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Enumerating Enumerable: Enumerable#first

Enumerating Enumerable

Welcome to another installment of Enumerating Enumerable, my series of articles in I attempt to do a better job of documenting Ruby’s Enumerable module than Ruby-Doc.org. In this installment, I cover the first method.

In case you missed any of the previous articles, they’re listed and linked below:

  1. all?
  2. any?
  3. collect / map
  4. count
  5. cycle
  6. detect / find
  7. drop
  8. drop_while
  9. each_cons
  10. each_slice
  11. each_with_index
  12. entries / to_a
  13. find_all / select
  14. find_index

Enumerable#first Quick Summary

Graphic representing the "first" method in Ruby's "Enumerable" module

In the simplest possible terms What are the first n items in the collection?
Ruby version 1.8 and 1.9
Expects An optional integer n that specifies the first n items of the collection to return. If this integer is not given, n is 1 by default.
Returns If first is applied to a collection containing m elements:

  • The first item in the collection, if m > 0 and no argument n is provided.
  • An array containing the first n items in the collection, if m > 0 and an argument n is provided.
  • nil if the collection is empty and no argument n is provided.
  • The empty array [] if the collection is empty and an argument n is provided.
RubyDoc.org’s entry Enumerable#first

Enumerable#first and Arrays

When used on an array without an argument, first returns the first item in the array:

posts = ["First post!", "Second post!", "Third post!"]
=> ["First post!", "Second post!", "Third post!"]

# What's the first item in posts?
posts.first
=> "First post!"

# Here's the equivalent using array notation:
posts[0]
=> "First post!"

When used on an array with an integer argument n, first returns an array containing the first n items in the original array:

# What are the first 2 items in posts?
posts.first 2
=> ["First post!", "Second post!"]

# Note that when you provide an argument of 1,
# the result is still an array -- with just one element.
# If you want a scalar, don't use an argument.
posts.first 1
=> ["First post!"]

# Here's the equivalent using array slice notation:
posts[0..1]
=> ["First post!", "Second post!"]

posts[0...2]
=> ["First post!", "Second post!"]

When used on an empty array, first returns:

  • nil if no argument n is provided
  • The empty array, [], if an argument n is provided

[].first
=> nil

[].first 2
=> []

Enumerable#first and Hashes

In Ruby 1.8 and previous versions, hash order is seemingly arbitrary. Starting with Ruby 1.9, hashes retain the order in which they were defined, which makes the first method a little more applicable.

When used on a hash without an argument, first returns the first item in the hash as a two-element array, with the key as the first element and the corresponding value as the second element.

# Let's see what stages of partying our friends are in
party_stages = {"Alice" => :party_mineral,
                "Bob"   => :party_animal,
                "Carol" => :party_reptile,
                "Dave"  => :party_vegetable}
=> {"Alice"=>:party_mineral, "Bob"=>:party_animal, "Carol"=>:party_reptile, "Dave"=>:party_vegetable}

# Who's the first partier and what state is s/he in?
party_stages.first
=> ["Alice", :party_mineral]

When used on a hash with an integer argument n, first returns an array containing the first n items in the hash, with each item represented as a two-element array:

party_stages.first 2
=> [["Alice", :party_mineral], ["Bob", :party_animal]]

# Note that when you provide an argument of 1,
# the result is still an array -- with just one element.
# If you want a scalar, don't use an argument.
party_stages.first 1
=> [["Alice", :party_mineral]]

When used on an empty hash, first returns:

  • nil if no argument n is provided
  • The empty array, [], if an argument n is provided

{}.first
=> nil

{}.first 2
=> []

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Enumerating Enumerable: Enumerable#find_index

Enumerating Enumerable

Once again, it’s Enumerating Enumerable, my series of articles in which I attempt to outdo Ruby-Doc.org’s documentation of Ruby’s Enumerable module. In this article, I cover the find_index method, which was introduced in Ruby 1.9.

