WEBVTT

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In this session,
we'll talk about loops in Pharo.

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In Pharo, loops are messages
sent to objects.

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There are many types of loops.

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They're messages
sent to numbers, collections,

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or even to blocks.

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In some cases, they are iterators
over the elements of a collection.

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This is an example:

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I send the message timesRepeat:

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to the integer 4.

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I pass it a block as a parameter.

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We studied blocks
in the previous course.

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This blocks starts here and ends there.

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This message repeats
the evaluation of this block

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several times: in this example,
four times

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as the receiver is 4.

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There are other types of messages
to create loops.

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In this example,
it's the message to:do:.

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The message to:do:
is defined on the class Number.

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I'll also pass it a block
as a parameter,

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the last argument of the message.

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This block gets executed
a number of times

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with a loop cursor
that goes from the receiver

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to the first argument of the message.

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For instance,

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with the exact same example:

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I send the message to:do:
to the integer 1.

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In the block,
you display the parameter of the block,

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the parameter i at each loop.

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In the transcript, you have all
the integers between 1 and 100.

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There are other messages
to create loops.

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For instance, the message to:by:do

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to increment by more than 1.

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You control the scale of the increment.

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You can increment by 3 in this example.

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With every loop,
i takes values by step of 3.

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You can see what happens
when you execute this bit of code.

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You can see i went from 1 to 100

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with an increment of 3 each time.

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There are also iterators
to create loops.

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Iterators are messages
that must be sent to collections,

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to collection objects.

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There are many types such as do:.

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do: iterates
over every element of a collection.

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collect: iterates
over the elements of a collection

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but also collects the results

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to put them in a new collection.

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select: selects matching elements
in a collection:

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you build a new collection
with those you selected.

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reject: rejects the elements
of a collection you don't want.

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detect: helps to determine

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whether an element exists
in a given collection, etc.

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We'll study some of these
in the next slides.

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do:, for instance,
is the most common message.

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You send the message do:
to a collection and pass it a block.

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With every loop,

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the parameter of the block, :each,

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receives the first element
of the collection,

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then the second, until the final element
of the collection.

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This is an example:
it's a defined collection.

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A reminder of the syntax: open with #(

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and close with )
for literal collections.

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This collection contains four integers:
15, 10, 19, and 68.

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You send the message do:
to the collection and pass it a block.

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i, the parameter of the block,

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takes the value 15 on the first loop,

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then 10, 19, and 68.

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This is another message: whileTrue:.

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whileTrue: can also create loops,

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but it is a message you send to a block.

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It is defined over the class Block.

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This is the receiver block.

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There's a block of arguments here.

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I give you a concrete example
from the class Color.

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This is whileTrue:.

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In the receiver block,
there's a condition.

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The block is evaluated to true or false.

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Depending on the evaluation
of the first block, the receiver,

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you trigger or don't trigger
the execution

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of the second block,
which is the parameter of whileTrue:.

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There's another version of whileTrue:.

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It's the method whileTrue
without a parameter.

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It takes the receiver block
and evaluates it.

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If it returns true or false,

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it is re-evaluated once more, and so on.

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You re-evaluate the block
as long as its value is true.

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You have the twins of whileTrue
with the methods whileFalse,

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with and without parameters.

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Every variation exists
when it comes to classes and blocks.

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To sum up, in this course,

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we saw that loops are expressed
as normal messages in Pharo.

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They're sent to objects

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(integers, collections, blocks...).

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You can use them to build
plenty of different loops.

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You can even add some if you wish.