Question

I have a simple array:

arr = ["apples", "bananas", "coconuts", "watermelons"]

I also have a function f that will perform an operation on a single string input and return a value. This operation is very expensive, so I would like to memoize the results in the hash.

I know I can make the desired hash with something like this:

h = {}
arr.each { |a| h[a] = f(a) }

What I’d like to do is not have to initialize h, so that I can just write something like this:

h = arr.(???) { |a| a => f(a) }

Can that be done?

Answer 1

Say you have a function with a funtastic name: “f”

def f(fruit)
   fruit + "!"
end

arr = ["apples", "bananas", "coconuts", "watermelons"]
h = Hash[ *arr.collect { |v| [ v, f(v) ] }.flatten ]

will give you:

{"watermelons"=>"watermelons!", "bananas"=>"bananas!", "apples"=>"apples!", "coconuts"=>"coconuts!"}

Updated:

As mentioned in the comments, Ruby 1.8.7 introduces a nicer syntax for this:

h = Hash[arr.collect { |v| [v, f(v)] }]