How to duplicate elements in array?

[Chris Chris]

Hi,

something I couldn't quite figure out (sorry for posting so soon after
my last question)...

a = [["ABC", "30", "1"]]

I want to duplicate/copy the element. This is what I'm trying to get:

a2 = [["ABC", "30", "1"], ["ABC", "30", "1"]]

What I did was simply

a2 = a + a

=> [["ABC", "30", "1"], ["ABC", "30", "1"]]

That works.

I then tried changing a single element.

a2[0][1] = "test"

puts a2.inspect
=> [["ABC", "test", "1"], ["ABC", "test", "1"]]

Of course, I just intended to change the first value, not both of them.


Can anybody explain please what I can do about this?

Thank you!

Chris

 

[phlip]

a = [["ABC", "30", "1"]]

a2 = a + a

a2[0][1] = "test"

=> [["ABC", "test", "1"], ["ABC", "test", "1"]]

a is an object, not a value. a + a sez "make an array containing two links to
this object". Changing one object changes the other.

You need a2 = a.dup + a.clone

I forget the difference between .dup and .clone, though... (-:

 

[Marc Heiler]

a2 = a.dup + a.clone

That gives the same result as his example.

Dunno if you can understand german.. at least the examples are in
english, maybe you understand what is going on despite being unable to
understand anything

http://forum.ruby-portal.de/viewtopic.php?t=1214

(Hopefully that link works)

 

[Leslie Viljoen]

Hi,

something I couldn't quite figure out (sorry for posting so soon after
my last question)...

a = [["ABC", "30", "1"]]

I want to duplicate/copy the element. This is what I'm trying to get:

a2 = [["ABC", "30", "1"], ["ABC", "30", "1"]]

What I did was simply

a2 = a + a

=> [["ABC", "30", "1"], ["ABC", "30", "1"]]

That works.

I then tried changing a single element.

a2[0][1] = "test"

puts a2.inspect
=> [["ABC", "test", "1"], ["ABC", "test", "1"]]

Of course, I just intended to change the first value, not both of them.


Can anybody explain please what I can do about this?

You need to use .dup:

a = ["ABC", "30", "1"] => ["ABC", "30", "1"]
b = a.dup => ["ABC", "30", "1"]
b[0] = "DEF" => "DEF"
a => ["ABC", "30", "1"]
b

=> ["DEF", "30", "1"]

If you just assign an object, you get another reference to the same
object. DUP will give you a copy, although not a deep copy. That means
that if

a = [["ABC", "30", "1"]], a.dup will not give you a copy of the
element within the array - it will give you a copy of the array with
the inner element being a reference again:

a = [["ABC", "30", "1"]] => [["ABC", "30", "1"]]
b = a.dup => [["ABC", "30", "1"]]
b[0][0] = "DEF" => "DEF"
a => [["DEF", "30", "1"]]
b

=> [["DEF", "30", "1"]]

The easy way to do deep copies is to use Marshal.

Les

 

[phlip]

a2 = a.dup + a.clone

That gives the same result as his example.

Indeed - it only cloned the top level of a, which is an array. The rest remained
a linked-too object.

How to deep clone?

Dunno if you can understand german.. at least the examples are in
english, maybe you understand what is going on despite being unable to
understand anything

http://forum.ruby-portal.de/viewtopic.php?t=1214

Or try this savage hack:

a2 = eval(a.inspect) + a

(-:

 

[Chris Chris]

a = [["ABC", "30", "1"]]

a2 = a + a

a2[0][1] = "test"

=> [["ABC", "test", "1"], ["ABC", "test", "1"]]

Thanks guys. Yeah, I speak German ... Going through that discussion
solved it for me after tweaking it a little bit...

Changed
a = [["ABC", "30", "1"]] TO a = ["ABC", "30", "1"]

a2 = [a.dup, a.dup]

a2[0][1] = "test"

=> [["ABC", "test", "1"], ["ABC", "30", "1"]]

Chris

 

[David A. Black]

Hi --

Indeed - it only cloned the top level of a, which is an array. The rest
remained a linked-too object.

How to deep clone?

The most common idiom for deep copying is:

copy = Marshal.load(Marshal.dump(original))


David