WEBVTT

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Hello. Today, we will learn
about variables

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and the types available in Pharo.

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In fact, there are two big types:

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local variables always start
with lowercase by convention;

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we will see the local types available.

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The second big group
is shared variables.

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They start with uppercase by convention.

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There are also several types.

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Let's start with local variables
which start with lowercase.

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Local variables have one method.

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They are declared between pipes,

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the vertical bars.

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Here, there's a local variable, c:

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it's local to the method
and exists only for its execution.

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Therefore, it must always
start with lowercase.

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By the way, all the class names
start with uppercase

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because they are shared in the system.

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Other types of local variables:

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for example, there are
instance variables in the object

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which are local to it.
There are two of them here: x and y.

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They always start with lowercase.

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As for local variables,

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there are also
the methods' arguments, here.

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In aPoint, "a" is lowercase.

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because this variable
is also local to the method.

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There are also block parameters.

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For :x, x is a local variable
to a block.

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A block argument, sorry.

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So here, it starts with lowercase.

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In Pharo, there are
six specific variables,

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called special variables;
you can't change their names.

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They are reserved words in Pharo.

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There are more than 50 of them
in Java or JavaScript;

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in Pharo, only six.

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These are true, false, nil, self, super,
and thisContext.

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Let's figure out
what these reserved words are for.

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true and false
are instances of Booleans:

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true is the unique instance
of the class True;

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false, the unique instance
of the class False.

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nil is the unique instance
of the class UndefinedObject.

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So true, false, and nil
are objects like any other,

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except they are the unique instances
of a specific class in the system.

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Next, there are three pseudo-variables:

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self refers to the current receiver,
and it's equivalent to this in Java.

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super always refers
to the current receiver,

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but this time, when you send
a message via super,

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it means the method lookup,
the selection method algorithm,

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which we will see in another class,

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will start in the superclass
where the keyword super is written.

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thisContext is a pseudo-variable
which refers to the object

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that reifies the program's call stack.

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It's an advanced concept
we'll study at the end of this MOOC.

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We will explain it to you later.

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For now, you just need to know
it's a pseudo-variable.

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Now, shared or global variables.

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First, you have all the classes.

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All class names start with uppercase

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because I can reference a class
from anywhere in the system.

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Be careful: what starts
with uppercase is not always a class.

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For example, Transcript:

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it's an object name
shared in the system.

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Everyone can reference this object.

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And it's standard output,
stdout in other languages.

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It's an instance object of a class,
globally accessible.

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So its name starts with uppercase.

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There are also class variables,

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which are declared here when declaring
a class in classVariableNames;

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here, the class #CombinedChar
defines two class variables,

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one called Compositions
and the other Decompositions.

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These two class variables

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are accessible by the instances
of the class #CombinedChar

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and of its subclasses.

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That's why they're called
shared variables.

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All the instances of the class
and its subclasses share them.

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So both of them start with uppercase.

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To summarize, you must remember:

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all local variables, local to an object,
a block, or a method,

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start with lowercase;

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all global variables,
such as class names, etc.,

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start with uppercase.