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Today we're going to review
something you've already learned,

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but I really want to bring it home.

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Characters, Strings and Symbols.

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What you will learn
is how to handle characters,

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how strings are represented,

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and what a symbol is in relation
to a string.

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Let's take it from the start:
Characters.

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In Pharo,
characters start with dollar sign,

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followed by a letter.

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This is to represent literal characters.

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Non-printable characters
that do not have a form

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use space, tab or carriage return,

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which jumps to a new line.

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We send a message
to this character class

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to get this character.

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Now for strings.

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Strings of characters
are delimited by single quotes.

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This character right here.

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Here we have the string:
'eclair au chocolat'.

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It starts here and ends here.

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We can send it a message,
for example, size.

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And it sends back "18".

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Indeed, this string
contains 18 characters.

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We can even play with this string.

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I can say character space split.

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This means I will cut up this string
according to spaces,

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and I get an OrderedCollection
with 3 elements.

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The string was cut up into 3 elements.

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Strings of characters
are collections like any other in Pharo.

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A string is really
a collection of characters,

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but the string class will inherit
from the collection class.

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So I can use all of the classic methods.

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Remember,
we have common collection classes.

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For example, "at:"
for all the indexed collections.

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'eclair au chocolat' at: 1

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means I want element "1"
of this collection and I get "e".

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It corresponds to this "e".

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I can also use "do:" which runs through
the elements of a collection.

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And after every loop, the block setting

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equals the first element
of the collection, then the second, etc.

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One small subtlety.

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Since strings of characters
are delimited by single quotes,

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if I want to put code in a string,

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the trick is to use two quotes,
side by side.

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This means I want to insert one quote
into my string of characters...

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But be careful, the subtlety is that

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it counts as only
one element in the string.

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For example:
'L''eclair au chocolat' at: 2

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means I want element 2
of this collection.

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And it sends back the quote character.

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And if I ask for at: 3
I get the "e" that is here.

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So even if I enter two single quotes,
they count as one.

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Strings of characters
are treated like any other collection.

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I can say that I want
the last element of a string.

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If I ask for "str at: str size"
I get the very last element.

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Don't forget, in Pharo
strings begin with index value 1,

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therefore the last element
is the size of the string.

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Or we can simply use
"last" to recover the last element.

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To generate strings,
there are various techniques.

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The easiest one
is the conversion method.

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I convert a symbol into
a string of characters with asString.

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Or else I can send
printString to any object.

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This will give me
a string representation of the object.

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Or I can generate a creation method
directly from a collection.

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Remember, you can send “with”
to any collection class

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to generate a new collection that
automatically contains certain objects.

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If I use String with: $A,

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I get a string
that contains one single character: "A."

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For concatenation,

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you've already seen this,
we use the "comma" message.

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I send "," to a string
and run it through another string

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to create a new string that is
the concatenation of the two.

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Nonetheless, be careful.

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If I take the same example,
you must understand

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that I generated
intermediate versions that are useless.

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So I have this first message
that was sent

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to this string, with this value.

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We have a first concatenation string
that was generated.

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This concatenation sent the message
"," with this value,

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and another string was created:
The final result.

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An intermediate string
we haven't seen was generated,

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and it makes you lose time
in your calculations.

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You had a session on benchmarking.

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You can use benchmarking
to really see what's going on

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and whether it wouldn't be
worthwhile to use a stream.

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I generate a string of characters

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and I say that I will define
a streamContent with :s

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And I can directly send strings to this
stream to generate the final string.

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This way, I avoid intermediary strings.

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Now for symbols.

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Syntactically speaking,
symbols start with a hashtag,

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then a string of characters.

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This is a literal symbol.

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It's a kind of string, but be careful,
they're unique in the system.

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If I write hashtag "blah blah"
anywhere in my program,

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anywhere else
in the program this appears,

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I'm designating the same object.

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It's the same instance.

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So symbols are unique.

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Here we have #calvin == #calvin
and it sends back true.

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It's the same object.

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This is not the case for strings.
For strings, it depends.

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It depends on
the compiler's optimizations.

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So this is the real difference
between symbols and strings.

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A symbol is solely in read-only.

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Symbols cannot be modified,
you must create a new one.

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It's a unique object,
so it always points to the same object.

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Strings are mutable objects,
you can modify their content.

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It says "for now" because
this point is under discussion.

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Symbols are often used as

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method selectors.

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And symbols
are good candidates for keys

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in dictionaries,

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namely identityDictionary.

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You remember this?

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We compare elements,

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and we are easily able
to calculate hashes starting with a symbol,

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in order to generate it in a dictionary.

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The important thing to remember
from this session,

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is that strings
are like any other collection.

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And that symbols are unique,

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in read-only mode, and immutable.