In case you missed any of the previous articles, they’re listed and linked below:

  1. all?
  2. any?
  3. collect / map
  4. count
  5. cycle
  6. detect / find
  7. drop
  8. drop_while
  9. each_cons
  10. each_slice
  11. each_with_index
  12. entries / to_a
  13. find_all / select

Enumerable#find_index Quick Summary

Graphic representation of the "find_index" method in Ruby's "Enumerable" module

In the simplest possible terms What’s the index of the first item in the collection that meets the given criteria?
Ruby version 1.9
Expects A block containing the criteria.
Returns
  • The index of the item in the collection that matches the criteria, if there is one.
  • nil, if no item in the collection matches the crtieria.
RubyDoc.org’s entry Enumerable#find_index

Enumerable#find_index and Arrays

When used on an array, find_index passes each item in the array to the given block and either:

  • Stops when the current item causes the block to return a value that evaluates to true (that is, anything that isn’t false or nil) and returns the index of that item, or
  • Returns nil if there is no item in the array that causes the block to return a value that evaluates to true.

Some examples:

# How about an array of the name of the first cosmonauts and astronauts,
# listed in the chronological order of the missions?
mission_leaders = ["Gagarin", "Shepard", "Grissom", "Titov", "Glenn", "Carpenter",
"Nikolayev", "Popovich"]
=> ["Gagarin", "Shepard", "Grissom", "Titov", "Glenn", "Carpenter", "Nikolayev",
"Popovich"]

# Yuri Gagarin was the first in space
mission_leaders.find_index{|leader| leader == "Gagarin"}
=> 0

# John Glenn was the fifth
mission_leaders.find_index{|leader| leader == "Glenn"}
=> 4

# And James Tiberius Kirk is not listed in the array
kirk_present = mission_leaders.find_index{|leader| leader == "Kirk"}
=> nil

Enumerable#find_index and Hashes

When used on a hash, find_index passes each key/value pair in the hash to the block, which you can “catch” as either:

  1. A two-element array, with the key as element 0 and its corresponding value as element 1, or
  2. Two separate items, with the key as the first item and its corresponding value as the second item.

As with arrays, find_index:

  • Stops when the current item causes the block to return a value that evaluates to true (that is, anything that isn’t false or nil) and returns the index of that item, or
  • Returns nil if there is no item in the array that causes the block to return a value that evaluates to true.

Some examples:

require 'date'
=> true

# These are the names of the first manned spaceships and their launch dates
launch_dates = {"Kedr"              => Date.new(1961, 4, 12),
                "Freedom 7"         => Date.new(1961, 5, 5),
                "Liberty Bell 7"    => Date.new(1961, 7, 21),
                "Orel"              => Date.new(1961, 8, 6),
                "Friendship 7"      => Date.new(1962, 2, 20),
                "Aurora 7"          => Date.new(1962, 5, 24),
                "Sokol"             => Date.new(1962, 8, 11),
                "Berkut"            => Date.new(1962, 8, 12)}
=> {"Kedr"=>#<Date: 4874803/2,0,2299161>, "Freedom 7"=>#<Date: 4874849/2,0,2299161>,
"Liberty Bell 7"=>#<Date: 4875003/2,0,2299161>, "Orel"=>#<Date: 4875035/2,0,2299161>,
"Friendship 7"=>#<Date: 4875431/2,0,2299161>, "Aurora 7"=>#<Date: 4875617/2,0,2299161>,
"Sokol"=>#<Date: 4875775/2,0,2299161>, "Berkut"=>#<Date: 4875777/2,0,2299161>}

# Where in the list is John Glenn's ship, the Friendship 7?
launch_dates.find_index{|ship, date| ship == "Friendship 7"}
=> 4

# Where in the list is the first mission launched in August 1962?
launch_dates.find_index{|ship, date| date.year == 1962 && date.month == 8}
=> 6

# The same thing, expressed a little differently
launch_dates.find_index{|launch| launch[1].year == 1962 && launch[1].month == 8}
=> 6

