Enumerating Enumerable: Enumerable#group_by

Enumerating Enumerable

Once again, it’s Enumerating Enumerable time! This is the latest in my series of articles where I set out to make better documentation for Ruby’s Enumerable module than’s. In this installment — the seventeenth in the series — I cover the group_by 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
  15. first
  16. grep

Enumerable#group_by Quick Summary

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

In the simplest possible terms Break a collection into groups based on some given criteria.
Ruby version 1.9 only
Expects A block containing the criteria by which the items in the collection will be grouped.
Returns A hash where each key represents a group. Each key’s corresponding value is an array containing the members of that group.’s entry Enumerable#group_by

Enumerable#group_by and Arrays

When used on an array, group_by iterates through the array, passing each element to to the block. The result value of the block is the group into which the element will be placed.

Example 1

For the first example, I’ll use some code similar to the example given in’s writeup of group_by:

(0..15).group_by {|number| number % 3}
=> {0=>[0, 3, 6, 9, 12, 15], 1=>[1, 4, 7, 10, 13], 2=>[2, 5, 8, 11, 14]}

In the code above, the numbers 0 through 15 are passed to the block, which receives each number as the parameter number. The group that each number is placed into is determined by the result value of the block, number % 3, whose result can be one of 0, 1 or 2. This means that:

  • The resulting hash will have three groups, represented by the keys 0, 1 and 2
  • The key 0‘s corresponding value is an array containing the numbers in the range (0..15) that are evenly divisible by 3 (i.e. the numbers for which number % 3 is 0.
  • The key 1‘s corresponding value is an array containing the numbers in the range (0..15) that when divided by 3 leave a remainder of 1 (i.e. the numbers for which number % 3 is 1.
  • The key 2‘s corresponding value is an array containing the numbers in the range (0..15) that when divided by 3 leave a remainder of 2 (i.e. the numbers for which number % 3 is 2.

Example 2

In the first example, the keys in the resulting hash are the same type as the values in the array whose contents we’re grouping. In this example, I’ll show that the keys in the resulting hash don’t have to be the same type as the values in the array.

simpsons = %w(Homer Marge Bart Lisa Abraham Herb)
=> ["Homer", "Marge", "Bart", "Lisa", "Abraham", "Herb"]

simpsons.group_by{|simpson| simpson.length}
=> {5=>["Homer", "Marge"], 4=>["Bart", "Lisa", "Herb"], 7=>["Abraham"]}

In the code above, each Simpson name is passed to the block, which receives it as the parameter simpson. The block’s result is the length of simpson, and this result is the group into which the name will go.

In the resulting hash:

  • Note that the keys are integers while the names in the groups are strings.
  • The key 5‘s array contains those names in Simpsons that are 5
    characters in length.
  • The key 4‘s array contains those names in Simpsons that are 4 characters in length.
  • The key 7‘s array contains those names in Simpsons that are 7 characters in length.

Example 3

In the previous two examples, the keys for the resulting array were calculated from the values in the initial array. In this example, I’ll demonstrate that the keys for the groupings can be determined in a completely arbitrary fashion that has nothing to do with the values:

# Put the Simpsons into randomly determined groups
simpsons.group_by{rand(3) + 1}
=> {3=>["Homer", "Bart", "Abraham", "Herb"], 1=>["Marge", "Lisa"]}

# Let's try that again. The results are very likely to be different:
simpsons.group_by{rand(3) + 1}
=> {1=>["Homer", "Bart"], 2=>["Marge", "Lisa", "Herb"], 3=>["Abraham"]}

# One more time!
simpsons.group_by{rand(3) + 1}
=> {2=>["Homer", "Bart", "Lisa"], 3=>["Marge", "Herb"], 1=>["Abraham"]}

Enumerable#group_by and Hashes

When used on a hash, group_by 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.

Example 1

In this example, we’ll group the cast of Family Guy by the item that they’re bringing to a potluck dinner:

potluck = {"Peter" => "lasagna",
           "Lois"  => "potato salad",
           "Chris" => "lasagna",
           "Meg"   => "brownies",
           "Stewie" => "chateaubriand",
           "Brian" => "potato salad",
           "Evil Monkey" => "potato salad"}
=> {"Peter"=>"lasagna", "Lois"=>"potato salad", "Chris"=>"lasagna", "Meg"=>"brownies",
"Stewie"=>"chateaubriand", "Brian"=>"potato salad", "Evil Monkey"=>"potato salad"}

# Here's one way to do it:
potluck.group_by{|person, bringing| bringing}
=> {"lasagna"=>[["Peter", "lasagna"], ["Chris", "lasagna"]], "potato salad"=>[["Lois", "potato salad"],
["Brian", "potato salad"], ["Evil Monkey", "potato salad"]], "brownies"=>[["Meg", "brownies"]],
"chateaubriand"=>[["Stewie", "chateaubriand"]]}

# Here's another way to do it:
potluck.group_by{|person| person[1]}
=> {"lasagna"=>[["Peter", "lasagna"], ["Chris", "lasagna"]], "potato salad"=>[["Lois", "potato salad"],
["Brian", "potato salad"], ["Evil Monkey", "potato salad"]], "brownies"=>[["Meg", "brownies"]],
"chateaubriand"=>[["Stewie", "chateaubriand"]]}

