From: Robert Klemme <shortcutter@...>
Date: 2012-02-08T17:52:10+09:00
Subject: Re: Appending to an Array

On Tue, Feb 7, 2012 at 11:22 PM, Peter Vandenabeele
<peter@vandenabeele.com> wrote:
> 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]

Judge for yourself:

irb(main):001:0> a = [:x]
=> [:x]
irb(main):002:0> b = Array(a)
=> [:x]
irb(main):003:0> b.equal? a
=> true
irb(main):004:0> a.object_id == b.object_id
=> true

Kind regards

robert


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