Using find_index as a Membership Test

Although Enumerable has a method for checking whether an item is a member of a collection (the include? method and its synonym, member?), find_index is a more powerful membership test for two reasons:

  1. include?/member? only check membership by using the == operator, while find_index lets you define a block to set up all sorts of tests. include?/member? asks “Is there an object X in the collection equal to my object Y?” while find_index can be used to ask “Is there an object X in the collection that matches these criteria?”
  2. include?/member? returns true if there is an object X in the collection that is equal to the given object Y. find_index goes one step further: not only can it be used to report the equivalent of true if there is an object X in the collection that is equal to the given object Y, it also reports its location in the collection.

A quick example of this use in action:

# Once again, the mission leaders
mission_leaders = ["Gagarin", "Shepard", "Grissom", "Titov", "Glenn", "Carpenter",
"Nikolayev", "Popovich"]
=> ["Gagarin", "Shepard", "Grissom", "Titov", "Glenn", "Carpenter", "Nikolayev",
 "Popovich"]

# Yuri Gagarin is in the list
gagarin_in_list = mission_leaders.find_index {|leader| leader == "Gagarin"}
=> 0

# Captain James T. Kirk is not
kirk_in_list = mission_leaders.find_index {|leader| leader == "Kirk"}
=> nil

# gagarin_in_list is 0, which as a non-false and non-nil value evaluates as true.
# We can use it as both a membership test *and* as his location in the list.
p "Gagarin's there. He's number #{gagarin_in_list + 1} in the list." if gagarin_in_list
"Gagarin's there. He's number 1 in the list."
=> "Gagarin's there. He's number 1 in the list."

# kirk_in_list is nil, which is one of Ruby's two "false" values.
# Let's use it with the "something OR something else" idiom that
# many Ruby programmers like.
kirk_in_list || (p "You only *think* he wasn't there.")
"You only *think* he wasn't there."
=> "You only *think* he wasn't there."

Parts that Haven’t Been Implemented Yet

Ruby-Doc.org’s documentation is generated from the comments in the C implementation of Ruby. It mentions a way of calling find_index that is just like calling include?/member?:

# What the docs say (does not work yet!)
["Alice", "Bob", "Carol"].find_index("Bob")
=> 1

# What happens with the current version of Ruby 1.9
["Alice", "Bob", "Carol"].find_index("Bob")
ArgumentError: wrong number of arguments(1 for 0)
...


Ruby 1.9 is considered to be a work in progress, so I suppose it’ll get implemented in a later release.

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Enumerating Enumerable: Enumerable#find_all / Enumerable#select

Enumerating Enumerable

We’re at lucky number 13 — the thirteenth article in the Enumerating Enumerable series, which covers the methods of Ruby’s Enumerable module. I started this series after being disappointed with the documentation at Ruby-Doc.org.

In this article, I’m covering the find_all — a.k.a. select — method.

In case you missed any of the previous articles, they’re listed and linked below:

  1. all?
  2. any?
  3. collect / map
  4. count
  5. cycle
  6. detect / find
  7. drop
  8. drop_while
  9. each_cons
  10. each_slice
  11. each_with_index
  12. entries / to_a

Enumerable#find_all / Enumerable#select Quick Summary

Graphic representation of the \"find_all\" / \"select\" method in Ruby\'s \"Enumerable\" module

In the simplest possible terms Which items in the collection meet the given criteria?
Ruby version 1.8 and 1.9
Expects A block containing the criteria.
Returns An array containing the items in the collection that meet the given criteria. If no items in the collection meet the given criteria, this is an empty array.
RubyDoc.org’s entry Enumerable#find_all / Enumerable#select

Enumerable#find_all / Enumerable#select and Arrays

When used on an array, find_all and its synonym select passes each item from the array to the block, returning an array containing only those elements in the original array for which the block returns a value that doesn’t evaluate to false.

If no item in the array causes the block to return a value that doesn’t evaluate to false, find_all/select returns an empty array.