Example 2

In the previous example, the groupings were based on a calculation performed on the objects in the original hash. In this example, the groupings will be random: a random number generator will determine whose car each potluck attendee will ride to the potluck dinner:

potluck.group_by {[:peters_car, :quagmires_car, :clevelands_car][rand(3)]}
=> {:peters_car=>[["Peter", "lasagna"], ["Chris", "lasagna"], ["Evil Monkey", "potato salad"]],
:quagmires_car=>[["Lois", "potato salad"], ["Meg", "brownies"], ["Stewie", "chateaubriand"]],
:clevelands_car=>[["Brian", "potato salad"]]}

# Let's try another random grouping
potluck.group_by {[:peters_car, :quagmires_car, :clevelands_car][rand(3)]}
=> {:peters_car=>[["Peter", "lasagna"], ["Meg", "brownies"]], :quagmires_car=>[["Lois", "potato salad"],
["Stewie", "chateaubriand"], ["Brian", "potato salad"], ["Evil Monkey", "potato salad"]],
:clevelands_car=>[["Chris", "lasagna"]]}

# One more time!
potluck.group_by {[:peters_car, :quagmires_car, :clevelands_car][rand(3)]}
=> {:peters_car=>[["Peter", "lasagna"], ["Chris", "lasagna"], ["Stewie", "chateaubriand"]],
:quagmires_car=>[["Lois", "potato salad"], ["Evil Monkey", "potato salad"]], :clevelands_car=>[["Meg", "brownies"],
["Brian", "potato salad"]]}


Blast from the Past: My Old Notes on the GNUtella Protocol

Found these photos while digging through my archives on the backup hard drive and thought you folks might find them interesting. They’re flipchart pages from either June 2000, back when I was working at Cory Doctorow’s startup, OpenCola. These are notes that I wrote on the GNUtella protocol; Chris Cummer and I worked off these notes to produce an app called COLAvision, which scored Gnutellanet for audio and video files, captured them and cued them up to be served as streams (oh, the assignments we got during the bubble…).

I expect to be doing a fair bit of doodling, since the b5media tech office walls are largely floor-to-ceiling whiteboards and I’ve also been diving into Dan Roam’s book, The Back of the Napkin, a book on solving problems and selling ideas with hand-drawn pictures on whiteboards, flipcharts, notepads, scrap paper or even — as the title implies — the back of a napkin (or serviette to you Brits out there).

Joey deVilla's GNUtella protocol notes, circa summer 2000.
My notes on the GNUtella protocol, part 1.

Joey deVilla's GNUtella protocol notes (part 2), circa summer 2000.
My notes on the GNUtella protocol, part 2.


Pair Programming Chairs

The development tool company Cenqua have a cute gag ad for a pair programming chair called the “PairOn” — an Aeron built for two:

Cenqua's "PairOn" chair

They list the PairOn’s “key features” as:

  • Fully unit-tested in our ego-free ergonomics lab
  • Essential office furniture for any eXtreme XP Pair (XXPP)
  • Fully adjustable via individual or pair control
  • Can be levered to standup-meeting height
  • 40-hour-week alarm buzzer built in
  • Available in a range of attractive colours

Here’s a pair programming chair that really exists: it’s a set of drawers that doubles as a stool:

TSOT's "Pair Programming" filing cabinate/chair

These are drawer sets mounted on some very good wheels and topped with well-padded upholstery. If you wanted to pair program with someone, you’d simply pull the drawer unit out from under their desk and sit on it. These things were surprisingly comfortable; I’ve spent upwards of two hours sitting on one of these with nary a complaint.

Alas, these aren’t available in stores. They were custom-built for a startup for whom I briefly worked, whose CEOs was more in love with the idea of running a start-up than actually running a start-up. Their last programmer recently ditched them, and I’m hoping to pick up one of these in their bankruptcy sale.


Taking IE8 Beta 2 for a Test Drive, Part 1: “Porn Mode” (a.k.a. InPrivate Browsing)

The IE8 USB key in my computer

Last night, I attended a special sneak preview for Internet Explorer 8 Beta 2 organized by the folks at High Road Communications, who do the PR for Microsoft here in Toronto. Pete LePage, Product Manager of Internet Explorer Developer Division, did the presentation, and also present were Elliot Katz, Senior Product Manager for Microsoft Canada, Daniel Shapiro, Microsoft Canada’s Audience Manager, and my friend and fellow DemoCamp steward David Crow, Tech Evangelist for Microsoft Canada.

Let me get the disclosure part out of the way. Attending this event got me:

  • Free drinks and snacks during the presentation and a free dinner afterwards,
  • One Internet Explorer 8 gym water bottle with a tag inside it saying “BPA Free”,
  • and one 1GB USB key containing installers for IE8 (pictured in my laptop above) and the IE8 Evaluators’ Guide (a Word document that walks you through IE8’s features).

