Pushing a string into an array within an array yields unexpected result

Question

I have an array in Ruby that consists of 5 empty arrays. I am trying to use the << operator to push a string into the first array, but the result is that the string gets pushed into ALL of the arrays. Please help me understand this.

The expected output is:

# => [["car"], [], [], [], []]

but instead I get:

# => [["car"], ["car"], ["car"], ["car"], ["car"]]

irb dump:

1.9.3-p194 :001 > output = Array.new(5, [])
 => [[], [], [], [], []] 
1.9.3-p194 :002 > output.inspect
 => "[[], [], [], [], []]" 
1.9.3-p194 :003 > output[0].inspect
 => "[]" 
1.9.3-p194 :004 > output[0] << "car"
 => ["car"] 
1.9.3-p194 :005 > output.inspect
 => "[[\"car\"], [\"car\"], [\"car\"], [\"car\"], [\"car\"]]" 

Answer 1

They're all the same object:

ree-1.8.7-2012.02 :001 > output = Array.new(5, [])
 => [[], [], [], [], []] 
ree-1.8.7-2012.02 :002 > output[0]
 => [] 
ree-1.8.7-2012.02 :003 > output[0].object_id
 => 2219989240 
ree-1.8.7-2012.02 :004 > output[1].object_id
 => 2219989240 
ree-1.8.7-2012.02 :005 > output[2].object_id
 => 2219989240 
ree-1.8.7-2012.02 :006 > output[3].object_id
 => 2219989240 
ree-1.8.7-2012.02 :007 > output[4].object_id
 => 2219989240 
ree-1.8.7-2012.02 :008 > 

Try this:

ree-1.8.7-2012.02 :008 > output = []
 => [] 
ree-1.8.7-2012.02 :009 > 5.times{output << []}
 => 5 

Answer 2

They all have the same object id, as pointed out by Pedro Nascimento, if the arrays are initialized in that manner. You can get around this by using a similar syntax to create your nested arrays:

irb(main):047:0> output = Array.new(5) {[]}
=> [[], [], [], [], []]
irb(main):048:0> output.each {|i| puts i.object_id}
10941700
10941680
10941660
10941640
10941620

So your append to output[0] will work as expected:

irb(main):049:0> output[0] << "cat"
=> ["cat"]
irb(main):050:0> output
=> [["cat"], [], [], [], []]