In the examples that follow (which are based on my examples for the detect/find method), I’ll be using the find_all method. select does exactly the same thing; it’s just that I prefer find_all.

# Once again,  I shall establish my "old fart" credentials
classic_rock_bands = ["AC/DC", "Black Sabbath", "Queen", \
"Ted Nugent and the Amboy Dukes", "Scorpions", "Van Halen"]
=> ["AC/DC", "Black Sabbath", "Queen", "Ted Nugent and the Amboy Dukes",
"Scorpions", "Van Halen"]

# Of the bands in the array, which ones have
# a name longer than 8 characters?
classic_rock_bands.find_all {|band| band.length > 8}
=> ["Black Sabbath", "Ted Nugent and the Amboy Dukes", "Scorpions", "Van Halen"]

# Which ones have a name exactly 5 characters long?
classic_rock_bands.find_all {|band| band.length == 5}
=> ["AC/DC", "Queen"]

# If no band in the array meets the criteria,
# find_all returns an empty array.
# Which ones have names shorter than 5 characters?
classic_rock_bands.find_all {|band| band.length < 5}
=> []

Enumerable#find_all / Enumerable#select and Hashes

When used on a hash, find_all/select passes each key/value pair in the hash to the block, which you can “catch” as either:

  1. A two-element array, with the key as element 0 and its corresponding value as element 1, or
  2. Two separate items, with the key as the first item and its corresponding value as the second item.

As with arrays, find_all/select passes each item from the hash to the block, returning an array containing only those items in the original array for which the block returns a value that doesn’t evaluate to false.

Note that each item in the result array is a two-element array corresponding to an item from the original hash. In this array, element 0 contains the key and element 1 contains the corresponding value.

If no item in the hash causes the block to return a value that doesn’t evaluate to false, find_all/select returns an empty array.

years_founded = {"AC/DC" => 1973, \
                 "Black Sabbath" => 1968, \
                 "Queen" => 1970, \
                 "Ted Nugent and the Amboy Dukes" => 1967, \
                 "Scorpions" => 1965, \
                 "Van Halen" => 1972}
=> {"AC/DC"=>1973, "Black Sabbath"=>1968, "Queen"=>1970,
"Ted Nugent and the Amboy Dukes"=>1967, "Scorpions"=>1965, "Van Halen"=>1972}

# Ruby 1.9 preserves hash order so that hashes keep the order in which
# you defined them, while Ruby 1.8 puts them in some mysterious order.
# All these examples are in Ruby 1.9

# Which bands were founded in 1970 or later?
years_founded.find_all {|band| band[1] >= 1970}
=> [["AC/DC", 1973], ["Queen", 1970], ["Van Halen", 1972]]

# Here's another way of phrasing it:
years_founded.find_all {|band, year_founded| year_founded >= 1970}
=> [["AC/DC", 1973], ["Queen", 1970], ["Van Halen", 1972]]

# Which bands were founded after 1980?
years_founded.find_all {|band, year_founded| year_founded > 1980}
=> []

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Enumerating Enumerable: Enumerable#each_with_index

Enumerating Enumerable

Here’s number 11 in the Enumerating Enumerable series, in which I’m trying to outdo RubyDoc.org in their documentation of Ruby’s key Enumerable module. In this article, I cover the each_with_index method.

In case you missed the earlier articles, they’re listed and linked below:

  1. all?
  2. any?
  3. collect / map
  4. count
  5. cycle
  6. detect / find
  7. drop
  8. drop_while
  9. each_cons
  10. each_slice

Enumerable#each_with_index Quick Summary

Graphic representation of the \"each_with_index\" method in Ruby\'s \"Enumerable\" module

In the simplest possible terms Think of each_with_index as a version of each that includes an extra piece of information: a number representing the current iteration’s element’s position in the collection.
Ruby version 1.8 and 1.9
Expects An optional block to receive the values from each iteration.
Returns
  • The original collection, if used with a block.
  • An Enumerator object that outputs the collection’s items and indexes, if used without a block.
RubyDoc.org’s entry Enumerable#each_with_index