I’ve been to a couple of these Microsoft events before. The one about their “Windows Live” sites didn’t interest me at all, and the Vista one I attended was largely for people who did IT at companies with 1000 or more employees, which really isn’t my area of interest either (and the Vista preview installer they gave me resulted in disaster). This one was a considerably more interesting, as Pete put on a good presentation and it appears that Microsoft is making an effort to match the competing browsers.

Over the next little while, I’ll post articles covering my experiences as I take IE8 for a spin. In this article, I’ll mostly be talking about InPrivate Browsing, which is colloquially known as “Porn Mode”.

“Porn Mode”, a.k.a. “InPrivate Browsing”

The implementation of a browser session in which history, cache and other “trails of breadcrumbs” are deleted as soon as the session is over isn’t new: Apple’s Safari has a “Private Browsing” feature and there’s a Firefox extension that provides the same utility. However, for those not using Macs and especially those who aren’t the type to download and install Firefox and then install a plugin — and there are lots of these people out there — IE8 may be their first opportunity to try out such a feature.

Banking, Not Wanking

In his presentation, Pete was careful to take the “Banking, not wanking” approach when covering InPrivate Browsing, suggesting all sorts of non-saucy uses for the feature, including doing online banking, shopping for surprise presents for your spouse, surfing from a public terminal and so on. The Microsoft people present took my constant referring to it as “Porn Mode” in great stride, and I thank them for having a sense of humor about it.

The Problem

Convenience features like history, cache, automatic username and password field-filling are handy, but they sometimes have unintended consequences. For instance, suppose you, as a healthy, open-minded adult, like to look at videos featuring ladies without pants sitting on cakes at Let’s also suppose that a friend asks to borrow your computer for a moment to see a funny cat video at As your friend types in the letters for “” in the address bar, this happens:

Screen capture: A user starts to type in "" and as "you" is formed, my "" history appears.

This sort of browser-assisted embarrassment takes place more often than you might think. I’ve seen it happen firsthand, and it’s done everything from causing a little red-facedness to actually thwarting romantic possibilities. And you thought computers were supposed to make our lives easier!

The IE8 solution, InPrivate Browsing, is accessible through the Safety menu (shown below) or through the control-shift-P key combo:

Screen Shot: IE8's "Safety" menu, with "InPrivate Browsing" selected

This opens up a new, separate browser window for InPrivate Browsing, which does not keep “breadcrumbs” like history, cache data, cookies and so on. The address bar for InPrivate Browsing windows has the InPrivate logo as a visual cue that this particular session won’t leave a trail that will embarrass you or give away your secrets:

Screen Shot: A new "InPrivate Browsing" window appears

Maybe it’s me, but I think the “InPrivate” graphic in the address bar is a bit too subtle. Then again, a more obvious visual indicator (say, giving the InPrivate browser window a different color) might be an invitation to shoulder-surf.

Hey man, I had to see if it works, right?

Screen Shot: YouPorn's title page

I swear, I had to poke about the site a little bit in order to test if my History was being saved. It’s all in the name of application testing!

Screen Shot: Blurred-out YouPorn video page

After a little “research”, I closed not just the InPrivate Browsing window, but the whole browser, then started it up again. Then I proceeded to type “You” into the address bar. Under normal circumstances, my history would be there for all to see. But it wasn’t!

Screen shot: None of my InPrivate browsing history shows up

For those of you who need to clear the cache, cookies, history or other data for any reason, there’s also the Delete Browsing History item in the Safety menu:

"Safety" menu with "Delete Browsing History" item selected

And it provides a number of deletion options:

The "Delete Browsing History" dialog box

And there you have it: a quick tour of IE8’s much-snickered-about “Porn Mode”.

Keep watching the blog for more posts about IE8 as I use it more and cover its features. Perhaps I’ll cover the development tools next.


The Funniest “Unboxing” Video I’ve Seen

This one’s for the Samsung Omnia (a.k.a. the i900):


b5media is a Red Herring Canada Top 50 Finalist

Red Herring "Top 50 Canada" badge and b5media logo

b5media, the company where I work and hold the title of “Nerd Wrangler”, is one of the finalists in Red Herring Canada’s “Top 50” Awards, which is described as…

…celebrating the most innovative and promising companies in Canada. Adding to the success of our yearly North America 100, Europe 100 and Global 100 events, Red Herring Canada will provide an opportunity for the nation’s Top 50 companies to deliver their message to the top VCs and other participants throughout North America and abroad.

The Top 50 will be announced at the Red Herring Canada ’08 event in Mont Tremblant (near Montreal), which takes place from September 15th through 17th.


I’m on “Future Tense” Tonight

John Gordon, host of "Future Tense"On Friday, Jon Gordon from the radio show Future Tense interviewed me about my recent posting here titled Seinfeld and Celebrity Computer Endorsements, which features my comments on Microsoft’s hiring of Jerry Seinfeld as a pitchman and a collection of celebrity computer endorsements. In the interview, which is being broadcast tonight, we talk about whether celebrity endorsements for computers work, as well as some of our favorite celebrity computer advertisements.

Future Tense is broadcast on American Public Media stations. For a schedule of when it’s on, click here.