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Enumerating Enumerable: Enumerable#collect/Enumerable#map

Here we are the third installment of Enumerating Enumerable, my attempt to do a better job of documenting Ruby’s Enumerable module than RubyDoc.org does. In this installment, I cover Enumerable#collect and its syntactic sugar twin (and the one I prefer), Enumerable#map.

In case you missed the earlier installments, they’re listed (and linked) below:

  1. all?
  2. any?

Enumerable#collect/Enumerable#map Quick Summary

Graphic representation of Ruby\'s \"Enumerable#collect / Enumerable#map\" methods

In the simplest possible terms Create a new array by performing some operation on every item in the given collection. collect and map are synonyms — you can use either. I personally prefer map as it’s shorter and makes sense if you think of the method as being like functional mapping.
Ruby version 1.8 and 1.9
Expects A block containing the criteria. This block is optional, but you’re likely to use one in most cases.
Returns An array made up of items created by performing some operation on the given collection.
RubyDoc.org’s entry Enumerable#collect / Enumerable#map

Enumerable#collect/Enumerable#map and Arrays

When used on an array and a block is provided, collect/map passes each item to the block, where the operation in the block is performed on the item and the result is then added to the result array. Note the the result array has the same number of elements as the given array.

[1, 2, 3, 4].map {|number| number ** 2}
=> [1, 4, 9, 16]

["Aqua", "Bat", "Super", "Wonder Wo"].map {|adjective| adjective + "man"}
=> ["Aquaman", "Batman", "Superman", "Wonder Woman"]

When the block is omitted, collect/map uses this implied block: {|item| item}, which means when applied on an array without a block, collect/map is the identity function — the resulting array is the same as the given array.

[1, 2, 3, 4].map
=> [1, 2, 3, 4]

["Aqua", "Bat", "Super", "Wonder Wo"].map
=> ["Aqua", "Bat", "Super", "Wonder Wo"]

Enumerable#collect/Enumerable#map and Hashes

When used on a hash and a block is provided, collect and map pass 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.

Each key/value pair is passed to the block, where the operation in the block is performed on the item and the result is then added to the result array. Note the the result array has the same number of elements as the given array.

burgers = {"Big Mac" => 300, "Whopper with cheese" => 450, "Wendy's Double with cheese" => 320}

# What if I had just half a burger?
burgers.map {|burger| burger[1] / 2}
=> [160, 150, 225]

burgers.map {|sandwich, calories| calories / 2}
=> [160, 150, 225]

burgers.map {|burger| "Have a tasty #{burger[0]}!"}
=> ["Have a tasty Wendy's Double with cheese!", "Have a tasty Big Mac!",
 "Have a tasty Whopper with cheese!"]

burgers.map {|sandwich, calories| "Have a tasty #{sandwich}!"}
=> ["Have a tasty Wendy's Double with cheese!", "Have a tasty Big Mac!",
 "Have a tasty Whopper with cheese!"]

burgers.map {|sandwich, calories| ["Half a #{sandwich}", calories / 2]}
=> [["Half a Wendy's Double with cheese", 160], ["Half a Big Mac", 150],
 ["Half a Whopper with cheese", 225]]

When the block is omitted, collect/map uses this implied block: {|item| item}, which means when applied on an hash without a block, collect/map returns an array containing a set of two-item arrays, one for each key/value pair in the hash. For each two-item array, item 0 is the key and item 1 is the corresponding value.

burgers = {"Big Mac" => 300, "Whopper with cheese" => 450, "Wendy's Double with cheese" => 320}

burgers.map
=> [["Wendy's Double with cheese", 320], ["Big Mac", 300], ["Whopper with cheese", 450]]

Special Case: Using Enumerable#collect/Enumerable#map on Empty Arrays and Hashes

When applied to an empty array or hash, with or without a block, collect and map always return an empty array.

# Let's try it with an empty array
[].map
=> []

[].map {|item| item * 2}
=> []

# Now let's try it with an empty hash
{}.map
=> []

{}.map {|sandwich, calories| "Have a tasty #{sandwich}!"}
=> []

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How Map/Reduce/Filter Can Rock Your World

Better late than never: C# 3.0 will feature map, reduce and filter, and Dare “Carnage4Life” Obasanjo explains how to rock these features. He concludes the article with “If your programming language doesn’t support lambda functions or have map/reduce/filter functions built in, you just might be a Blub Programmer who is missing out on being more productive because your programming language doesn’t support ‘esoteric’ or ‘weird’ features,” which sounds kind of weird in an article about C#, a language that could only be Blubbier if its name were BlubLang or Blub#. I suspect I’ve woken up in some upside-down parallel universe again!