Enumerable#each_with_index and Arrays

When used with on an array and given a block, each_with_index iterates through the array, passing each item in the array and the number representing its order in the array to the block. Once again, I’ll use the “Justice League” example:

justice_league = ["Aquaman", "Batman", "Black Canary", \
                  "Flash", "Green Arrow", "Green Lantern", \
                  "Martian Manhunter", "Superman", \
                  "Vixen", "Wonder Woman"]
justice_league = ["Aquaman", "Batman", "Black Canary", "Flash", "Green Arrow",
"Green Lantern", "Martian Manhunter", "Superman", "Vixen", "Wonder Woman"]

justice_league.each_with_index {|member, index| puts "#{index}: #{member}"}
=> 0: Aquaman
1: Batman
2: Black Canary
3: Flash
4: Green Arrow
5: Green Lantern
6: Martian Manhunter
7: Superman
8: Vixen
9: Wonder Woman
=> ["Aquaman", "Batman", "Black Canary", "Flash", "Green Arrow", "Green Lantern",
"Martian Manhunter", "Superman", "Vixen", "Wonder Woman"]

Note that unlike the each_cons and each_slice, each_with_index doesn’t return nil, but the original array, just like each does. each_cons and each_slice are newer methods that were introduced in Ruby 1.9; perhaps Matz and company decided that it made more sense for “each” methods to return nil from now on. each and each_with_index have return the original array as they always have, since making changes to these methods might break a lot of existing Ruby code. Hooray for those little inconsistencies that creep into every programming language!

Enumerable#each_with_index and Hashes

When used with on a hash and given a block, each_with_index iterates through the hash, converting each item in the hash into a two-element array (where the first element is the key and the second element is the corresponding value), and then passing that array and the number representing its order in the original array to the block. Once again, I’ll use the “Enterprise Crew” example:

enterprise_crew = {:captain => "Picard",
                   :first_officer => "Riker",
                   :science_officer => "Data",
                   :tactical_officer => "Worf",
                   :chief_engineer => "LaForge",
                   :chief_medical_officer => "Crusher",
                   :ships_counselor => "Troi",
                   :annoying_ensign => "Crusher",
                   :attractive_ensign => "Ro",
                   :expendable_crew_member => "Smith"}
=> {:captain=>"Picard", :first_officer=>"Riker", :science_officer=>"Data",
:tactical_officer=>"Worf", :chief_engineer=>"LaForge",
:chief_medical_officer=>"Crusher", :ships_counselor=>"Troi",
:annoying_ensign=>"Crusher", :attractive_ensign=>"Ro",
:expendable_crew_member=>"Smith"}

enterprise_crew.each_with_index {|member, index| puts "#{index}: #{member}"}
=> 0: [:captain, "Picard"]
1: [:first_officer, "Riker"]
2: [:science_officer, "Data"]
3: [:tactical_officer, "Worf"]
4: [:chief_engineer, "LaForge"]
5: [:chief_medical_officer, "Crusher"]
6: [:ships_counselor, "Troi"]
7: [:annoying_ensign, "Crusher"]
8: [:attractive_ensign, "Ro"]
9: [:expendable_crew_member, "Smith"]
=> {:captain=>"Picard", :first_officer=>"Riker", :science_officer=>"Data",
:tactical_officer=>"Worf", :chief_engineer=>"LaForge",
:chief_medical_officer=>"Crusher", :ships_counselor=>"Troi",
:annoying_ensign=>"Crusher", :attractive_ensign=>"Ro",
:expendable_crew_member=>"Smith"}

Enumerable#each_with_index Oddities

Blocks with a Single Parameter

What happens if you use each_with_index with a block that has only one parameter? You’d think that the block would accept the item and index passed to it as a two-element array, with the item as the first element and the corresponding index as the second element, but that’s not the case:

# For each item in justice_league, this line will output the following:
# {second letter of member's name} : {first letter of member's name} 
justice_league.each_with_index {|member| puts "#{member[1]}: #{member[0]}"}
=> q: A
a: B
l: B
l: F
r: G
r: G
a: M
u: S
i: V
o: W
=> ["Aquaman", "Batman", "Black Canary", "Flash", "Green Arrow", "Green Lantern"
, "Martian Manhunter", "Superman", "Vixen", "Wonder Woman"]

When each_with_index passes an item and its index to a block that takes only one parameter, the item goes into the paramter, and the index is discarded. Simply put, in this case, each_with_index behaves just like each.

