From: Brian Candler <B.Candler@...>
Date: 2004-09-29T03:45:05+09:00
Subject: Re: weird behavior pushing an array onto an array

> > In my program, I fill an array, a, with data and push it onto an array b
> > which is initially empty.
> 
> You push the same array over and over. You want to push a copy instead.
> Use #clone or #dup to get a copy of the Array.

Beware that may not be enough by itself, because the array contains strings;
even if you dup the array you will still have references to the same
strings. This will become apparent if you later modify one of the strings:

  a = ["one","two","three"]
  b = []
  b << a.dup << a.dup
  b[0][0] << "XXX"
  p b
  # => [["oneXXX", "two", "three"], ["oneXXX", "two", "three"]]

But it's not an issue if you only ever replace the array elements with a
reference to another object, e.g.

  b[0][0] = "newstring"

A safer way is to fill the array directly:

  b = []
  5.times do
    b << ["ACINX", "28.53", "15.63", "3.95", "7.76", "1.05", 0]
  end

Or more compactly,

  b = (1..5).collect { ["ACINX", "28.53", "15.63", "3.95", "7.76", "1.05", 0] }

That's because each time a constant like "ACINX" is executed, it creates a
brand new String object; equally, each time [ ... ] is executed it creates a
brand new Array object.

Alternatively, you can do a "deep copy" of the array and its contents. A
quick and fairly convenient way to do this is with Marshal, to serialise the
object and then create a fresh object tree from the serialised form.

  a = ["one","two","three"]
  b = []
  b << Marshal.load(Marshal.dump(a)) << Marshal.load(Marshal.dump(a))
  b[0][0] << "XXX"   
  p b
  # => [["oneXXX", "two", "three"], ["one", "two", "three"]]

Regards,

Brian.