How to remove elements of array in place returning the removed elements

Question

I have an array arr. I want to destructively remove elements from arr based on a condition, returning the removed elements.

arr = [1,2,3] arr.some_method{|a| a > 1} #=> [2, 3] arr #=> [1]

My first try was reject!:

arr = [1,2,3] arr.reject!{|a| a > 1}

but the returning blocks and arr's value are both [1].

I could write a custom function, but I think there is an explicit method for this. What would that be?

Update after the question was answered:

partition method turns out to be useful for implementing this behavior for hash as well. How can I remove elements of a hash, returning the removed elements and the modified hash?

hash = {:x => 1, :y => 2, :z => 3} comp_hash, hash = hash.partition{|k,v| v > 1}.map{|a| Hash[a]} comp_hash #=> {:y=>2, :z=>3} hash #=> {:x=>1}

robinst · Accepted Answer · 2012-12-06 11:38:32Z

I'd use partition here. It doesn't modify self inplace, but returns two new arrays. By assigning the second array to arr again, it gets the results you want:

comp_arr, arr = arr.partition { |a| a > 1 }

See the documentation of partition.

krichard · Accepted Answer · 2012-12-06 16:59:56Z

All methods with a trailing bang ! modify the receiver and it seems to be a convention that these methods return the resulting object because the non-bang do so.
What you can to do though is something like this:

b = (arr.dup - arr.reject!{|a| a>1 }) b # => [2,3] arr #=> [1]

Here is a link to a ruby styleguide which has a section on nameing - although its rather short

Vadym Tyemirov · Accepted Answer · 2014-02-05 17:42:56Z

To remove (in place) elements of array returning the removed elements one could use delete method, as per Array class documentation:

a = [ "a", "b", "b", "b", "c" ] a.delete("b") #=> "b" a #=> ["a", "c"] a.delete("z") #=> nil a.delete("z") { "not found" } #=> "not found"

It accepts block so custom behavior could be added, as needed

delete doesn't allow specify a condition (a > 1) and – as shown in your example – it also only returns the last deleted item, even if multiple items have been removed.

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