Classification

Ruby
1

I have to classify a bunch of products into 4 boxes, according to the
value of one of their attribute (product.code , an integer between 0
and 4)

I wrote that but it’s seems wrong :

box[0] = box[1] = box[2] = box[3] = []

for product in products
tag = Tag.new(product.title, product.price)
box[ product.code ] << tag
end

then I’ll reuse each box[] for further processing…

what’s wrong ?

tfyl

joss

2

Alle lunedì 29 gennaio 2007, Josselin ha scritto:

box[ product.code ] << tag
end

then I’ll reuse each box[] for further processing…

what’s wrong ?

tfyl

joss

The problem is that writing something like

a = b = c = []

will store the same array in all the variables. If you use the object_id
methods on the four elements of box, you’ll see that the returned value
is
the same. This means that modifiying, for instance box[0] will also
modify
box[1], box[2] and box[3], because they all contain the same object. You
should replace the line

box[0] = box[1] = box[2] = box[3] = []

with, for example,

box=Array.new(4){[]}

This will create a new Array with four elements, each of which is a
different
Array. Note that writing box=Array.new(4,[]) will produce the same
(wrong)
result as your code.

I hope this helps

Stefano

3

On 2007-01-29 18:18:31 +0100, Stefano C. [email protected]
said:

tag = Tag.new(product.title, product.price)

should replace the line
)
result as your code.

I hope this helps

Stefano

thansk a lot … it’s so DRY…

btw : I tried also

    box = []
    0.upto(4) do |i|
      box[i] = []
    end
4

On 29.01.2007 19:20, Josselin wrote:

box[0] = box[1] = box[2] = box[3] = []
tfyl

box=Array.new(4){[]}
Stefano

thansk a lot … it’s so DRY…

btw : I tried also

   box = []
   0.upto(4) do |i|
     box[i] = []
   end

Much more convenient is this:

box = Hash.new {|h,k| h[k] = []}

Now you can just do

for product in products
tag = Tag.new(product.title, product.price)
box[ product.code ] << tag
end

Kind regards

robert