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\"]]"

possible duplicate of Why doesn't terse way of defining new hashes in Ruby work (they all refer to same object) — the Tin Man
– the Tin Man, Commented May 31, 2012 at 4:45
Thanks all! This was REALLY stumping me. And now I learned a new method: object_id — will_d
– will_d, Commented May 31, 2012 at 5:06
Can anyone help me understand why this is happening, so I can understand Ruby better? — will_d
– will_d, Commented May 31, 2012 at 5:09
You are creating five instances of the same [] array, which is why you see duplicates. — the Tin Man
– the Tin Man, Commented May 31, 2012 at 5:16

Pedro Nascimento · Accepted Answer · 2012-05-31 03:54:56Z

6

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

answered May 31, 2012 at 3:54

Pedro Nascimento

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Comments

Makoto · Accepted Answer · 2012-05-31 04:04:31Z

5

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"], [], [], [], []]

answered May 31, 2012 at 4:04

Makoto

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Comments

Shamith c · Accepted Answer · 2012-05-31 04:30:53Z

1

The code

output = Array.new(5, [])

is trying to create only one copy of the object,

So

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

If you want to create multiple copies of object, use this

     output = Array.new(5) {[]}    #=> [[], [], [], [], []]

The code

 output.each {|i| puts i.object_id}

will show you

1.9.3-p194 :005 >  output.each {|i| puts i.object_id}
13417360
13417340
13417320
13417300
13417280
 => [[], [], [], [], []]

answered May 31, 2012 at 4:30

Shamith c

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