iterate over an array and delete elements conditionally

Question

The following logic:

Iterate over the URLs array and If the HTTP request with the given URL does not timeout, continue. If the URL times out, remove it from the array and continue with the next one until the array is exhausted.

Has been implemented in the following way:

@urls.delete_if do |url|
 begin
 doc = perform_request(some_params)
 break
 rescue TimeoutError
 Rails.logger.warn("URL #{url} times out, will be removed from list")
 true
 end
end

Anyone for a cleaner solution? Basically, I want to iterate over an array and remove elements from it if there is a timeout, but if the URL works loop should end and app should continue processing.

• Emo: Specifically, hate the break in there - maybe someone can propose a cleaner solution.
• Emo: Btw "true" in the rescue can be removed, not needed.
• tokland: if not needed just edit the question! I have a doubt, why do you want to modify inplace @urls? wouldn't do a functional solution?

Answer 1

note : your break is useless, because if the block returns a falsey value, the url is kept.

Maybe it can just be a bit more expressive ?

def timeouts?( url )
 perform_request( url ) && false 
rescue TimeoutError
 true
end
@urls.delete_if {|url| timeouts?( url ) }

You could even make timeouts? a method on your @urls objects, for extra readability :

@urls.delete_if( &:timeouts? )

if you want to keep @tokland's functionnal approach, just use :

@kept_urls = @urls.reject{|url| timeouts?( url ) }

however, it would not be fully "functionnal", because timeouts may not return the same results with the same arguments, depending on the execution environment.

• jox: Maybe calling the function timesout? instead of timeouts? would semantically be better.

Answer 2

You may use drop_while to get rid of iterative operator break:

@urls = @urls.drop_while do |url|
 begin
 doc = perform_request(some_params)
 false
 rescue TimeoutError
 Rails.logger.warn("URL #{url} times out, will be removed from list")
 true
 end
end

• tokland: the problem is doc won't be visible outside the block's scope.

Answer 3

I'd take a functional approach (no in-place updates):

url, doc = @urls.map_detect do |url|
 begin
 [url, perform_request(some_params)]
 rescue TimeoutError
 Rails.logger.warn("URL #{url} timed out")
 false
 end
end

This snippet uses the custom abstraction Enumerable#map_detect (see also Facets)

You can then use url to update @urls, but it's better not to mix the algorithm (even if it's so simple) with the updates required by the app.