Without a Block

What happens if you use each_with_index without a block to create an Enumerator object that spits out the next item when its next method is called? This:

hero = justice_league.each_with_index
=> #<Enumerable::Enumerator:0x159cb98>

hero.next
=> "Aquaman"

hero.next
=> "Batman"

hero.next
=> "Black Canary"

Note that calling next only yields the next item in the collection; the index information is lost. In this case, each_with_index behaves just like each.

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“Rails to Victory”

Here’s a still from what I assume is a propaganda film from World War II titled Rails to Victory. If any of you are planning to do presentations covering the topic of Rails and are looking for some graphics for your slides, you might want to consider this one:

Opening shot from the film \"Rails to Victory\"
Click the photo to see a larger version.
Photo courtesy of Miss Fipi Lele.

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Take Hampton’s Ruby Survey!

\"Take Hampton\'s Ruby Survey\": Hampton Catlin pointing at his survey, projected on a screen
Photo by rcoder.
Click the photo to see the original on its Flickr page.

Hampton “HAML” Catlin of Unspace (you know, the development shop that put RubyFringe together) has created a survey that he’d like as many people who code in Ruby to take. It’s quick and painless, and he’ll share the data once he’s compiled it.

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Enumerating Enumerable: Enumerable#each_slice

Enumerating Enumerable

Ten installments already? That’s right, this is the tenth Enumerating Enumerable article. As I’m fond of repeating, this is my little contribution to the Ruby community: a series of articles where I attempt to do a better job at documenting Ruby’s Enumerable module than Ruby-Doc.org does, with pretty pictures and more in-depth examples!

In this article, I’m going to cover each_slice, which got introduced in Ruby 1.9.

If you missed any of the earlier articles, I’ve listed them all below:

  1. all?
  2. any?
  3. collect / map
  4. count
  5. cycle
  6. detect / find
  7. drop
  8. drop_while
  9. each_cons

Enumerable#each_slice Quick Summary

Graphic representation of the "each_slice" method in Ruby's "Enumerable" module

In the simplest possible terms Given a number n, split the array into n-element slices (if the number of elements in the array isn’t evenly divisible by n, just put the remaining elements in the last slice), then iterate through those slices.
Ruby version 1.9 only
Expects
  • A number n describing the size of the iteration slice.
  • An optional block to receive the values from each iteration.
Returns
  • nil, if used with a block.
  • An Enumerator object that outputs n-sized slices of the collection, if used without a block.
RubyDoc.org’s entry Enumerable#each_slice

Enumerable#each_slice and Arrays

When used on an array with a block, each_slice takes a numeric argument n and splits the array into slices of length n (if the number of elements in the array isn’t evenly divisible by n, the remaining elements are put into the last slice). each_slice then iterates through the set of slices, passing each slice to the block. After the final iteration, each_slice returns nil.

Since this is yet another case when an showing an example makes things very clear, I’ll do just that, using the same example data I used in the article on each_cons:

justice_league = ["Aquaman", "Batman", "Black Canary", 
                  "Flash", "Green Arrow", "Green Lantern", 
                  "Martian Manhunter", "Superman", 
                  "Vixen", "Wonder Woman"]
justice_league = ["Aquaman", "Batman", "Black Canary", "Flash", "Green Arrow",
"Green Lantern", "Martian Manhunter", "Superman", "Vixen", "Wonder Woman"]

justice_league.each_slice(3) {|team| p team}
=> ["Aquaman", "Batman", "Black Canary"]
["Flash", "Green Arrow", "Green Lantern"]
["Martian Manhunter", "Superman", "Vixen"]
["Wonder Woman"]
=> nil

