Each loop not working as expected - ruby

Question

I have an array called #results that is made up only of arrays. I want to iterate through #results and permanently delete any of the inner arrays that are smaller than a given size:
My code:
def check_results limit
#results.each_with_index do |result, index|
#results.delete_at(index) if result.size < limit
end
end
Unfortunately, this only deletes the first item where the array length is less than limit. For example if limit = 4 and #results = [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1], [1, 1]] then check_results returns [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1]]
I can't figure out why this is happening. Am I using the wrong loop?

Answer 1

You should do this, as delete_at modifies the array, and you will get unexpected behavior if you are deleting elements while iterating it
#results.reject { |i| i.size < limit }
Above code will exclude all array elements whose size is smaller than limit

Answer 2

It's not a good idea to modify the #results array in place as that will conflict with the outer iteration.
What you should do instead is use select to build a new array.
def check_results(limit)
#result.select { |result| result.size > limit }
end

Answer 3

As per the documentation, #delete_at returns the element at that index.
a = ["ant", "bat", "cat", "dog"]
a.delete_at(2) #=> "cat"
a #=> ["ant", "bat", "dog"]
a.delete_at(99) #=> nil
I added some debug statements to show you what is happening at each step, assuming limit is 4:
#results = [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1], [1, 1]]
#results.each_with_index do |r, i|
puts "RESULT: #{r.to_s}"
puts "INDEX: #{i}"
#results.delete_at(i) if r.size < 4
puts "ARRAY: #{#results.to_s}"
end
RESULT: [1, 1, 1, 1]
INDEX: 0
ARRAY: [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1], [1, 1]]
RESULT: [1, 1, 1, 1]
INDEX: 1
ARRAY: [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1], [1, 1]]
RESULT: [1, 1, 1]
INDEX: 2
ARRAY: [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1]]
# #results == [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1]]
As you can see, the element originally at index 2 has been removed. Because you are modifying #results while you are iterating through it, an index of 3 no longer exists, and an index of 2 has already been analyzed. This is why you should not modify an object while iterating through it.
Ideally, you want to use #delete_if. Similar to methods ending in !, #delete_if will modify the array (not return a copy of the result), based on conditions from a block (as an argument). The following would be how you would implement the method:
def check_results(limit)
#results.delete_if { |arr| arr.length < limit }
end
#results = [ ['foo', 'bar'], ['bizz', 'bazz'], ['kaboom'] ]
check_results(2)
# => #results == [ ['foo', 'bar'], ['bizz', 'bazz'] ]
If you do not want to modify #results, then I suggest a similar method, #reject. Again, #results will not be modified, and instead a copy of the results will be returned.
def check_results(limit)
#results.reject { |arr| arr.length < limit }
end
#results = [ ['foo', 'bar'], ['bizz', 'bazz'], ['kaboom'] ]
check_results(2)
# => [ ['foo', 'bar'], ['bizz', 'bazz'] ]
# => #results == [ ['foo', 'bar'], ['bizz', 'bazz'], ['kaboom'] ]