From: Brian Candler <lists@...>
Date: 2012-08-31T20:53:35+09:00
Subject: Re: Load an array of 10 ratings by position

Joao Silva wrote in post #1073988:
> for i in 4..n-1
>     aux[i]=rating[i]
> end

Where does '4' come from? You are iterating over the whole array later, 
so you need to copy the whole array. Just changing this to '0' makes 
your program work.

Of course there are simpler ways:

    aux = rating.reverse

would do the same.

> puts "#{rating}"
> puts "#{result}"

You don't need the string interpolation, puts calls to_s on the object 
automatically.

In any case I would do:

puts rating.inspect
puts result.inspect

because in ruby 1.8, Array#to_s joins the elements together with no 
separator.

As you develop in familiarity, you'll find that "for" loops are almost 
never needed in ruby. For low-level iteration you can use each, or 
each_with_index if you need the index for some reason:

  rating.each_with_index do |elem,i|
    result[i] = elem + aux[i]
  end

But then there are other iterators which wrap up common patterns, such 
as building a new array from the results of applying a function to the 
elements of another array (map/collect).

  arr2 = arr1.map { |x| x*2 }

So we don't need to assign to result[i] explicitly:

  result = rating.each_with_index.map { |e,i| e + aux[i] }

And as someone else has shown, given that you reversed the elements into 
an aux array, you can zip the elements together and iterate over them 
together without using an index.

  result = rating.zip(rating.reverse).map { |a,b| a+b }

You'll get more comfortable with these over time :-)

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