Creating a dictionary Object from an Array

authors.md

@@ -20,6 +20,7 @@ The following people are totally rad and awesome because they have contributed r
 * Jeff Pickhardt *pickhardt (at) gmail (dot) com*
 * Frederic Hemberger
 * Mike Hatfield *oakraven13@gmail.com*
+* [Anton Rissanen](http://github.com/antris) *hello@anton.fi*
 * ...You! What are you waiting for? Check out the [contributing](/contributing) section and get cracking!
 
 # Developers

chapters/arrays/creating-a-dictionary-object-from-an-array.md

@@ -0,0 +1,63 @@
+---
+layout: recipe
+title: Creating a dictionary Object from an Array
+chapter: Arrays
+---
+## Problem
+
+You have an Array of Objects, such as:
+
+{% highlight coffeescript %}
+cats = [
+  {
+    name: "Bubbles"
+    age: 1
+  },
+  {
+    name: "Sparkle"
+    favoriteFood: "tuna"
+  }
+]
+{% endhighlight %}
+
+But you want to access it as a dictionary by key, like `cats["Bubbles"]`.
+
+## Solution
+
+You need to convert your array into an Object. Use reduce for this.
+
+{% highlight coffeescript %}
+# key = The key by which to index the dictionary
+Array::toDict = (key) ->
+  @reduce ((dict, obj) -> dict[ obj[key] ] = obj if obj[key]?; return dict), {}
+{% endhighlight %}
+
+To use this:
+
+{% highlight coffeescript %}
+  catsDict = cats.toDict('name')
+  catsDict["Bubbles"]
+  # => { age: 1, name: "Bubbles" }
+{% endhighlight %}
+
+## Discussion
+
+Alternatively, you can use an Array comprehension:
+
+{% highlight coffeescript %}
+Array::toDict = (key) ->
+  dict = {}
+  dict[obj[key]] = obj for obj in this when obj[key]?
+  dict
+{% endhighlight %}
+
+If you use Underscore.js, you can create a mixin:
+
+{% highlight coffeescript %}
+_.mixin toDict: (arr, key) ->
+    throw new Error('_.toDict takes an Array') unless _.isArray arr
+    _.reduce arr, ((dict, obj) -> dict[ obj[key] ] = obj if obj[key]?; return dict), {}
+catsDict = _.toDict(cats, 'name')
+catsDict["Sparkle"]
+# => { favoriteFood: "tuna", name: "Sparkle" }
+{% endhighlight %}