From: Ara.T.Howard@...
Date: 2004-10-13T17:24:34+09:00
Subject: Re: The Challenge of R

On Wed, 13 Oct 2004, trans.  (T. Onoma) wrote:

> Here's a coding challenge you.
>
> The R language has the following nifty features for working with arrays. To
> what degree can you get Ruby to do the same?
>
> The solution that is closest/nicest gets forever embedded in my growing
> library of Ruby enhancements, and it's author's name wrapped in shiny
> asterisks and number signs! Joy, joy! :)
>
> Listing 1. Elementwise operations in R
>
>> a = 1:10            # Range of numbers 1 through 10
>> b = a+5             # Add 5 to each element
>> b                   # Show b vector
> [1]  6  7  8  9 10 11 12 13 14 15
>
> Or you can operate selectively only on elements with certain indices, by using
> an "index array":
>
>
> Listing 2. Using index arrays to select elements
>
>> c = b               # Make a (lazy) copy of b
>> pos = c(1,2,3,5,7)  # Change the prime indices
>> c[pos] = c[pos]*10  # Reassign to indexed positions
>> c                   # Show c vector
> [1]  60  70  80   9 100  11 120  13  14  15
>
> Or, maybe best of all, you can use a syntax much akin to list comprehensions
> in Haskell or Python, and only operate on elements that have a desired
> property:
>
>
> Listing 3. Using predicates to select elements
>
>> d = c
>> d[d %% 2 == 0] = -1 # Reassign -1 to all even elements
>> d
> [1] -1 -1 -1  9 -1 11 -1 13 -1 15

i did not write it, but narray does all that you want, and more, including
being extremely fast, doing simply display of data in X, and working
seamlessly with memory mapping:

   jib:~ > cat a.rb
   require 'narray'

   a = NArray[1..10]
   b = a + 5
   p b

   c = b
   pos = 0,1,2,4,6           # adjust R's 1 based idx to ruby's 0 based
   c[pos] = c[pos] * 10
   p c

   d = c
   d[(d % 2).eq(0).where] = -1
   p d

   jib:~ > ruby a.rb
   NArray.int(10):
   [ 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 ]
   NArray.int(10):
   [ 60, 70, 80, 9, 100, 11, 120, 13, 14, 15 ]
   NArray.int(10):
   [ -1, -1, -1, 9, -1, 11, -1, 13, -1, 15 ]

you can thank Masahiro Tanaka for such a great work.

http://www.ir.isas.ac.jp/~masa/ruby/index-e.html

kind regards.

-a
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