I would like to be able to do this:
my_array = Array.new
my_array[12] += 1
In other words, somehow upon trying to access entry 12, finding it uninitialized, it is initialized to zero so I can add one to it. Array.new has a default: parameter, but that comes into play when you initialize the array with a known number of slots. Other than writing my own class, is there a ruby-ish way of doing this?
No need to create a new class :
my_hash = Hash.new(0)
my_hash[12] += 1
p my_hash
#=> {12=>1}
For many cases, hashes and arrays can be used interchangeably. An array with an arbitrary number of elements and a default value sounds like a hash to me ;)
Just to make it clear : Hash and Array aren't equivalent. There will be cases where using a hash instead of an array will be completely wrong.
a = []; a[12] = 1; puts a and you’ll see what I am saying.
Array, which are absent in case of using Hash. Side effects are the most hard-to-find evil-to-debug etc part of the code. This should be at least understood, taken into account and mentioned :)a[12] in the array results in a.count #⇒ 13. In hash count will return guess what?Something like:
a[12] = (a[12] ||= 0) + 1
a[12] = (a[12] || 0) + 1 works too.Making use of nil.to_i == 0
my_array = Array.new
my_array[12] = my_array[12].to_i + 1
Note, that unlike other solutions here so far, this one works for any arbitrary initial value.
my_array = Array.new.extend(Module.new {
def [] idx
super || 0
end
})
my_array[12] += 1
#⇒ 1
new works, but sadly it's not the case. Maybe subclassing Array into ArrayWithDefault is a better plan than a dynamic mixin. This could get punishingly expensive if a lot of these are created since each instance has its own pet module.This is not possible with the stock Array::new method.
https://docs.ruby-lang.org/en/2.0.0/Array.html#method-c-new
You will either need to monkey patch Array class, or monkey patch nil class. And they are not recommended.
If you have a specific use case, I would create a new wrapper class around Array
class MyArray < Array
def [](i)
super(i) ? super(i) : self[i] = 0
end
end
arr = MyArray.new
arr[12] += 1 # => 1
my_array[12] ||= 1? This will initialize if array[12] is nil