WEBVTT

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In this course,
we'll study the essential elements

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of the collections hierarchy in Pharo.

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We'll see that Pharo
has many types of collections.

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They make a programmer's life easier

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as they all share the same API.

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We'll also see the difference
between literal and dynamic collections.

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The collections' API is very diverse:
there are many types of collections.

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They all share a common API
which is well organized

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and make a programmer's life easier.

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What's specific about Pharo

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is that the first element is at index 1

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instead of 0 in other languages.

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Collections can contain
any types of objects in Pharo.

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It's not always the case
with other languages.

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Let's see some of the most remarkable
and widely used collections:

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OrderedCollection
is a dynamic collection

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which grows
whenever you add elements in it.

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Array is a collection of fixed size.

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You can access the elements
according to an index, Set,

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which contains elements
without duplicates.

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You can't insert the same element twice.

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Dictionaries are hash tables:
to one key is associated one value.

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This is an excerpt
of the collection hierarchy.

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It's only an excerpt:
in Pharo, it's bigger.

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There are many classes.
They all have collections

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with a shared API
for all the collections.

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In this course, I'll tell you
about those in bold type.

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There's a shared API
with seven elements:

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specific methods for the creation
of collections sent to the classes;

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specific methods
to access their properties

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(the size of the collections

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or even the elements they contain);

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testing methods
(is the collection empty?);

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elements adding and removing methods;
collection elements enumeration

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(I want to browse the elements
or know whether an element is there);

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and conversion methods
from one type to another.

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Let's start with an example:
I want to create a collection in Pharo.

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I'll just select
the class I'm interested in

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and send the message new
to instantiate this class.

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First option: I use new.

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Second option: I can't use new directly.

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I want to specify the size
of the collection.

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I could send new: 4.
I create a size-4 or size-2 array.

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It also work with OrderedCollection.
I could create one of size 1000.

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There are other existing methods
to create pre-initialized collections:

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withAll: for instance.

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I'll pass a literal collection.
A literal collection starts with #(.

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I'll create a new instance
of OrderedCollection

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which will contain all the elements
passed when it was created.

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I could do the same with Set.

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In a set, you can't have any duplicates.

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The number 7, which you have twice
in the literal collection,

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can't end up twice in Set.

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There are other types of messages
you can send

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to the classes to initialize them.

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This is another example: new: withAll:.

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I want a size-5 collection.

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Every element must be initialized
with a specific object:

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in this case, a string containing a.

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Remember that in Pharo,
every collection starts at index 1.

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If I ask this 3-element collection

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to return the element at index 2,
it'll be this one.

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This is 1; this is 2; this is 3.

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It's the same with OrderedCollection:

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If I convert this collection
and ask it to return its index 2,

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I'll get the same result: hates.

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Collections can contain
any types of objects.

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I'll show you one example:

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this literal collection
contains the string calvin

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and a collection
containing the numbers 1 2 3.

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I can create an array.

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This array is composed
of the elements 1 2

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and of Set at the end.

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I added the element 1 here,
then the element 2, then Set.

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You can browse
the elements of a collection

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by using the message do: for instance.

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This is a collection

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to which I'll send the message do:
before passing it a block.

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The block starts with "["
and ends with "]".

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The block's parameter is called :each.

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It is separated
from the block's body by |.

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With each loop, the value of :each
is the collection's 1st element,

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then the second, etc.

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And on the transcript you have:
Calvin hates Suzie.

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Arrays are fixed-size collections.

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You can query an array for its size:
I send the message size.

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You can directly access an array's
element by sending at:.

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I want the second element.

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You can modify the element
at index 2 in the collection

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by sending at: 1 put: 'Calvin':

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I insert the string Calvin
in cell 1.

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I can also ask the size.

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What's interesting about this example

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is that the same array
has been built in two ways:

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first a literal version,
then a dynamic version.

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Here, I instantiated
the class Array by myself

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and filled in every cell.

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You can send size to a collection
to learn about its size.

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You can access a collection's element
by using the method at:.

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I already mentioned it.

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Be careful:

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when you access an element
by providing an index,

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you must ensure that the index

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is within the collection's bounds:
it should be smaller than its size.

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If I ask this collection to return
the element at index 55, it won't exist.

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It'll return an error.