When used on an array without a block, each_slice takes a numeric argument n and returns an Enumerator that outputs slices of the array when its next method is called. Here’s an example:

teams_of_3 = justice_league.each_slice(3)
=> #<Enumerable::Enumerator:0x007fc73baa9830>

teams_of_3.next
=> ["Aquaman", "Batman", "Black Canary"]

teams_of_3.next
=> ["Flash", "Green Arrow", "Green Lantern"]

teams_of_3.next
=> ["Martian Manhunter", "Superman", "Vixen"]

teams_of_3.next
=> ["Wonder Woman"]

teams_of_3.next
=> StopIteration: iteration reached at end...
# (I'm skipping the rest of the error message)

teams_of_3.rewind
=> #<Enumerable::Enumerator:0x007fc73baa9830>

teams_of_3.next
=> ["Aquaman", "Batman", "Black Canary"]

Enumerable#each_slice and Hashes

When used on an hash with a block, each_slice takes a numeric argument n and splits the hash into arrays of length n (if the number of elements in the array isn’t evenly divisible by n, the remaining elements are put into the last slice). Within these arrays, each hash element is represented as a two-element array, with the first element being the key and the second element being the corresponding value.

each_slice then iterates through the set of arrays, passing each array to the block. After the final iteration, each_slice returns nil.

Here’s an example, using the same example data I used in the article on each_cons:

enterprise_crew = {:captain => "Picard",
                   :first_officer => "Riker",
                   :science_officer => "Data",
                   :tactical_officer => "Worf",
                   :chief_engineer => "LaForge",
                   :chief_medical_officer => "Crusher",
                   :ships_counselor => "Troi",
                   :annoying_ensign => "Crusher",
                   :attractive_ensign => "Ro",
                   :expendable_crew_member => "Smith"}
=> {:captain=>"Picard", :first_officer=>"Riker", :science_officer=>"Data",
:tactical_officer=>"Worf", :chief_engineer=>"LaForge",
:chief_medical_officer=>"Crusher", :ships_counselor=>"Troi",
:annoying_ensign=>"Crusher", :attractive_ensign=>"Ro",
:expendable_crew_member=>"Smith"}
=> nil

enterprise_crew.each_slice(3) {|team| p team}
=> [[:captain, "Picard"], [:first_officer, "Riker"], [:science_officer, "Data"]]
[[:tactical_officer, "Worf"], [:chief_engineer, "LaForge"],[:chief_medical_officer, "Crusher"]]
[[:ships_counselor, "Troi"], [:annoying_ensign, "Crusher"], [:attractive_ensign,  "Ro"]]
[[:expendable_crew_member, "Smith"]]
=> nil

When used on a hash without a block, each_slice takes a numeric argument n and returns an Enumerator that outputs arrays when its next method is called. Here’s an example:

away_teams_of_3 = enterprise_crew.each_slice(3)
=> #<Enumerable::Enumerator:0x007fc73ba44c50>

away_teams_of_3.next
=> [[:captain, "Picard"], [:first_officer, "Riker"], [:science_officer, "Data"]]

away_teams_of_3.next
=> [[:tactical_officer, "Worf"], [:chief_engineer, "LaForge"], [:chief_medical_officer, "Crusher"]]

away_teams_of_3.next
=> [[:ships_counselor, "Troi"], [:annoying_ensign, "Crusher"], [:attractive_ensign, "Ro"]]

away_teams_of_3.next
=> [[:expendable_crew_member, "Smith"]]

away_teams_of_3.next
=> StopIteration: iteration reached at end...
# (I'm skipping the rest of the error message)

away_teams_of_3.rewind
=> #

away_teams_of_3.next
=> [[:captain, "Picard"], [:first_officer, "Riker"], [:science_officer, "Data"]]