Appending to an Array

ruby-talk
1

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?

Thanks,

Anthony

2

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

Kind regards

robert

···

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?

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

3

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

···

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

4

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

···

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]

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/