How do I break out of a map/collect and return whatever has been collected up to that point?

Question

I'm rewriting this question in code:

many = 1000  # An expensive method. # # It returns some data or nil if no result is available. expensive_method = lambda do   rand(5) == 0 ? nil : "foo" end  # Now, let's collect some data and stop collecting when no more data is # available.  # This is concise but doesn't work. collection = many.times.map do   expensive_method.call || break end  puts collection.is_a? Array # false  # This is less concise but works. collection = [] many.times do   collection << (expensive_method.call || break) end  puts collection.is_a? Array # true  # My inner Rubyist ponders: Is it possible to accomplish this more concisely # using map? 

Answer 1

Sure seems the only way to do this in Ruby is a filter type method then passing results to map. I'm not sure if this works in 1.8, but in 1.9 you could:

[0,1,2,1,0].take_while {|val| val < 2}.map(&:some_function) 

Or in the times example

3.times.take_while {|count| count <= 1 } #=> [0,1] 

Answer 2

Instead of using map directly, build up your own collection and then use the fact that break returns a value to abort early:

result =    [0, 1, 2, 1, 0].each_with_object([]) { |val, accumulator|     if val < 2       accumulator << val     else       break accumulator     end   } result  # => [0, 1] 

If we did just break (instead of break accumulator) then nil would be implicitly returned and result would just be set to nil.

This solution has the advantage of only allocating a single accumulator Array and only having to loop once.