From: dagbrown@... (Dave Brown)
Date: 2004-04-07T14:39:20+09:00
Subject: Re: deciding between ruby and python

In article <jtt1709r6qpjqoft5gtl6gooeb34rnuevt@4ax.com>,
gabriele renzi  <surrender_it@remove.yahoo.it> wrote:
: In ruby you're forced to build a huge array any time you use zip, whle
: in python you can delay the creation of the yielded pair till they're
: actually useful. 

Nope.

    irb(main):001:0> a=[1,2,3,4,5]
    => [1, 2, 3, 4, 5]
    irb(main):002:0> b=[6,7,8,9,10]
    => [6, 7, 8, 9, 10]
    irb(main):003:0> a.zip(b)
    => [[1, 6], [2, 7], [3, 8], [4, 9], [5, 10]]
    irb(main):004:0> a.zip(b) { |x| p x }
    [1, 6]
    [2, 7]
    [3, 8]
    [4, 9]
    [5, 10]
    => nil

It's those blocks and yield again!  Aren't they great?  Stick a block
after a zip and instead of building a great big array, it yields pairs
for you to do what you want with.

If you want generators in ruby, just define an "each" method in your
class.  Remember that

    for i in STUFF
        # do things
    end

is just syntactical sugar for

    STUFF.each do 
        |i|
        # do things
    end

Here's the standard silly example: the Fibonacci sequence generator.

    class Fib
        def fib(x)
            @@fibs ||= {};
            if x <= 0 then return 1 end;
            return @@fibs[x] ||= fib(x - 2) + fib(x - 1)
        end

        # I didn't exactly put much thought into this method, and it shows
        def each(limit=10000,&block)
            counter=1
            while true
                r=fib(counter)
                break if r>limit
                yield r
                counter+=1
            end
        end
    end

    f=Fib.new

    # Ruby with a Python accent. "for", a generator, a colon, and the %
    # string operator!
    for i in f: 
        puts "%d" % i
    end

--Dave