2d array variable overwrite doesn't work in ruby?

Question

I can't seem to be able to override a variable in my ruby code.

i have two 2d arrays called tableand updated_table, updated_table inherits the values of table and that works, but then later in the code, changes are made to the values of the updated_table and when i try to set the updated_table back to the same values(state) of table this doesn't work.

why is that? very simple example of what im trying to do.

class SomeClass

  table = [[0,20,5,1000,1000], [20,0,10,8,20], [5,10,0,1000,40], [1000,8,1000,0,6], [1000,20,40,6,0]]
  updated_table = table
   ##
   then here i have some code that makes changes to the values in the updated_table
   ##  
   2.times do
     updated_table = table # this dosent work??
     ## do some more calculations and so on ##
   end
end

Why are you doing it twice? (Also you can just do 2.times) — Andrew Marshall
– Andrew Marshall, Commented Mar 5, 2011 at 3:05
this is just an example, there is logic in the actual code that determines how many times this loops and calculations are made to that table every time. — Mo.
– Mo., Commented Mar 5, 2011 at 3:07

AJM · Accepted Answer · 2011-03-05 03:57:20Z

2

You made updated_table a reference to table when you called updated_table = table. They both point to the exact same table. When you modified updated_table you actually updated table at the same time.

I believe you want updated_table = table.dup.

answered Mar 5, 2011 at 3:57

AJM

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Comments

Matheus Moreira · Accepted Answer · 2011-03-05 03:59:08Z

2

In Ruby, variables do not contain the object itself. They are said to point to or reference the object in memory. So, when you write updated_table = table, you are not making a copy of the object, you are just creating another reference to that object.

You can create a separate although identical object with the Object#dup method.

updated_table = table.dup

This is a shallow copy, due to the fact that the array holds references to objects, and these objects are not duplicated themselves. This means that both the original array and the duplicate effectively reference the same elements, so keep this in mind if you need to modify objects stored inside either array.

answered Mar 5, 2011 at 3:59

Matheus Moreira

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Comments

bluekeys · Accepted Answer · 2011-03-05 11:29:03Z

1

Will deep copying achieve your intended result?

I think the reason your code wasn't working as expected was each time you made a change to updated table, it was automatically changing the original because of how you'd copied (referenced) it in the first place.

I don't think object.dup will work as you want, because your array is 2 dimensional. See here - http://ruby.about.com/od/advancedruby/a/deepcopy.htm - for good further reading (with array based examples) on the subject of object.dup, clone and marshalling to find out why.

I've just simply added a snippet to deep copy the table rather than cloning it.

# Deep copy - http://www.ruby-forum.com/topic/121005
# Thanks to Wayne E. Seguin.
class Object
   def deep_copy( object )
     Marshal.load( Marshal.dump( object ) )
   end
end

table = [[0,20,5,1000,1000], [20,0,10,8,20], [5,10,0,1000,40], [1000,8,1000,0,6],     [1000,20,40,6,0]]

updated_table = deep_copy(table) # -- DON'T CLONE TABLE, DEEP COPY INSTEAD --

##
# then here i have some code that makes changes to the values in the updated_table
##  
2.times do
    updated_table = deep_copy(table) # -- USE DEEP_COPY HERE TOO --
    ## do some more calculations and so on ##
end

edited Mar 5, 2011 at 11:29

answered Mar 5, 2011 at 11:12

bluekeys

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3 Comments

Mo. Over a year ago

hi and thanks for your answer, would i have to use that every time i want to overwrite a variables content?

bluekeys Over a year ago

@Mo, No. You can make changes to individual elements as usual e.g. updated_table[0][0] = 100. only deep copy when you want to 'duplicate' the contents of table.

Mo. Over a year ago

you were right, the .dup didn't work. thanks, i was trying to debug that problem for about 2 hours.

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