You may have heard Ruby 2.3 dropped on Christmas this past year. Two new Enumerable instance methods were added, grep_v and chunk_while, both of which are variations on other Enumerable methods. We'll dissect both here below.

Triple Lindey

Grep allows you to select members of a collection that match an expression. That expression is some_object === item.

What does "triple equals" do? Well, in Ruby, as in most languages, that answer is complicated. Since we can define this method however we want in our own classes, it can mean anything. But, most commonly, it is a loose way of asking whether an object belongs to a certain group:

if a === b, then b is a subset of a

A few examples:

Symbol === :foo     # => true, :foo is a symbol
String === "foo"    # => true, "foo" is a string
Symbol === "foo"    # => false, "foo" is not a symbol
String === :foo     # => false, :foo is not a string

/foo/ === "foobar"  # => true, "foobar" matches /foo/
(1..10) === 2       # => true, 2 is in the range 1..10

We could use select to filter out items of a collection with the === like so:

1.upto(20).select { |i| (6..10) === i } # => [6, 7, 8, 9, 10]

But this is what grep is for:

1.upto(10).grep(6..8) # => [6, 7, 8]

So grep is like saying "gimme all the items in the given set".

To get all the items not included, use grep_v:

1.upto(10).grep_v(6..8)
=> [1, 2, 3, 4, 5, 9, 10]

It's probably most likely that you'd use grep and grep_v with a regular expression, like to select all the months that end in "er":

MONTHS.grep(/er$/)
=> ["September", "October", "November", "December"]

Define === on any class or object to take advantage of filtering with grep and grep_v in other contexts.

Chunking

The Enumerable module provides several methods for enumerating adjacent members of a collection, including slice_when, slice_before, slice_after, each_cons, and, not surprisingly, chunk. Ruby 2.3 offers yet another chunking method, chunk_while.

Before diving into chunk_while, let's look at its relatives, chunk and slice_when.

For the following examples, we'll enumerate over recurring events given by the Montrose gem. Montrose provides an api to create recurrences as enumerators.

Here's a recurrence that will enumerate over every other Tuesday at noon.

require "montrose"

r = Montrose.every(2.weeks, on: :tuesday, at: '12pm')

r.take(10).to_a
=> [2016-02-02 12:00:00 -0500,
 2016-02-16 12:00:00 -0500,
 2016-03-01 12:00:00 -0500,
 2016-03-15 12:00:00 -0400,
 2016-03-29 12:00:00 -0400,
 2016-04-12 12:00:00 -0400,
 2016-04-26 12:00:00 -0400,
 2016-05-10 12:00:00 -0400,
 2016-05-24 12:00:00 -0400,
 2016-06-07 12:00:00 -0400]

For calendaring, it may be useful to split this array into chunks by month.

We could use group_by to return a hash of month numbers to Tuesday time instances:

r.take(10).group_by(&:month)
=> {2=>[2016-02-02 12:00:00 -0500, 2016-02-16 12:00:00 -0500],
 3=>[2016-03-01 12:00:00 -0500, 2016-03-15 12:00:00 -0400, 2016-03-29 12:00:00 -0400],
 4=>[2016-04-12 12:00:00 -0400, 2016-04-26 12:00:00 -0400],
 5=>[2016-05-10 12:00:00 -0400, 2016-05-24 12:00:00 -0400],
 6=>[2016-06-07 12:00:00 -0400]}

The chunk method is similar to group_by as it will divide the collection into groups based on the given block/proc except it returns an enumerator instead of a hash:

r.take(10).chunk(&:month)
=> #<Enumerator: ...>

I recently described some great things you can do with enumerators in Ruby including transforming the result of chunk into an array of month, time pairs:

r.take(10).chunk(&:month).to_a
=> [[2, [2016-02-02 12:00:00 -0500, 2016-02-16 12:00:00 -0500]],
 [3, [2016-03-01 12:00:00 -0500, 2016-03-15 12:00:00 -0400, 2016-03-29 12:00:00 -0400]],
 [4, [2016-04-12 12:00:00 -0400, 2016-04-26 12:00:00 -0400]],
 [5, [2016-05-10 12:00:00 -0400, 2016-05-24 12:00:00 -0400]],
 [6, [2016-06-07 12:00:00 -0400]]]

Interestingly enough, passing the previous result to Hash[] returns the same result we got with group_by:

Hash[r.take(10).chunk(&:month).to_a]
=> {2=>[2016-02-02 12:00:00 -0500, 2016-02-16 12:00:00 -0500],
 3=>[2016-03-01 12:00:00 -0500, 2016-03-15 12:00:00 -0400, 2016-03-29 12:00:00 -0400],
 4=>[2016-04-12 12:00:00 -0400, 2016-04-26 12:00:00 -0400],
 5=>[2016-05-10 12:00:00 -0400, 2016-05-24 12:00:00 -0400],
 6=>[2016-06-07 12:00:00 -0400]}

If we just wanted the groups of times, not the month keys, we could ask for the group_by values:

r.take(10).group_by(&:month).values
=> [[2016-02-02 12:00:00 -0500, 2016-02-16 12:00:00 -0500],
 [2016-03-01 12:00:00 -0500, 2016-03-15 12:00:00 -0400, 2016-03-29 12:00:00 -0400],
 [2016-04-12 12:00:00 -0400, 2016-04-26 12:00:00 -0400],
 [2016-05-10 12:00:00 -0400, 2016-05-24 12:00:00 -0400],
 [2016-06-07 12:00:00 -0400]]

We can replace this computation with slice_when which will allow us to compare adjacent members to determine "when" to start a new slice (or chunk). That means the block we pass to slice_when accepts two arguments, the current and previous collection member, and should return true or false. In this case, we want a new slice to start when the month of the previous time does not match the month of the current time as we enumerate:

r.take(10).slice_when { |a, b| a.month != b.month }.to_a
=> [[2016-02-02 12:00:00 -0500, 2016-02-16 12:00:00 -0500],
 [2016-03-01 12:00:00 -0500, 2016-03-15 12:00:00 -0400, 2016-03-29 12:00:00 -0400],
 [2016-04-12 12:00:00 -0400, 2016-04-26 12:00:00 -0400],
 [2016-05-10 12:00:00 -0400, 2016-05-24 12:00:00 -0400],
 [2016-06-07 12:00:00 -0400]]

Note the return value of slice_when is an enumerator like we saw with chunk.

It turns out, the most common use cases of slice_when tend to be negative comparisons, i.e., "slice when the previous thing is not the same as the current thing". Since this is Ruby after all, wouldn't you prefer to stay positive?

Which brings us to chunk_while. Ruby 2.3 introduces this positive complement to slice_when so we can say, "keep the same chunk if the current thing does match the previous thing".

Back to our Tuesday recurrences. Let's replace slice_when with chunk_while:

r.take(10).chunk_while { |a, b| a.month == b.month }.to_a
=> [[2016-02-02 12:00:00 -0500, 2016-02-16 12:00:00 -0500],
 [2016-03-01 12:00:00 -0500, 2016-03-15 12:00:00 -0400, 2016-03-29 12:00:00 -0400],
 [2016-04-12 12:00:00 -0400, 2016-04-26 12:00:00 -0400],
 [2016-05-10 12:00:00 -0400, 2016-05-24 12:00:00 -0400],
 [2016-06-07 12:00:00 -0400]]

So in short, Enumerable in Ruby 2.3 gives us both a negative and a positive variation, grep_v and chunk_while respectively, on existing methods.


Looking for a way to handle recurring events in your app? Be sure to check out Montrose.


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Part of the Enumerable series. Published on Jan 18, 2016