I have an array, and I want the result of the first block that returns a truthy value (aka, not nil). The catch is that in my actual use case, the test has a side effect (I'm actually iterating over a set of queues, and pop'ing off the top), so I need to not evaluate the block beyond that first success.
a,b,c = [1,2,3]
[a,b,c].first_but_value{ |i| (i + 1) == 2 } == 2
a == 2
b == 2
c == 3
Any ideas?
break is ugly =P
If you want a functional approach, you want a lazy map:
[nil, 1, 2, 3].lazy.map{|i| i && i.to_s}.find &:itself
# => "1"
If you don't believe it is not iterating throughout the array, just print out and see:
[nil, 1, 2, 3].lazy.map{|i| (p i) && i.to_s}.find &:itself
# nil
# 1
# => "1"
Replace i.to_s by your block.
lazy is there. with lazy you don't have to map all elements. I even provided a version with print so you can test it yourself if you don't believe it.[1, 2, 3].detect { |i| i += 1; break i if i == 2 }
# => 2
[1, 2, 3].detect { |i| i += 1; break i if i == 10 }
# => nil
break like that (or that'd become the value of the loop expression)
break and return are essentially the same thing. One is used for loops and the other for function contexts. By breaking the loop we are returning early.. If there was no arg with the break we would simply return nil but with the "return value" we can get the desired return behavior. (The same will work with any loop)
[1, 2, 3].each{|i| i += 1; break i if i == 10} => [1, 2, 3]
#detect insteadfind_yield does what you want, check out ruby facets with many core extensions, and especially find_yield Enumberable method: https://github.com/rubyworks/facets/blob/master/lib/core/facets/enumerable/find_yield.rb
Is this what you want to do?
a, b, c = 1, 2, 3
binding.tap { |b|
break b.local_variable_get [ :a, :b, :c ].find { |sym|
b.local_variable_set( sym, b.local_variable_get( sym ) + 1 ) == 2
}
} #=> 2
a #=> 2
b #=> 2
c #=> 3
binding before, so neat! I'll have to look it up to understand your answer, but thanks!
Binding to be explicitly aware of its local variable symbols. I'm still waiting for the = hook :-)Similar to @ribamar's answer, you can alternatively use lazy, filter_map and first:
[nil, 1, 2, 3].lazy.filter_map{|i| i && i.to_s}.first
# => "1"
I find it easier to read than having to do the find(&:itself).
Note: filter_map was added in Ruby 2.7, so won't work if you need to support old versions of Ruby.
Here's my take, is this closer to your actual use case? Note the content of b is 3 instead of 2 because my_test_with_side_effect is called on b as well.
class MyQueue
def initialize(j)
@j = j
end
def my_test_with_side_effect
(@j+=1) == 2
end
end
(a,b,c) = [MyQueue.new(1),MyQueue.new(2),MyQueue.new(3)]
[a,b,c].each { |i| break i unless i.my_test_with_side_effect }
=> #<MyQueue:0x007f3a8c693598 @j=3>
a
=> #<MyQueue:0x007f3a8c693980 @j=2>
b
=> #<MyQueue:0x007f3a8c693598 @i=3>
c
=> #<MyQueue:0x007f3a8c693430 @i=3>
I doubt there's a way to do this. The problem being that Ruby creates a closure in the block and the variable i is local to it. Doing i+=1 can be expanded to i = i + 1 which creates a new variable i in the block's scope and doesn't modify the value in any of your a,b,c variables.
a, b, c variables? How do you know that? And, btw., in Ruby there is a way to do almost everything. But I need to know what the asker wants.a == 2 at the bottom half of the code snippetjob_data = queue_names.each{|q| job = Resque.pop(q); break job if job}. Does that tell you why I want to do something like this?
#findmethod normally does? I already wrote an answer, but I don't believe that you actually mean what you ask for.+=operator in your block does not make sense, sinceigets discarded after every block evaluation.==are crude asserts. And I do always forget that Ruby's+=doesn't always work the way I expect (hold over from the C++ days), so you're correct that I'm using incorrectly for what I'm trying to achieve. In the actual use case, it's a.pop, but I felt like that would have required additional background. The "result" would definitely be the return value of the block. I don't know any other way to interpret the "result". In my use case, the block will eval tonilrather thanfalse, and not everyone that reads these questions will knownilis falsey