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To modify the elements:

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at index 2, I want to insert
a new element in the collection.

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The string loves
will replace the string hates.

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You can see it in the result.

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Literal arrays:

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this is an example of a literal array.

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It starts with #(, as I told you.

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You can put anything inside:
a number, a string, etc.

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Every literal array in Pharo

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are instance of the class Array
by default.

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I can send the message class
to a literal array.

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It returns Array, which means
it's instance of the class Array.

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Dynamic and literal versions
are equivalent in Pharo.

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Literal arrays are simply shorter:
you can write them faster.

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Here you have a collection's
literal version.

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And that's the dynamic version
where I instantiate the class Array.

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They're equivalent
as they return the same results.

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The class OrderedCollection
defines a specific collection

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which can be extended.

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Whenever you add elements, it grows.

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I instantiate OrderedCollection
by sending the message new.

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I use the method add: to add
new elements in this collection.

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I could use addFirst: to add an element
at the beginning of the collection.

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By default, it's at the end.

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You can see what the collection returns.

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It is composed of three elements:
Pharo, Reef, Pharo.

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With add: 'Seaside',
'Seaside' is added at the end.

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I have conversion methods
between a collection type and another.

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Here I'm using a literal collection,
which is an array.

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The message asOrderedCollection
will turn this array

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into an ordered collection.

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Sets are a type of collection
without duplicates.

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They can be extended:
they grow with every added element.

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I can use a literal collection
I turn into a Set.

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I end up with a Set
without any duplicates.

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I could also choose the dynamic version
over the literal one:

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Set with: with: creates a Set
and fills it with two elements,

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that's two Sets each time.

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Conversion methods are convenient to
turn a collection into something else.

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It's always the same: as + the name
of the collection you want to get.

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Dictionaries are a group
of collections key/values:

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I associate a value to a key.

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They can be extended:
they grow with every added element.

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There's also an unusual API

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when it comes to this collection:

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typical at: message; at:ifabsent:

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(what do I return if I want to access
a specific key that doesn't exist?);

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at:put: insert a new value
into a specific key.

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I can iterate using typical messages
such as do:,

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but also new messages

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such as keysDo: which browses
all the dictionary's keys

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or keys and values.

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For instance, I create an instance
of the class Dictionary.

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Picture the Dictionary as an array:

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to the key January, I associate 31;
to the key February, 28;

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to the key March, 31.

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It's equivalent to a dynamic collection.

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You create a dynamic collection using {:

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{ and }.

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You use arrows to create associations.

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This is a symbol.

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To the symbol January
I associate the number 31.

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I have a collection of associations

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I turn into a dictionary
with asDictionary.

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These two ways of creating a dictionary
are equivalent.

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If I query association for its key,

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it will return the key,
so the beginning.

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It's the same.

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If I query an association for its value,
it'll only return the value.

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This is a pair or an association.

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Dictionaries:

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if I want to access a specific value
in a Dictionary,

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I just need to use at: and to specify
the key whose value I want.

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If the key doesn't exist,

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I will get an error in return.

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To avoid that,
I can use at: ifAbsent:.

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I write at: plus a key
that doesn't exist in the Dictionary.

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If it is absent,
it will return this value, 0.

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The key doesn't exist in the Dictionary,
so I get the value 0.

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I can iterate over a Dictionary:
if use do: over its elements,

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I get the Dictionary's values only.
You can't see the keys.

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One might wonder why
as it's quite strange.

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It's very logical, actually.

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If you look at the way do:, taking
a block, is implemented in Dictionary,

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what happens is ^self valuesDo:.

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By default, if you apply do:
to a Dictionary,

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you only browse its values,
not the keys.

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If I want to browse both,

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I must use a specific method
called keysAndValuesDo:

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which takes a block with two arguments
(:k and :v) as parameter.

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:k is a key
and v: is the value.

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The Dictionary is full.

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In this course, we learned that

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Pharo has plenty of different types
of collections available.

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The collections have a common vocabulary

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when it comes to creating them,

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accessing their elements,
their size, etc.

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It makes a programmer's life easier.

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It's easy to convert a collection
into another type.

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We even learned something more:

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when you have questions, it's easy
to look into the system, into Pharo,

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by reading the code of the classes
to learn new classes of collections.