From: Peter Vandenabeele <peter@...>
Date: 2012-02-08T07:22:52+09:00
Subject: Re: Appending to an Array

--485b397dd2fd2be06d04b86738b1
Content-Type: text/plain; charset=UTF-8

On Tue, Feb 7, 2012 at 11:04 PM, Robert Klemme
<shortcutter@googlemail.com>wrote:

> On Tue, Feb 7, 2012 at 10:27 PM, Anthony Simonelli
> <asimonelli01@gmail.com> wrote:
> > I am querying a database using DBI and ODBC.  After I run the execute, I
> > want to grab each row as an array, and add it to another array,
> essentially
> > creating an Array of Arrays.  I've tried this two ways with the same
> result:
> >
> > sth = dbh.execute("SELECT * FROM SOMETABLE")
> > rows = sth.fetch_all
> >
> > and
> >
> > rows = sth.fetch do |row|
> >    rows << row
> > end
> >
> > The result for both is that the last row is repeated in the array however
> > many times there are rows.  Why isn't it appending correctly?
>
> Probably because the DB driver optimizes object allocation and reuses
> the same instance.  Then this is what happens:
>
> irb(main):006:0> rows = []
> => []
> irb(main):007:0> a = %w{a b c d}
> => ["a", "b", "c", "d"]
> irb(main):008:0> rows << a
> => [["a", "b", "c", "d"]]
> irb(main):009:0> a[2] = 99
> => 99
> irb(main):010:0> rows << a
> => [["a", "b", 99, "d"], ["a", "b", 99, "d"]]
>
> The phenomenon is called "aliasing".
>
> You can solve it by copying row Arrays.
>
> rows = []
>
> sth.fetch do |row|
>   rows << row.to_a # if rows is not an Array
>  rows << row.dup # if rows is an Array
> end
>

Would `Array(row)` not be a clean implementation of this ?

peterv@ASUS:~$ irb
001:0> row = "a string"
=> "a string"
002:0> Array(row)
=> ["a string"]
003:0> row = [:array_entry]
=> [:array_entry]
004:0> Array(row)
=> [:array_entry]

Peter

--485b397dd2fd2be06d04b86738b1--