Can I create an array in Ruby with default values?

Question

Perl is pretty nice about default values:

: jmglov@laurana; perl -e '@foo; printf "%d\n", $foo[123]' 0 : jmglov@laurana; perl -e '%foo; printf "%d\n", $foo{bar}' 0 

Ruby can do the same, at least for hashes:

>> foo = Hash.new(0) => {} >> foo[:bar] => 0 

But the same seemingly does not work for arrays:

>> foo = Array.new(0) => [] >> foo[123] => nil >> foo[124] = 0 => 0 >> foo[456] = 0 => 0 >> foo[455,456] => [nil, 0] 

Is it possible to supply a default value for arrays, so when they are auto-extended, they're filled with 0 instead of nil?

Of course I can work around this, but at a cost to expressiveness:

>> foo[457,458] = 890, 321 => [890, 321] >> foo[456] += 789 NoMethodError: You have a nil object when you didn't expect it! You might have expected an instance of Array. The error occurred while evaluating nil.+ >> foo.inject(0) {|sum, i| sum += (i || 0) } => 1211 >> foo.inject(:+) NoMethodError: You have a nil object when you didn't expect it! You might have expected an instance of Array. The error occurred while evaluating nil.+ 

Update 1: One of my colleagues pointed out that I can use #compact to solve the #inject issue, and #to_i to solve the standard element-at-index issue:

>> foo.include? nil => true >> foo.compact.inject(:+) => 1211 >> foo[456,457] => [0, 890, 321] >> foo[455..457] => [nil, 0, 890] >> foo[455..457].map(&:to_i) => [0, 0, 890] 

Update 2: Thanks to Andrew Grimm for a solution to the += issue:

>> foo = [] => [] >> def foo.[](i) >>   fetch(i) {0} >> end => nil >> foo[4] => 0 >> foo => [] >> foo[4] += 123 => 123 >> foo => [nil, nil, nil, nil, 123] 

Update 3: this is starting to look like whack-a-mole!

>> foo => [nil, nil, nil, nil, 123] >> foo[-2..-1] TypeError: can't convert Range into Integer 

But we can deal with that:

>> def foo.[](index) >>   if index.is_a? Range >>     index.map {|i| self[i] } >>   else ?>     fetch(index) { 0 }  # default to 0 if no element at index; will not cause auto-extension of array >>   end >> end => nil >> foo => [nil, nil, nil, nil, 123] >> foo[-2..-1] => [nil, 123] 

I now have to admit (sheepishly) that I'll subclass Array to avoid cluttering my code:

class MyClass   class ArrayWithDefault < Array     def [](index)       if index.is_a? Range         index.map {|i| self[i] }       else         fetch(index) { 0 }  # default to 0 if no element at index; will not cause auto-extension of array       end     end   end end 

Thanks for all the creative solutions. TIMTOWTDI indeed!

Answer 1

Not auto extended, but initialized to the specified length with a default value:

>> Array.new(123, 0)   => [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]