Why doesn't Array#delete remove an element from my array of arrays?

Question

To practice working with arrays and hashes I use a fun lotto app. Could someone please explain to me why result.delete(i) deleted the whole result array? Isn't is suppose to delete just THAT element and return the rest of the array?( using result - [i] works as intended):

for i in 1..number_of_balls do    # taking each possible ball
  results.each do |result|        # going through results
    if result.include?(i)         # find result that contains that ball
      #result.delete(i)           # delete itself from the result
      matched_numbers[i]<< result - [i] 
    end 
  end
end

Here are my inputs:

number_of_balls = 49 
results = [[3,11,12,14,41,43,13],[8,33,36,37,39,41,9], [1,6,23,24,27,39‌,34],
           [3,9,10,13,20,4‌3,34], [5,14,21,31,34‌,47,45],[8,20,21,25,‌31,41,33],
           [18,25,28,‌33,36,42,7],[7,16,17‌,31,40,48,26],[5,10,‌23,27,37,38,33],
           [4,1‌5,30,37,46,48,3], [7,‌9,21,33,38,42,45], [1‌1,17,19,20,36,43,9],‌
           [7,14,17,20,37,47,34‌],[25,28,29,30,35,44‌,3],[8,18,36,39,41,4‌7,31],
           [9,12,13,14,44‌,48,18],[4,14,18,40,‌43,44,5], [13,16,18,3‌4,35,36,26],
           [11,23,2‌5,28,29,36,27]]

Desired output:

    [[11, 12, 14, 41, 43, 13], [9, 10, 13, 20, 43, 34], [4, 15, 30, 37, 46, 48],
     [25, 28, 29, 30, 35, 44], [4, 19, 33, 34, 48, 39], [4, 9, 10, 11, 43, 46],
     [5, 6, 33, 38, 39, 8],    [2, 7, 21, 22, 30, 33],  [1, 19, 31, 32, 47, 37], 
     [17, 20, 30, 35, 48, 26]]

can you provide the inputs you used so we can reproduce the code, and also the desired result based on those inputs. And delete actually deletes all the element in the array that matches i. — davidhu
– davidhu, Commented Oct 15, 2016 at 17:29
If you're going through all the possible balls and deleting results that include them then it makes sense that the whole results array will be deleted. What am I missing? — jcm
– jcm, Commented Oct 15, 2016 at 17:55
@CodeGnome I want to thank you for providing refactored example. I study by myself and your example is invaluable for me. Your explanation of why the code does not work, made me thinking differently and I solved another problem. just thought I would mention this. — clonecq
– clonecq, Commented Oct 16, 2016 at 18:22

Todd A. Jacobs · Accepted Answer · 2016-10-15 18:32:25Z

TL;DR

Your code doesn't work because you're incorrectly comparing objects. You would be better served by using a different data structure. For example, using a Hash that keys off of your ball values simplifies things a lot.

Why Your Code Doesn't Work

Your current code doesn't work because the variable i in your code is an Integer, not an Array. As a result, the Array#delete method doesn't match anything when performing its equality check. Consider the following minimal example:

# Define a simplified array of arrays.
arrays = [[1, 2, 3]]
#=> [[1, 2, 3]]

# Doesn't delete anything because there's no match. This is correct
# behavior, since `1 == [1, 2, 3] #=> false`.
arrays.delete 1
#=> nil

# Deletes the specified array
arrays.delete [1, 2, 3]
#=> [1, 2, 3]

# Arrays is now empty, as expected.
arrays
#=> []

Refactored Example

The following provides a method for storing matching draws into a Hash object. The hash keys are the possible ball values.

# Your sample data reformatted.
number_of_balls = 49 
results = [
  [3,11,12,14,41,43,13],
  [8,33,36,37,39,41,9],
  [1,6,23,24,27,39,34],
  [3,9,10,13,20,43,34],
  [5,14,21,31,34,47,45],
  [8,20,21,25,31,41,33],
  [18,25,28,33,36,42,7],
  [7,16,17,31,40,48,26],
  [5,10,23,27,37,38,33],
  [4,15,30,37,46,48,3],
  [7,9,21,33,38,42,45],
  [11,17,19,20,36,43,9],
  [7,14,17,20,37,47,34],
  [25,28,29,30,35,44,3],
  [8,18,36,39,41,47,31],
  [9,12,13,14,44,48,18],
  [4,14,18,40,43,44,5],
  [13,16,18,34,35,36,26],
  [11,23,25,28,29,36,27],
]

# Returns a Hash of draws matching each available ball.
def matched_draws avail_balls, seqs_drawn 
  draws = {}
  (1..avail_balls).each do |key|
     draws[key] = seqs_drawn.select { |ary| ary.include? key }
  end 
  draws
end

# Invoke using your existing variable names.
h = matched_draws number_of_balls, results

Sample Results

The Hash allows you to show all draws that contain a given ball value. Note that the value may be an empty array if no draws matched.

h[1]
#=> [[1, 6, 23, 24, 27, 39, 34]]

h[49]
#=> []

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Why doesn't Array#delete remove an element from my array of arrays?

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Why Your Code Doesn't Work

Refactored Example

Sample Results

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