returns a new Hash with `leet` itself as the block for handling defaults. This is the behaviour which allows `autonew` to work for Hashes of arbitrary depth. It also assigns this new Hash to `hsh[key]` so that next time you request the same key you will get the existing Hash rather than a new one being created.

Sweet deal!

I found a script that explains everything really well.
Credit goes to:
# CLOSURES IN RUBY Paul Cantrell http://innig.net
# Email: username “cantrell”, domain name “pobox.com”
Along with this blog entry, it’s made the whole deal much easier to figure out.

Now.. Why didn’t I get that when I was looking at LISP?

Wow. I just watched Dave Thomas’ talk Ruby Metaprogramming: Extending Ruby for Fun and Profit and it explained so many things.

It’s quite worth watching if you like Ruby and don’t know about metaprogramming and Ruby hooks and what ‘self’ means, fully.

Someday I’ll come out with a proper revision numbering scheme for it.
I’ve implemented some Cucumber testing, the code now actually parses SGF files properly, including the KGS ones, it’ll work on Ruby 1.8.6, 1.8.7, 1.9.1, so .. It’s all good on that front.

The code’s not as clean as I’d like it to be, and probably not as commented, either.
http://github.com/Trevoke/SGFParser

I am not -saving- SGF files with it yet, but heck if I’m not parsing them. And now YOU can, too !

If you get this error : Mysql::Error: query: not connected: or one just like it, it’s because you need an older DLL.. Which I just happen to have found, because I needed to fix this problem too!
libmySQL.dll

Enjoy.

So, I play weiqi (known as the game of go, but I prefer using the chinese name).
There are some SGF parsers out there for Ruby, but I wanted to write my own – the other ones didn’t seem comprehensive enough, didn’t seem Ruby-like enough in their objectifiying the women data.
My original goal was twofold: write a script to rename all the SGF files in my collection so they can be easily parsed later, and … Learn more Ruby!
Here is the github link: http://github.com/Trevoke/SGFParser
It’s a work in progress. Right now it parses the SGF file properly, but the user has a fair amount of work to do to use the data.. Which, maybe, is the way it needs to be. But I’m gonna try to follow the principle of least surprise to the bitter end! ðŸ™‚