Last night I lost hours of my life to strange behavior with Ruby arrays. Now that I know what’s going on it all makes sense but I can see how someone can easily be confused by this. I’m sharing it in hopes that others might avoid the same pitfall and as a reminder to myself in the future.

The issue I ran into involves iteration. While iterating over an array I was also selectively deleting that item from the array if it matched some criteria. What I found is that I was ‘randomly’ failing to iterate over some values in my array.

Below is an example of what I was doing. For context, I was writing AWS Network ACL support for chef/chef-provisioning-aws.

current_rules = [
  {
    rule_number: 100,
    action: :deny,
    protocol: -1,
    cidr_block: '10.0.0.0/24',
    egress: false,
    port_range: -1
  }
]
desired_rules = [
  {
    rule_number: 100,
    action: :deny,
    protocol: -1,
    cidr_block: '10.0.0.0/24'
  },
  {
    rule_number: 200,
    action: :allow,
    protocol: -1,
    cidr_block: '0.0.0.0/0'
  },
  {
    rule_number: 300,
    action: :allow,
    protocol: 6,
    port_range: 22..23,
    cidr_block: '172.31.0.0/22'
  }
]

desired_rules.each do |desired_rule|
  matching_rule = current_rules
    .select { |r| r[:rule_number] == desired_rule[:rule_number]}.first

  if matching_rule
    # Anything unhandled will be removed
    current_rules.delete(matching_rule)
    # Anything unhandled will be added
    desired_rules.delete(desired_rule)

    ...
  end
end

Since current_rules and desired_rules both have rule_number: 100 they are deleted from both arrays. That seems fine, but what happened to desired_rules (remember, the array I’m currently iterating over)? desired_rules now looks like this:

[
  {
    rule_number: 200,
    action: :allow,
    protocol: -1,
    cidr_block: '0.0.0.0/0'
  },
  {
    rule_number: 300,
    action: :allow,
    protocol: 6,
    port_range: 22..23,
    cidr_block: '172.31.0.0/22'
  }
]

All the remaining elements in the array shifted forwarded by one index. Element with rule_number: 200 shifted from index 1 to index 0. The next iteration over the array is index 1, now rule_number: 300. Whoops!

Knowing the issue here is most of the battle. The solution is rather easy - just clone the array before iterating:

desired_rules.clone.each do |desired_rule|
  ...
end

desired_rules elements still shift one index forward but we’re iterating over a clone that is unaffected by the delete action.

My friend Stuart (@McCroden) is working with Swift and came across an interesting Stack Overflow answer suggesting reverse iteration to avoid this problem. That’s a neat way to get around this, too.

Special thanks to Christopher Webber (@cwebber) for suggesting this post. It’s a great, small topic to get my blog off the ground. I will remember to blog about small issues like this more in the future.

Unfortunately I don’t have comments enabled on my blog yet. If you have comments, please send them to @drewblessing on Twitter.