How to Create an Array in Ruby with Default Values

Can I create an array in Ruby with default values?

Given that Ruby returns nil for a non-existing element (as opposed to index-out-of-bounds type error), you could just use an "or":

a = [1,2,3]
puts a[5]  # => nil
puts a[5] || "a default"  # => a default

You could take the monkey patch approach, but you probably would not want to do this in anything larger than a 1-file script:

a = [1,2,3]
def a.[](index)
  self.at(index) || "a default"
end
puts a[5]   # => "a default"

Creating a Hash with values as arrays and default value as empty array

Lakshmi is right. When you created the Hash using Hash.new([]), you created one array object.

Hence, the same array is returned for every missing key in the Hash.

That is why, if the shared array is edited, the change is reflected across all the missing keys.

Using:

Hash.new { |h, k| h[k] = [] }

Creates and assigns a new array for each missing key in the Hash, so that it is a unique object.

Create an array with a fixed size, and fill default content with another array?

def fixed_array(size, other)  
   Array.new(size) { |i| other[i] }
end
fixed_array(5, [1, 2, 3])
# => [1, 2, 3, nil, nil]

How do I assign default values for two-dimensional array?

Just supply a second argument with the value:

array = Array.new(10) { Array.new(10, 4) }

Here, the default value is 4 and thus, a 10*10 2D array is created with default value of 4.

How to initialize an array in one step using Ruby?

You can use an array literal:

array = [ '1', '2', '3' ]

You can also use a range:

array = ('1'..'3').to_a  # parentheses are required
# or
array = *('1'..'3')      # parentheses not required, but included for clarity

For arrays of whitespace-delimited strings, you can use Percent String syntax:

array = %w[ 1 2 3 ]

You can also pass a block to Array.new to determine what the value for each entry will be:

array = Array.new(3) { |i| (i+1).to_s }

Finally, although it doesn't produce the same array of three strings as the other answers above, note also that you can use enumerators in Ruby 1.8.7+ to create arrays; for example:

array = 1.step(17,3).to_a
#=> [1, 4, 7, 10, 13, 16]

Populate hash with array keys and default value

Do as below :

def solution(keys,default_val)
  Hash[keys.product([default_val])]
end

solution([:key1,:key2],12)  # => {:key1=>12, :key2=>12}

Read Array#product and Kernel#Hash.

Can't use an array as default values for Ruby Hash?

Try the following instead:

hash = Hash.new{|h, k| h[k] = []}
hash['a'] << 1 # => [1]
hash['b'] << 2 # => [2]

The reason you got your unexpected results is that you specified an empty array as default value, but the same array is used; no copy is done. The right way is to initialize the value with a new empty array, as in my code.

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